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  • 1
    Digitale Medien
    Digitale Medien
    Turbulence: An Old Challenge and New Perspectives (1998)
    Springer
    Meccanica 33 (1998), S. 445-468 
    Details
    ISSN: 1572-9648
    Schlagwort(e): Turbulence ; Wall-bounded turbulent shear flows ; Scaling laws in turbulence ; Local structure of turbulence ; Fluid mechanics
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract Turbulence at very large Reynolds numbers (often called developed turbulence) is widely considered to be one of the happier provinces of the turbulence realm, as it is widely thought that two of its basic results are well-established, and have a chance to enter, basically untouched, into a future complete theory of turbulence. These results are the von Kármán-Prandtl universal logarithmic law in the wall-region of wall-bounded turbulent shear flow, and the Kolmogorov-Obukhov scaling laws for the local structure of developed turbulent flow.However, doubts have been expressed over the years about the fluid mechanical assumptions that underlie these laws. After a concise review of the problem of turbulence as a whole we will show in the present paper that the von Kármán–Prandtl universal logarithmic law is based on an assumption which,though plausible, in fact is not quite correct. We will come to the conclusion, based on theoretical considerations and on processing of experimental data, that the universal logarithmic law does not describe the real features of developed turbulent wall-bounded flow of viscous fluid; it should be jettisoned and replaced by a different law, a scaling law. Experimental evidence for the local structure of turbulent flows is now not sufficiently well-established to allow a similarly definite conclusion. However, the application of the new approach presented here makes it very plausible that the classical, non-modified version of Kolmogorov–Obukhov ‘K-41’ laws gives an adequate description of the local features of developedturbulent flows. Sommario.La turbolenza agli altissimi numeri di Reynolds (spesso chiamata turbolenza sviluppata) è largamente ritenuta una delle regioni felici del regno della turbolenza: si pensa infatti che due suoi risultati fondamentali siano ben assodati e che abbiano speranza di entrare senza rilevanti modifiche in una futura teoria completa della turbolenza.Questi risultati sono la legge logaritmica universale di Kármán–Prandtl per la regione di parete dei flussi turbolenti confinati e la legge di simulitudine di Kolmogorov–Obukhov per la struttura locale del flusso turbolento sviluppato. Nel corso degli anni sono stati tuttavia espressi dubbi sulle ipotesi fluidodinamiche che sottendono queste leggi. Nel presente lavoro, dopo un breve esame del problema della turbolenza nel suo insieme,dimostreremo che la legge logaritmica universale di Kármán–Prandtl è basata su un'assunzione che, per quanto plausibile, non è del tutto corretta. Giungeremo alla conclusione, basata su considerazioni teoriche esull'elaborazione di dati sperimentali, che la legge logaritmica universale non descrive le caratteristiche reali del flusso turbolento di un fluido viscoso sviluppato e confinato da una parete; essa dovrebbe essere sostituita de una legge differente, una legge di similitude. L'evidenza sperimentale per la stzuttura locale di flusso turbolento non è al momento sufficientemente assodata perpermettere una conclusione altrettanto definita. L'impiegodel nuovo approccio qui presentato, tuttavia, rende assai plausible che la classica versione non modificata della legge ‘K-41’ di Kolmogorov-Obukhov fornisca una adeguata descrizione delle caratteristiche locali del flussoturbolento sviluppato.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1004312409376
    Standort Signatur Erwartet Verfügbarkeit
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  • 2
    Digitale Medien
    Digitale Medien
    A Computational Algorithm for Second-Moment Closure Turbulence Modelling (1998)
    Springer
    Meccanica 33 (1998), S. 29-46 
    Details
    ISSN: 1572-9648
    Schlagwort(e): Finite difference methods ; Free shear layers ; Turbulence ; Fluid mechanics
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract A computational formulation is proposed for second-moment closure turbulence models, especially suited to models intended to ensure physical realizability. It enables to cast the quite complicated model equations in a compact form. It is specifically applied here to a two-dimensional parabolized flow, though it lends itself to extension to more complex flows. An effective computational algorithm is proposed, based on a staggered grid and a block tridiagonal solver. The algorithm is applied to a turbulent mixing layer, and the comparison between the predictions obtained by standard modelling tools and a realizable second-moment closure clearly points out the superiority of the latter.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1004295401311
    Standort Signatur Erwartet Verfügbarkeit
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  • 3
    Digitale Medien
    Digitale Medien
    Quantitative Flow Visualization (1998)
    Springer
    Meccanica 33 (1998), S. 503-516 
    Details
    ISSN: 1572-9648
    Schlagwort(e): Flow visualization ; Speckle photography ; Particle image velocimetry ; Turbulence ; Fluid mechanics
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract Modern developments in laser and computer technology, electronic cameras, and digital image processing techniques allow to generate planar distributions of quantitative data in turbulent flows. Large amounts of data can be processed easily and analyzed statistically. With these tools, it is possible to quantitatively visualize turbulent coherent structures, even in flows of high Reynolds number, and measure characteristic spatial quantities like vorticity, length scales, spatial correlation functions, etc. These potentials in analyzing spatial characteristics of turbulent flows are demonstrated with two different methods of quantitative flow visualization: speckle photography as a representative of the line-of-sight methods, and particle image velocimetry belonging to the methods that rely on the scattering of laser light from tracer particles. Sommario.I moderni sviluppi nelle tecnologie del laser e dei computers, delle telecamere elettroniche e le tecniche di analisi digitale delle immagini permettono di ottenere distribuzioni quantitative, in un piano, di dati relativi a flussi turbolenti. Una gran quantità di dati può essere con facilità analizzata statisticamente. Con questi mezzi è possibile visualizzare quantitativamente strutture coerenti turbolente anche in flussi da alto numero di Reynolds, e misurare caratteristiche spaziali, come vorticità scale e funzioni di correlazione. Questa potenzialità di studiare caratteristiche spaziali di flussi turbolenti viene qui mostrata per due differenti metodi di visualizzazione quantitativa: fotografia ‘speckle’ e PIV (particle image velocimetry).
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1004372627123
    Standort Signatur Erwartet Verfügbarkeit
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  • 4
    Digitale Medien
    Digitale Medien
    The Solar Wind - Inner Heliosphere (1998)
    Springer
    Space science reviews 83 (1998), S. 75-86 
    Details
    ISSN: 1572-9672
    Schlagwort(e): Solar wind ; Heliosphere ; Ulysses ; SOHO ; MHD waves ; Turbulence
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract The solar wind in the inner heliosphere, inside ~ 5 AU, has been almost fully characterized by the addition of the high heliographic latitude Ulysses mission to the many low latitude inner heliosphere missions that preceded it. The two major omissions are the high latitude solar wind at solar maximum, which will be measured during the second Ulysses polar passages, and the solar wind near the Sun, which could be analyzed by a Solar Probe mission. Here, existing knowledge of the global solar wind in the inner heliosphere is summarized in the context of the new results from Ulysses.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1005069328058
    Standort Signatur Erwartet Verfügbarkeit
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  • 5
    Digitale Medien
    Digitale Medien
    Computer modelling of the meteorological and spraying parameters that influence the aerial dispersion of agrochemical sprays (1998)
    Springer
    International journal of biometeorology 41 (1998), S. 194-199 
    Details
    ISSN: 1432-1254
    Schlagwort(e): Key words Meteorology ; Turbulence ; Modelling ; Spray drift ; Wind
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geographie , Physik
    Notizen: Abstract  An insight into the nature of prevailing meteorological conditions and the manner in which they interact with spraying parameters is an important prerequisite in the analysis of the dynamics of agrochemical sprays. Usually, when these sprays are projected from hydraulic nozzles, their initial velocity is greater than that of the ambient wind speed. The flowfield therefore experiences changes in speed and direction which are felt upstream as well as downstream of the spray droplets. The pattern of the droplet flow, i.e. the shape of the streamlines marking typical trajectories, will be determined by a balance of viscous forces related to wind speed, inertial forces resulting from the acceleration of the airstream and pressure forces which can be viewed in terms of the drag forces exerted on the spray droplets themselves. At a certain distance in the ensuing motion, when the initial velocity of the spray droplets has decreased sufficiently for there to be no acceleration, their trajectories will be controlled entirely by the random effects of turbulence. These two transport processes in the atmosphere can be modelled mathematically using computers. This paper presents a model that considers the velocity of spray droplets to consist of a ballistic velocity component superimposed by a random-walk velocity component. The model is used to study the influence of meteorological and spraying parameters on the three-dimensional dynamics of spray droplets projected in specified directions in neutral and unstable weather conditions. The ballistic and random-walk velocity components are scaled by factors of (1–ξ) and ξ respectively, where ξ is the ratio of the sedimentation velocity and the relative velocity between the spray droplets and the surrounding airstream. This ratio increases progressively as the initial velocity of the spray droplet decreases with air resistance and attains a maximum when the sedimentation velocity has been reached. As soon as this occurs, the random-walk process predominates. The computed effects of the release height of spray droplets, atmospheric turbulence intensity, evaporation, drop size spectrum, wind velocity and wind direction on the transport process have been studied and an analysis of spray drift is provided.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s004840050074
    Standort Signatur Erwartet Verfügbarkeit
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  • 6
    Digitale Medien
    Digitale Medien
    Horizontal, Two Point Coherence for Separations Greater Than the Measurement Height (1998)
    Springer
    Boundary layer meteorology 87 (1998), S. 459-480 
    Details
    ISSN: 1573-1472
    Schlagwort(e): Coherence model ; Spatial coherence ; Turbulence
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract Wind speed measurements from the test site at Rutherford Appleton Laboratory have been evaluated with respect to the spatial coherence function. The experimental arrangement provides coherence information for separation distances of 62, 80 and 102 m. These are at least three times greater than the measurement heights of 18 m and 18.7 m. Based on these experimental data and data published in the literature, different theoretical formulations are compared and a new, but simple, model for longitudinal and lateral coherence is proposed. At large separations the turbulent wind field is not isotropic, theoretical models to describe the coherence function for such distances are not available. The new model we propose builds on the classical exponential approach. It takes into account the influence of turbulence intensity and models the angular dependence of horizontal coherence. It is found that, for constant turbulence intensity, the lateral coherence decay becomes independent of the mean wind speed.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1000997610233
    Standort Signatur Erwartet Verfügbarkeit
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  • 7
    Digitale Medien
    Digitale Medien
    Landscape Roughness Parameters for Sherwood Forest – Experimental Results (1998)
    Springer
    Boundary layer meteorology 89 (1998), S. 285-316 
    Details
    ISSN: 1573-1472
    Schlagwort(e): Boundary layer ; Heterogeneous terrain ; Roughness length ; Surface fluxes ; Turbulence
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract The aim of this work is to present experimentally evaluated effective roughnesses (zoe) of a partly forested landscape. Although the ratio of boundary-layer height to obstacle size was only of the order of 50, there still seemed to exist a height range of 75–200 m where surface-layer similarity was approximately valid. Attempts were made to use conventional wind profile analysis to evaluate zoe, but the small height range and the large number of variables initially led to unacceptable uncertainties. Fixing the displacement height zd, rather than fitting it, reduced the data scatter to an acceptable level. The profile-derived roughness lengths zop obtained in this way were in good agreement with previous work, and with an alternative roughness length estimate zof for which flux-derived profile parameters u* and θ* were used. This implies that the profile-derived roughnesses were consistent with the measured surface-layer momentum flux. Comparison of both roughness estimates also yielded an improved estimate of the displacement height. Besides this, the authors tested a landscape roughness evaluation method which makes use of the gustiness parameter Tu = σu/U in the surface layer. The results obtained by this method were in fair agreement with the profile-derived data. In previous work, the gustiness method was advocated because it could be used at relatively low levels, perhaps even within the roughness sub-layer. At the present measuring site, this was not the case as the gustiness method was only valid in an approximate way, and for a limited height range.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1001743525309
    Standort Signatur Erwartet Verfügbarkeit
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  • 8
    Digitale Medien
    Digitale Medien
    Analysis of Astex-Stratocumulus Observational Data Using a Mass-Flux Approach (1998)
    Springer
    Boundary layer meteorology 86 (1998), S. 63-87 
    Details
    ISSN: 1573-1472
    Schlagwort(e): Stratocumulus ; Mass flux ; Lateral entrainment ; Turbulence ; Conditional sampling
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract A mass-flux approach is applied to observational data obtained in a convective boundary layer topped with stratocumulus clouds. The observational data were obtained from aircraft measurements during the Atlantic Stratocumulus Transition Experiment (ASTEX). A conditional sampling method is used to calculate average updraft and downdraft values. The vertical fluxes calculated with the mass-flux approach are found to be proportional to the real (measured) fluxes, with a proportionality factor being about 0.6. This value is predicted by theory for two variables having a joint Gaussian distribution function; proportionality factor = 2π-1 ≈ 0.637. The horizontal fractional entrainment and detrainment rates calculated from the data (ε ≈ 1–2 × 10-2 m-1) are an order of magnitude higher than the rates obtained by large eddy simulations for cumulus convection (ε ≈ 2–3 × 10-3 m-1) and two orders of magnitude higher than those used in modelling cumulus convection with a mass-flux scheme in an operational weather forecast model (ε ≈ 3 × 10-4 m-1). A numerical mass-flux model for the thermodynamics was developed and showed that results are in good agreement when compared with measured profiles of the liquid water content.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1000624004907
    Standort Signatur Erwartet Verfügbarkeit
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  • 9
    Digitale Medien
    Digitale Medien
    Atmospheric Turbulence Characteristics Over a Temperature-Inhomogeneous Land Surface. Part II: The Effect of Small-Scale Inhomogeneities of Surface Temperature on Some Characteristics of the Atmospheric Surface Layer (1998)
    Springer
    Boundary layer meteorology 86 (1998), S. 103-124 
    Details
    ISSN: 1573-1472
    Schlagwort(e): Turbulence ; Surface temperature ; Surface inhomogeneity ; Source area
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract Turbulence characteristics, vertical profiles of wind velocity u(z) and air temperature T(z), and also spatial variations in steppe surface radiation temperature Tr(x) are measured simultaneously. A marked effect of Tr(x) characteristics on the turbulence characteristics and T(z) profiles is observed in the lower part of the atmospheric surface layer. We suggest that variability in Tr(x) noticeably influences the surface-layer temperature field and leads to scatter in the values of the universal functions obtained by different authors; effects of Tr(x) are not accounted for in similarity theory. The introduction of the value of temperature zero-plane displacement dT in the calculation formulae (to determine temperature flux) noticeably improves the agreement between calculated and measured (by eddy-correlation method) results. The influence of footprint (or Source Area) on the obtained results leads to noticeable scatter in the data obtained from measurements of atmospheric turbulence.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1000652913936
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  • 10
    Digitale Medien
    Digitale Medien
    Turbulence and Coherent Organizations in the Atmospheric Boundary Layer: A Radar-Aircraft Experimental Approach (1998)
    Springer
    Boundary layer meteorology 86 (1998), S. 147-179 
    Details
    ISSN: 1573-1472
    Schlagwort(e): Turbulence ; Convective boundary layer ; Clear air radar observations ; Coherent organizations ; Radar-aircraft joint observations
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract The scientific objective of the TRAC experiment (Turbulence Radar Aircraft Cells) was to investigate the respective roles played by small-scale turbulence and coherent structures in the vertical transfer within the atmospheric boundary layer (ABL). Field research held in June 1993 in France was based on coupled aircraft and Doppler radar measurements. The results discussed here are mainly focused on the evaluation of the performance of the radar in the 3D description of the clear air ABL, which was the technical goal of TRAC. During the experiment, the radar was able to provide continuous and coherent echo fields over a range of several tens of kilometres, extending up to about 3 km. Good agreement was obtained in the ABL between the radar-derived turbulent quantities and airborne measurements. As depicted by the reflectivity fields, coherent organizations were found to be a common feature of the eleven ABL cases analyzed. These organizations evolved during the day between a banded structure and a cellular pattern. A very weak correlation was found between the reflectivity field and the atmospheric parameters measured by the aircraft. However, in terms of characteristic scale, the reflectivity field appeared to be strongly related to the water vapour field. The inhomogeneity induced by the coherent circulations questions the representativity of one-dimensional sampling of these 3D fields and suggests the need to adapt the traditional statistical approach of the ABL.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1000613232592
    Standort Signatur Erwartet Verfügbarkeit
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  • 11
    Digitale Medien
    Digitale Medien
    Coherent Doppler Lidar Measurements of Wind Field Statistics (1998)
    Springer
    Boundary layer meteorology 86 (1998), S. 233-256 
    Details
    ISSN: 1573-1472
    Schlagwort(e): Lidar ; Remote sensing ; Turbulence ; Velocity statistics
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract Coherent Doppler lidar measurements of wind statistics in the boundary layer are presented. The effects of the spatial averaging by the lidar pulse are removed using theoretical corrections and computer simulations. This permits unbiased estimates of velocity variance, spatial velocity structure functions, energy dissipation rate, and other point statistics of the velocity field.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1000676021745
    Standort Signatur Erwartet Verfügbarkeit
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  • 12
    Digitale Medien
    Digitale Medien
    On the Formulation of Lagrangian Stochastic Models of Scalar Dispersion Within Plant Canopies (1998)
    Springer
    Boundary layer meteorology 86 (1998), S. 333-344 
    Details
    ISSN: 1573-1472
    Schlagwort(e): Turbulence ; Dispersion model ; Non-uniqueness ; Plant canopy
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract Lagrangian stochastic models, quadratic in velocity and satisfying the well-mixed condition for two-dimensional Gaussian turbulence, are used to make predictions of scalar dispersion within a model plant canopy. The non-uniqueness associated with satisfaction of the well-mixed condition is shown to be non-trivial (i.e. different models produce different predictions for scalar dispersion). The best agreement between measured and predicted mean concentrations of scalars is shown to be obtained with a small sub-class of ‘optimal’ models. This sub-class of ‘optimal’ models includes Thomson's model (J. Fluid Mech. 180, 529–556, 1987), the simplest model that satisfies the well-mixed condition for Gaussian turbulence, but does not include two other models identified recently as being in optimal agreement with the measured spread of tracers in a neutral boundary layer. It is therefore demonstrated that such models are not universal, i.e. applicable to a wide range of flows without readjustment of model parameters. Predictions for scalar dispersion in the model plant canopy are also obtained using the model of Flesch and Wilson (Boundary-Layer Meteorol. 61, 349–374, 1992). It is shown that, when used with a Gaussian velocity distribution or a maximum-missing-information velocity distribution, which accounts for the measured skewness and kurtosis of velocity statistics, the agreement between predictions obtained using the model of Flesch and Wilson and measurements is as good as that obtained using Thomson's model.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1000673418479
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  • 13
    Digitale Medien
    Digitale Medien
    A Lagrangian Decorrelation Time Scale in the Convective Boundary Layer (1998)
    Springer
    Boundary layer meteorology 86 (1998), S. 525-534 
    Details
    ISSN: 1573-1472
    Schlagwort(e): Lagrangian decorrelation time scales ; Structure function constant ; Turbulence ; Lagrangian spectrum constant
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract A new method for deriving the Lagrangian decorrelation time scales for inhomogeneous turbulence is described. The expression for the time scales here derived for the convective boundary layer is compared to those estimated by Hanna during the Phoenix experiment. Then the values of C0, the Lagrangian velocity structure function constant, and of Bi, the Lagrangian velocity spectrum constant, were evaluated from the Eulerian velocity spectra and from the Lagrangian time scales derived, under unstable conditions, from Taylor's statistical diffusion theory. The numerical coefficient of the lateral and vertical Lagrangian spectra in the inertial subrange was found equal to 0.21, in good agreement with previous experimental estimates.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1000734626931
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  • 14
    Digitale Medien
    Digitale Medien
    The Use of State-of-the-Art Kites for Profiling the Lower Atmosphere (1998)
    Springer
    Boundary layer meteorology 87 (1998), S. 1-25 
    Details
    ISSN: 1573-1472
    Schlagwort(e): Kites ; Atmospheric measurements ; Turbulence ; Profiling ; Ozone
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract This paper presents the capabilities and limitations of using state-of-the-art kites for atmospheric research. A brief historical review of the subject is first presented, followed by an outline of the current status of kite-borne measurement technology. The utility of the technique is then illustrated by presenting a series of recent measurements made using kite-borne technology. A summary of the advantages and limitations of kite-based measurements relative to other technologies is provided for reference.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1000812511429
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  • 15
    Digitale Medien
    Digitale Medien
    Evolution of Magnetic Fields in Galaxies with Spiral Structure (1998)
    Springer
    Studia geophysica et geodaetica 42 (1998), S. 382-390 
    Details
    ISSN: 1573-1626
    Schlagwort(e): Magnetohydrodynamics (MHD) ; Turbulence ; Magnetic fields ; Galaxies
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Architektur, Bauingenieurwesen, Vermessung , Geologie und Paläontologie , Physik
    Notizen: Abstract We present simulations of the 3D nonlinear induction equation in order to investigate the temporal evolution of large-scale magnetic fields in spiral galaxies. Our model includes differential rotation, ambipolar diffusion and, based on small-scale turbulence, eddy diffusivity and the tensorial α-effect with magnetic feedback. The nonaxisymmetric spiral pattern and – if considered – the vertical stratification of the galaxy are represented in its density and turbulence profile. Neglecting vertical stratification the lifetime and geometry of an initial magnetic field depend on the correlation time of interstellar turbulence τ corr . Short correlation times increase the lifetime of the initial magnetic field, but the field is rapidly wound up. Its pitch-angles develop to zero. The magnetic field has disappeared after at most 1 to 1.5 Gyr. A resonance like phenomenon is found by tuning the pattern velocity of the galactic spiral. The simulations then show an exceptional amplification of the magnetic field in the case that the pattern speed and a magnetic drift velocity have similar values. Considering a vertical stratification we achieve sufficiently long living grand-designed magnetic fields excited by dynamo action. The behaviour and geometry of the resulting field is again significantly influenced by the correlation time τ corr . Small values of τ corr lead to axisymmetric fields with small pitch-angles and field-concentration between the spiral arms. Increasing the correlation time the solutions show larger pitch-angles; and depending on very large correlation times the galactic dynamo rather generates fields clearly within the spiral arms and having a bisymmetric structure.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1023373224265
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  • 16
    Digitale Medien
    Digitale Medien
    A Case Study of Rayleigh-Bénard Convection with Clouds (1998)
    Springer
    Boundary layer meteorology 88 (1998), S. 211-237 
    Details
    ISSN: 1573-1472
    Schlagwort(e): Aircraft measurements ; Compositing ; Rayleigh-Bénard convection ; Spectral energetics ; Thermals ; Turbulence
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract A turbulence data set collected by the research aircraft Hercules and Falcon in the planetary boundary layer (PBL) over the North Sea during Rayleigh-Bénard convection (RBC) is analysed. Altogether nearly three hundred cell passages at different levels and in two different flight directions were sampled. The convective boundary-layer height (H) was about 1 km, and the RBC cells had a diameter D of roughly 2–3 km, resulting in an aspect ratio A = D/H ≈ 2–3. This value is also found in the case of RBC in laboratory-scale flows, whereas most of the recent PBL experimental work reports convection PBL rolls with A ≈ 3 and mesoscale cellular convection (MCC) with A ≈ 10–40 over the oceans. The large number of RBC cell passages made it possible to composite their average structure. Due to the more complex three-dimensional structure and the importance of thermals to the RBC dynamics, spectral, temporal and spatial decompositions and model calculations were necessary to illuminate structure, dynamics, energetics and organisation. The final impression is that the structure of RBC in the PBL is given by a honeycomb-like arrangement of short-lived mixed-layer thermals with more passive downward motions in between. The regularity of the Cu-cloud cover results partly from the more stationary flow in the cloud-free cell centres. On the other hand it is shown that active as well as inactive clouds contribute to the cloud cover. Thus, the PBL flow and the cloud cover are decoupled, at least temporarily and locally. Due to sparse observational and measured information about RBC occurrence and structure in the PBL, additional material was gathered, resulting in the impression that RBC is one additional realised mode of organised convection in the PBL, as has already been clarified for PBL rolls and MCC by recent investigations.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1001145803614
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  • 17
    Digitale Medien
    Digitale Medien
    Evidence of Organized Large Eddies by Ground-Based Doppler Lidar, Sonic Anemometer and Sodar (1998)
    Springer
    Boundary layer meteorology 88 (1998), S. 343-361 
    Details
    ISSN: 1573-1472
    Schlagwort(e): Heterodyne Doppler lidar ; Organized large eddies ; Planetary boundary laye ; Turbulence
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract In an experiment investigating the planetary boundary layer (PBL) wind and temperature fields, and PBL inversion height recorded by various instruments, the results reveal the presence of organized large eddies (OLE) or rolls. The measurements by lidars, anemometers, soundings and sodar gave an overview of the characteristics of the rolls and sources of energy production that maintain them. The experimental results obtained on two consecutive days are compared to model outputs. The agreement is excellent, showing that thermal stratification and wind shear are important factors in the structure and dynamics of OLE. A heterodyne Doppler lidar (HDL) is shown to be a useful tool in the study of OLE.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1001167212584
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  • 18
    Digitale Medien
    Digitale Medien
    An Investigation of Higher-Order Closure Models for a Forested Canopy (1998)
    Springer
    Boundary layer meteorology 89 (1998), S. 47-74 
    Details
    ISSN: 1573-1472
    Schlagwort(e): Canopy closure model ; Pine forest ; Turbulence
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract Simultaneous triaxial sonic anemometer velocity measurements vertically arrayed at six levels within and above a uniform pine forest were used to examine two parameterization schemes for the triple-velocity correlation tensor employed in higher-order closure models. These parameterizations are the gradient-diffusion approximation typically used in second-order closure models, and the full budget for the triple-velocity correlation tensor typically employed in third-order closure models. Both second- and third-order closure models failed to reproduce the measured profiles of the triple-velocity correlation within and above the canopy. However, the Reynolds stress tensor profiles (including velocity variances) deviated greatly from the measurements only within the lower levels of the canopy. It is shown that the Reynolds stresses are most sensitive to the parameterization of the triple-velocity correlation in these lower canopy regions where local turbulent production is negligible and turbulence is mainly sustained by the flux transport term. The failure of the third-order closure model to reproduce the measured third moments in the upper layers of the canopy-top contradicts conclusions from a previous study over shorter vegetation but agrees with another study for a deciduous forest. Whether the third-order closure model failure is due to the zero-fourth-cumulant closure approximation is therefore considered. Comparisons between measured and predicted quadruple velocity correlations suggest that the zero-fourth-cumulant approximation is valid close to the canopy-atmosphere in agreement with recent experiments.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1001509106381
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  • 19
    Digitale Medien
    Digitale Medien
    Radiative Effects on Temperature in the Stable Surface Layer (1998)
    Springer
    Boundary layer meteorology 89 (1998), S. 141-159 
    Details
    ISSN: 1573-1472
    Schlagwort(e): Radiation ; Stable conditions ; Turbulence
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract The interaction between longwave radiation and temperature fluctuations plays a role in the dissipation of temperature variance. This interaction is most easily described by spectral models of atmospheric turbulence and a spectral radiative dissipation function which gives the intensity of the damping at each radiative wavelength λ and wavenumber k. We have used a Corrsin–Pao closure for the spectral budgets of turbulent kinetic energy and temperature to study the coupling of radiation to turbulence. The spectral radiative dissipation function and a related integral have been fitted by analytical approximations with the correct asymptotic behavior. This resulted in a simple analytical formula for the dimensionless temperature spectrum as a function of Monin-Obukhov stability, and a new dimensionless parameter describing the relative importance of radiation in the temperature spectral budget. The radiative effects both on the temperature spectrum and on the dimensionless temperature variance can then be calculated. Based on typical values of the radiative dimensionless parameters for the surface layer, we conclude that radiative dissipation is probably negligible there.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1001574605472
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  • 20
    Digitale Medien
    Digitale Medien
    Turbulent Flux Measurements Above and Below the Overstory of a Boreal Aspen Forest (1998)
    Springer
    Boundary layer meteorology 89 (1998), S. 109-140 
    Details
    ISSN: 1573-1472
    Schlagwort(e): BOREAS ; Eddy correlation ; Eddy covariance ; Turbulence ; Understory
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract Turbulent flux measurements both above and beneath the canopy of a boreal aspen forest are described. Velocity skewness showed that, beneath the aspen canopy, turbulence was dominated by intermittent, downward penetrating gusts. Eulerian horizontal length scales calculated from integration of the autocorrelation function or spectral peaks were 9.0 and 1.4 times the mean aspen height of 21.5 m respectively. Above-canopy power spectral slopes for all velocity components followed the -2/3 power law, whereas beneath-canopy slopes were closer to -1 and showed a spectral short cut in the horizontal and vertical components. Cospectral patterns were similar both above and beneath the canopy. The Monin–Obukhov similarity function for the vertical wind velocity variance was a well-defined function of atmospheric stability, both above and beneath the canopy. Nocturnal flux underestimation and departures of this similarity function from that expected from Monin–Obukhov theory were a function of friction velocity. Energy balance closure greater than 80% was achieved at friction velocities greater than 0.30 and 0.10 m s-1, above and below the aspen canopy, respectively. Recalculating the latent heat flux using various averaging periods revealed a minimum of 15 min were required to capture 90% of the 30-min flux. Linear detrending reduced the flux at shorter averaging periods compared to block averaging. Lack of energy balance closure and erratic flux behaviour led to the recalculation of the latent and sensible heat fluxes using the ratio of net radiation to the sum of the energy balance terms.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1001557022310
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  • 21
    Digitale Medien
    Digitale Medien
    Computing the Scaling Exponents in Fluid Turbulence from First Principles: Demonstration of Multiscaling (1998)
    Springer
    Journal of statistical physics 93 (1998), S. 797-832 
    Details
    ISSN: 1572-9613
    Schlagwort(e): Turbulence ; universal statistics ; anomalous scaling ; multi-fractals
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract We develop a consistent closure procedure for the calculation of the scaling exponents ζ n of the nth-order correlation functions in fully developed hydro-dynamic turbulence, starting from first principles. The closure procedure is constructed to respect the fundamental rescaling symmetry of the Euler equation. The starting point of the procedure is an infinite hierarchy of coupled equations that are obeyed identically with respect to scaling for any set of scaling exponents ζ n . This hierarchy was discussed in detail in a recent publication by V. S. L'vov and I. Procaccia. The scaling exponents in this set of equations cannot be found from power counting. In this paper we present in detail the lowest non-trivial closure of this infinite set of equations, and prove that this closure leads to the determination of the scaling exponents from solvability conditions. The equations under consideration after this closure are nonlinear integro-differential equations, reflecting the nonlinearity of the original Navier–Stokes equations. Nevertheless they have a very special structure such that the determination of the scaling exponents requires a procedure that is very similar to the solution of linear homogeneous equations, in which amplitudes are determined by fitting to the boundary conditions in the space of scales. The renormalization scale that is necessary for any anomalous scaling appears at this point. The Hölder inequalities on the scaling exponents select the renormalization scale as the outer scale of turbulence L. We demonstrate that the solvability condition of our equations leads to non-Kolmogorov values of the scaling exponents ζ n . Finally, we show that this solutions is a first approximation in a systematic series of improving approximations for the calculation of the anomalous exponents in turbulence.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/B:JOSS.0000033164.35116.35
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  • 22
    Digitale Medien
    Digitale Medien
    The Menschutkin reaction of benzyl p-toluenesulfonates with N,N-dimethylanilines. Evidence for the duality of SN1 and SN2 mechanisms (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 254-260 
    Details
    ISSN: 0894-3230
    Schlagwort(e): Menschutkin reaction ; duality of SN1, SN2 mechanisms ; benzyl tosylates ; Yukawa-Tsuno equation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The rate data for the Menschutkin reaction between strongly activated Z-substituted benzyl p-toluenesulfonates and Y-substituted N,N-dimethylanilines in acetonitrile at 35 °C fit the equation, kobs  =  k1 + k2 [DMA], which is consistent with concurrent first- and second-order processes. The k1 and k2 values for each substrate were separated based on the above equation. The SN1 rate constant, k1, is unaffected by the nature of the nucleophile, whereas the SN2 rate constant, k2, increased with the electron-donating substituent of the N,N-dimethylaniline. The substituent effect on the k1 values is linearly correlated by the Yukawa-Tsuno equation with ρ  =  -5.2 and r  =  1.3. The unimolecular reaction can be regarded as a classical SN1 mechanism. In contrast, that on the k2 values shows an upward curvature when analyzed by the Brown σ+ treatment. These results are ascribed to the simultaneous and independent occurrence of SN1 and SN2 mechanisms in the present Menschutkin reaction. © 1998 John Wiley & Sons, Ltd.
    Zusätzliches Material: 6 Ill.
    Materialart: Digitale Medien
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  • 23
    Digitale Medien
    Digitale Medien
    Editorial (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 231-231 
    Details
    ISSN: 0894-3230
    Schlagwort(e): Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: No Abstract
    Materialart: Digitale Medien
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  • 24
    Digitale Medien
    Digitale Medien
    Solvent effects on infrared spectroscopic and calorimetric characteristics of aliphatic ketones in binary solvent mixtures (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 283-289 
    Details
    ISSN: 0894-3230
    Schlagwort(e): calorimetry ; IR spectra ; ketones ; solvation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Solution enthalpies of n-hexane, acetone, butan-2-one and octan-2-one in a series of tetrachloromethane-acetonitrile mixtures and the carbonyl stretching absorption frequencies in the IR spectra of these ketones were determined. It was found that over the whole range of concentrations (varying from neat tetrachloromethane up to neat acetonitrile) the solvation enthalpy of these compounds can be obtained additively from the contributions of the alkyl and carbonyl fragments. The solvent effect on the solvation enthalpy of the carbonyl group was found to be satisfactorily correlated with the corresponding IR frequency shifts of the C=O group. It was also found that the sensitivities of the carbonyl IR frequencies to the solvent composition are different for various ketones. From both IR and calorimetric data, the preferential solvation parameters were evaluated. The differences between the IR spectroscopic and calorimetric data are discussed. © 1998 John Wiley & Sons, Ltd.
    Zusätzliches Material: 4 Ill.
    Materialart: Digitale Medien
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  • 25
    Digitale Medien
    Digitale Medien
    Solvent effects on C=O stretching frequencies of some 1-substituted 2-pyrrolidinones (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 261-272 
    Details
    ISSN: 0894-3230
    Schlagwort(e): 1-substituted 2-pyrrolidinones ; C=O stretching frequencies ; solvent effects ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: In an effort to model solute-solvent interactions, the C=O stretching frequencies of five 1-substituted 2-pyrrolidinones and four other carbonyl-containing compounds were measured for 30 common solvents. These were then correlated with four empirical parameter sets and one theoretical (computational) parameter set. While an empirical parameter set gave the best correlation equations, the theoretical parameter equations are physically and statistically significant. Solvent volume, polarizability and hydrogen bond donor acidity (capacity) terms are significant in the correlation equations. © 1998 John Wiley & Sons, Ltd.
    Zusätzliches Material: 14 Tab.
    Materialart: Digitale Medien
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  • 26
    Digitale Medien
    Digitale Medien
    Molecular orbital calculations on the P - S bond cleavage step in the hydroperoxidolysis of nerve agent VX (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 232-240 
    Details
    ISSN: 0894-3230
    Schlagwort(e): P - S bond cleavage ; hydroperoxidolysis ; nerve agent VX ; molecular orbital calculations ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The P - S bond cleavage process in the hydroperoxidolysis of a model system for the nerve agent VX was studied using ab initio and semiempirical molecular orbital methods. Aqueous solvation effects were included through single-point calculations using the semiempirical SM5.2PD/A continuum solvation model and geometries optimized at the HF/MIDI! level of theory. The predominant pathway for P - S bond cleavage involves pseudorotation of a low-energy trigonal bipyramidal intermediate followed by apical ligand ejection. In aqueous solution, the free energy barriers for these processes are found to be 14.3 and 4.6 kcal mol-1, respectively, with electronic energies calculated at the MP2/cc-pVDZ//HF/MIDI! level of theory. By comparison with another continuum model of solvation (PCM), it is concluded that the SM5.2PD/A model performs well even for hypervalent phosphorus species, in spite of not having included any such molecules in the model's parameterization set. © 1998 John Wiley & Sons, Ltd.
    Zusätzliches Material: 4 Ill.
    Materialart: Digitale Medien
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  • 27
    Digitale Medien
    Digitale Medien
    Gas-phase thermolysis of N-methyl-N-phenyl-tert-butylsulfenamide and morpholinyl-tert-butylsulfenamide (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 407-410 
    Details
    ISSN: 0894-3230
    Schlagwort(e): gas-phase thermolysis ; N-methyl-N-phenyl-tert-butylsulfenamide ; morpholinyl-tert-butylsulfenamide ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: N-Methyl-N-phenyl-tert-butylsulfenamide (MPSA) and morpholinyl-tert-butylsulfenamide (MOSA) were thermolyzed in a stirred-flow reactor at temperatures of 340-390 °C and pressures of 7-13 Torr, using toluene as carrier gas, at residence times of 0.3-1.3 s. Isobutene was formed in 99% yield through first-order reactions having the following Arrhenius parameters (A,s-1, Ea, kJ mol-1): MPSA, log A  =  12.41 ± 0.02, Ea  =  158.8 ± 0.2; MOSA, log A  =  12.91 ± 0.22, Ea  =  159 ± 3. It is proposed that the elimination of isobutene takes place by unimolecular reaction mechanisms involving polar, four-center cyclic transition states, forming S-unsubstituted thiohydroxylamines as co-products. Thermochemical parameters, estimated by semiempirical AM1 calculations, are reported for the latter and for the parent molecules. © 1998 John Wiley & Sons, Ltd.
    Zusätzliches Material: 1 Ill.
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  • 28
    Digitale Medien
    Digitale Medien
    NMR and semi-empirical study of the tautomerism of­2,2′-bisbenzimidazolyl (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 411-418 
    Details
    ISSN: 0894-3230
    Schlagwort(e): 2,2′-bisbenzimidazolyl ; tautomerism ; NMR ; semi-empirical calculations ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: A dynamic NMR study of the tautomerism of 2,2′-bisbenzimidazolyl in DMSO-d6 and a mechanistic interpretation of the process, based on a stepwise, single-proton transfer and formation of a zwitterionic intermediate, are presented. This interpretation is substantiated by semi-empirical calculations of the postulated intermediate and transition state, that yield results which are compared with previous studies on related aliphatic systems. © 1998 John Wiley & Sons, Ltd.
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
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  • 29
    Digitale Medien
    Digitale Medien
    Effects of sulfenyl, sulfinyl and sulfonyl groups on acidities and homolytic bond dissociation energies of adjacent C - H and N - H bonds (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 397-406 
    Details
    ISSN: 0894-3230
    Schlagwort(e): bond dissociation energy ; acidity ; phenylsulfenylamides ; phenylsulfenylanilides ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Acidities and bond dissociation energies (BDEs) of the N-H bond in two phenylsulfenylamides, PhSNHBz and PhSNH-t-Bu, and four phenylsulfenylanilides, 4-GC6H4NHSPh, where G  =  MeO, H, Br and CN, were measured in order to compare the effects of substituents on acidities and BDEs of N - H bonds with those of C - H bonds. The effects of PhS groups on acidities and BDEs in a series of C - H acids were found to be comparable to those on acidities and BDEs of PhS in a similar series of N - H acids. Comparisons were also made of the effects of changing the oxidation state of sulfur in the series PhS, PhSO and PhSO2 on the acidities and BDEs of adjacent N - H and C - H bonds in weak acids. Hammett-type plots of pKHA values for phenyl benzyl sulfones (4-GC6H4CH2SO2Ph) and phenylsulfenylanilides (4-GC6H4NHSPh) were linear vs σp- values. A linear plot was obtained and explained for a plot of BDE of the N - H bonds in remotely substituted phenylsulfonylanilides with σ+ values. Plots of BDEs vs Eox(A-) were also linear for 4-substituted phenylsulfenylanilides (4-GC6H4NHSPh), phenylsulfonylanilides (4-GC6H4NHSO2Ph) and phenyl benzyl sulfones (4-GC6H4CH2SO2Ph). © 1998 John Wiley & Sons, Ltd.
    Zusätzliches Material: 7 Tab.
    Materialart: Digitale Medien
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  • 30
    Digitale Medien
    Digitale Medien
    17O NMR studies on α-diamides (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 387-391 
    Details
    ISSN: 0894-3230
    Schlagwort(e): α-diamides ; 17O NMR ; conformation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: 17O NMR spectra of several α-diamides were obtained at natural isotopic abundance in acetonitrile solution in order to study the conformations of these compounds in solution. The 17O NMR shifts vary with the intercarbonyl dihedral angle α in a sense opposite to that observed for α-diketones. © 1998 John Wiley & Sons, Ltd.
    Zusätzliches Material: 2 Ill.
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  • 31
    Digitale Medien
    Digitale Medien
    Theoretical study of the tautomerism of 8-azaadenine (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 392-396 
    Details
    ISSN: 0894-3230
    Schlagwort(e): 8-azaadenine ; tautomerism ; theoretical study ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The prototropic tautomerism of 8-azaadenine (azaade) was studied theoretically by means of ab initio methods, in both the gas phase and aqueous solution. A number of tautomeric forms were not included in the calculations after applying a stepwise elimination procedure based on both AM1 and HF/6-31G* energy values. The tautomers 9H-azaade, 8H-azaade and 7H-azaade survived to this elimination and their optimized geometries and energies were calculated at the MP2/6-31*//HF/6-31G* level. To include the solvent effects, two self-consistent reaction field method were used: (1) Onsager's SCRF with multipolar expansion up to the hexadecapolar term and (2) the isodensity polarizable continuum method (IPCM). Both methods produce similar results, although the latter represents better the situation in aqueous solution. The stability order in solution, 8H- 〉 9H- 〉 7H-azaade, differs slightly from that found in the gas phase, implying that in general the electrostatic effects in solution are important, but the intrinsic stability of these species in the gas phase overcomes the solvent effect. © 1998 John Wiley & Sons, Ltd.
    Zusätzliches Material: 1 Ill.
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  • 32
    Digitale Medien
    Digitale Medien
    Editorial (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 377-377 
    Details
    ISSN: 0894-3230
    Schlagwort(e): Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: No abstract.
    Materialart: Digitale Medien
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  • 33
    Digitale Medien
    Digitale Medien
    Electrochemical study of the π* probe dye 5-nitro-5′-dimethylamino-2,2′-bisthiophene (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 419-425 
    Details
    ISSN: 0894-3230
    Schlagwort(e): 5-nitro-5′-dimethylamino-2,2′-bisthiophene ; π* probe dye ; cyclic voltammetry ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Cyclic voltamograms of the solvatochromic dye 5-nitro-5′-dimethylamino-2,2′-bisthiophene (1), introduced recently as a sensitive π* probe, were recorded in different solvents. An EPR spectrum of the anionic radical of 1 in DMSO was obtained and compared with other spectra of analogous substituted bisthiophene radicals. It was found that the presence of a donor- acceptor pair of substituents in 1-⋅ reduces significantly the rotational barrier of the radical compared with the unsubstituted bithienyl radical anion 2-⋅. This is the result of an electronic repulsion between the donor ring fragment and the added electron in the coplanar radical, which does not exist in 2-⋅. © 1998 John Wiley & Sons, Ltd.
    Zusätzliches Material: 3 Ill.
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  • 34
    Digitale Medien
    Digitale Medien
    Host-guest interactions of calix[4]resorcinarenes with benzene derivatives in conditions of reversed-phase high-performance liquid chromatography. Determination of stability constants (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 426-437 
    Details
    ISSN: 0894-3230
    Schlagwort(e): calix[4]resorcinarenes ; organophosphorus compounds ; solvatophobic interactions ; host-guest complexation ; high-performance liquid chromatography ; x-ray analysis ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Reversed-phase high-performance liquid chromatography [LiChrosorb RP-18,UV detection at 254 nm and acetonitrile-water (86:14,v/v) as mobile phase] was applied to studies of the host-guest complexation of tetraalkylcalix[4]resorcinareneoctols and their upper rim phosphoryl, sulfonyl and dialkylaminomethyl derivatives with some aromatic guests in the mobile phase. It was shown that the formation of the inclusion complexes results in changes in the retention of aromatic guests and improves their separation. Stability constants of the complexes were calculated from the dependences of the l/k′ values of the aromatic guest on the concentration of the calix[4]resorcinarene in the mobile phase. The molecular structure of 4,6,16,18-tetrahydroxy-10,12,22,24-tetrakis(dipropoxyphosphoryloxy)-2,8,14,20-tetramethylcalix[4]resorcinarene (12) was determined. Crystal data for 12 are P21/n, a  =  16.708(9) Å, b  =  18.683(6) Å, c  =  20.243(5) Å, β  =  95.75(3)°, V  =  6287(4) Å3 and Z  =  4. Compound 12 exists in a boat conformation, in which two opposite unsubstituted resorcinol rings of the macrocyclic skeleton lie in the plane formed by four methine bridges and two diphosphorylated rings are perpendicular to the plane. © 1998 John Wiley & Sons, Ltd.
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  • 35
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    Digitale Medien
    Ab initio and molecular mechanics (MM3) calculations on alkyl- and arylboronic acids (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 378-386 
    Details
    ISSN: 0894-3230
    Schlagwort(e): alkylboronic acids ; arylboronic acids ; ab initio ; MM3 ; molecular mechanics ; force field parameters ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The boronic acid functional group has been incorporated into various biologically important compounds. In order to study this class of compounds better with molecular mechanics, five alkyl- and arylboronic acids were calculated using ab initio methods (Spartan) at the RHF/6-31G* level. MM3 force field parameters were developed based on the theoretically calculated geometries, vibrational spectra, and torsional profiles. © 1998 John Wiley & Sons, Ltd.
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  • 36
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    Digitale Medien
    Base-catalysed hydrolysis of γ-lactones: reactivity-structure correlations for 3-(substituted phenoxy- and thiophenoxymethylene)-(Z )-1(3H )-isobenzofuranones (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 467-474 
    Details
    ISSN: 0894-3230
    Schlagwort(e): γ-lactones ; base-catalysed hydrolysis ; reactivity-structure correlations ; isobenzofuranones ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Rate coefficients were measured for the base-catalysed hydrolysis of a series of substituted 3-(phenoxy- or thiophenoxymethylene)-(Z)-1(3H)-isobenzfuranones (3-phenoxy- or thiophenoxymethylenephthalides) in 70% (v/v) aqueous dioxane at 30.0 °C, in addition to the carbonyl stretching frequencies in chloroform and tetrachloromethane following deconvolution and band separation, when required. The Hammett reaction constants for the alkaline hydrolysis of the 3-/4-substituted phenoxy and thiophenoxy series are ca 0.75 and 1.10, respectively. These results are related to electrostatic field and resonance effects. Successful correlations between the carbonyl stretching frequencies and substituent constants and the rates of alkaline hydrolysis were found. Computational studies using the semi-empirical AM1 method correctly model both the details of the mechanistic pathway and the substituent effects. © 1998 John Wiley & Sons, Ltd.
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  • 37
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    A new approach to the theoretical estimation of inductive constants (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 437-447 
    Details
    ISSN: 0894-3230
    Schlagwort(e): inductive constants ; theoretical estimation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: A new model of the inductive effect is proposed, allowing highly accurate theoretical calculations of inductive constants for a diversity of substituents, using a simple and readily available system of mathematics. According to this approach, the inductive effect of a substituent is considered in terms of the additive influence of its constituent atoms. A constant inherent capacity for inductive interaction with a reactive center (with a four-coordinate carbon atom chosen for such a center), represented by an atomic constant σA, is ascribed to each atom. Values of σA for a wide variety of atoms are determined, and their physical meaning is revealed to elucidate to a certain extent the physical nature of the inductive effect. In addition, the proposed model permits the convenient use in calculations of group constants σG characterizing the inductive power of groups. Values of σG are determined for molecular fragments that are most widely dealt with in organic chemistry, and the use of σA or σG constants and of their superposition is shown to have, in most cases, little or no effect on the accuracy and reproducibility of the results obtained. It is also shown that, in terms of the developed approach, the inductive effect of a substituent is closely associated with its conformation. Theoretical inductive constants were calculated for 427 organic, aromatic, organometallic and charged substituents, and they showed perfect correlation with the corresponding experimental values. © 1998 John Wiley & Sons, Ltd.
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  • 38
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    Digitale Medien
    Oxidation of mandelic acid by alkaline potassium permanganate. A kinetic study (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 448-454 
    Details
    ISSN: 0894-3230
    Schlagwort(e): mandelic acid ; alkaline permanganate ; oxidation ; kinetics ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The kinetics of the oxidation of mandelic acid (MA) by permanganate in aqueous alkaline medium at a constant ionic strength of 1.0 mol dm-3 were studied spectrophotometrically. The reaction shows first-order kinetics in [permanganate ion] and fractional order dependences in [MA] and [alkali]. Addition of products, manganate and aldehyde have no significant effect on the reaction rate. An increase in ionic strength and a decrease in dielectric constant of the medium increase the rate. The oxidation process in alkaline medium under the conditions employed in the present investigation proceeds first by formation of an alkali permanganate complex, which combines with mandelic acid to form another complex. The latter decomposes slowly followed by a fast reaction between the free radical of mandelic acid and another molecule of permanganate to give products. The reaction constants involved in the mechanism were derived. There is good agreement between the observed and calculated rate constants under different experimental conditions. The reaction was studied at different temperatures and activation parameters were computed with respect to the slow step of the proposed mechanism. © 1998 John Wiley & Sons, Ltd.
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  • 39
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    Digitale Medien
    Selective arylation of exocyclic N-position of 2-pyrimidylnitrenium ion photolytically generated from tetrazolo-[1,5-a]pyrimidine in the presence of trifluoroacetic acid (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 478-484 
    Details
    ISSN: 0894-3230
    Schlagwort(e): Selective arylation ; exocyclic N-position ; 2-pyrimidylnitrenium ion ; tetrazolo[1,5a]pyrimidine ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Photo-reactions of tetrazolo[1,5-a]pyrimidine (1) with benzene (2a) and substituted benzenes (2b-f) in the presence of trifluoroacetic acid (TFA) gave 2-(2-, 3- and 4-substituted anilino)pyrimidines (3-5) together with 2-aminopyridine (6) and biphenyl (7) or diarylmethanes (8b, 8c and 8e). From the effect of heavy-atom solvent on the reactions, it is reasonable to assume that 3-5 are formed via a singlet species, but 6-8 via a triplet species. The intermediacy of 2-pyrimidylnitrenium ion is consistent with the evidence derived by the above effect, by a Hammett plot with ρ = -2.9 and by effects of solvent nucleophilicity and counter-anions. The selective exocyclic N-arylation giving 3-5 is discussed. © 1998 John Wiley & Sons, Ltd.
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  • 40
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    Digitale Medien
    Synthesis, structural, conformational and pharmacological study of esters derived from 3-methyl-2,4-diphenyl-3-azabicyclo[3.3.1]nonan-9α-ol as potential analgesics (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 125-132 
    Details
    ISSN: 0894-3230
    Schlagwort(e): 3-methyl-2, 4-diphenyl-3-azabicyclo[3.3.1]nonan-9α-ol esters ; analgesics ; synthesis ; structure ; conformation ; pharmacological assays ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: A series of esters derived from 3-methyl-2,4-diphenyl-3-azabicyclo[3.3.1]nonan-9α-ol (1) was synthesized and studied by 1H and 13C NMR spectroscopy, and the crystal structure of 3-methyl-2,4-diphenyl-9α-(3,5-dimethylbenzoyloxy)-3-azabicyclo[3.3.1]nonane (2) was determined by x-ray diffraction. The compounds studied display in CDCl3 a preferred flattened chair-chair conformation. This bicycle conformation is similar to that found for 2 in the crystal state. Pharmacological assays on mice were performed to evaluate drug-induced behavioral alteration, peripheral or central acute toxicity and analgesic activity. © 1998 John Wiley & Sons, Ltd.
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  • 41
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    Digitale Medien
    Stereospecific nucleophilic trapping of encounter complexes between photoexcited 1-cyanonaphthalene and norbornadiene (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 101-108 
    Details
    ISSN: 0894-3230
    Schlagwort(e): encounter complexes ; 1-cyanonaphthalene ; norbornadiene ; stereospecific ; nucleophilic trapping ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Photo-induced electron donor-acceptor reactions between 1-cyanonaphthalene (CNN) and norbornadiene (N) generate products of several structure types. Methanol adducts (1-3) formed in polar solvents are rationalized via the radical cation, N+·, and stereospecific (exo-) nucleophilic attack by methanol. In less polar solvents, CNN and N form [2 + 2]-cycloadducts, exclusively on the exo-face of N. In non-polar solvents containing methanol, CNN, N and methanol combine to form 1:1:1 adducts, containing the sensitizer on the endo- and the methoxy groups on the exo-face. The formation of these products is rationalized via the trapping of encounter complexes of different geometries. Any rearrangement of the norbornenyl system can be eliminated, since neither tricyclyl nor 7-methoxynorbornenyl structures are formed. Apparently, the alcohol captures an endo-encounter complex of CNN and N by attack from the exo-face, similar to the attack of methanol on N+·. © 1998 John Wiley & Sons, Ltd.
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  • 42
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    Gain or loss of aromaticity in Diels-Alder transition states and adducts: a theoretical investigationThis paper was presented at the Satellite Meeting on Structural and Mechanistic Organic Chemistry: A Tribute to Norman L. Allinger, held in Athens, GA, USA, June 3-7, 1997. (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 133-140 
    Details
    ISSN: 0894-3230
    Schlagwort(e): aromaticity ; Diels-Alder transition states ; Diels-Alder adducts ; reaction mechanism ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Semiempirical, ab initio and DFT investigations on the mechanism of the Diels-Alder reactions of a set of masked dienes (ring-fused dienes), viz. o-quinodimethane, anthracene and α-vinylnaphthalene, with ethylene, and reactions of α-vinylnaphthalene with maleic anhydride and p-benzoquinone were performed with a view to understanding the role of masking factors on the activation and reaction energies. The reactions were found to occur in a concerted fashion through synchronous transition states (TSs) in the first two reactions and through asynchronous TS in the other case. Of the compounds studied, o-quinodimethane is the most reactive diene both in a kinetic and a thermodynamic sense. α-Vinylnaphthalene reacts faster with maleic anhydride than p-benzoquinone, as expected, and two possible stereoselective TSs, one endo and the other exo, have been located for the reactions of α-vinylnaphthalene with the above dienophiles. Calculations show that the relative gain or loss of aromatic stabilization of the benzonoid ring in the transition state and in the product seems to play a major role in the kinetic and thermodynamic control of these reactions. © 1998 John Wiley & Sons, Ltd.
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  • 43
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    Digitale Medien
    Infrared spectra and conformation of methyl-substituted benzoic acidsPresented in part at the VIth International Conference on Correlation Analysis in Chemistry, Prague, 5-9 September 1994. (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 141-148 
    Details
    ISSN: 0894-3230
    Schlagwort(e): methylbenzoic acids ; infrared spectra ; conformation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Infrared spectra of all isomers of polymethyl-substituted benzoic acids were recorded in the carbonyl and hydroxyl regions in tetrachloromethane at various concentrations and interpreted in terms of conformation. According to a plot of ν(C=O) of the monomeric form vs Hammett substituent constants σ, these compounds may be classified into two classes. Derivatives with none or only one methyl group in the ortho position are concluded to exist in an equilibrium of two planar conformations, unless the equilibrium is degenerate. Derivatives with two ortho-situated methyl groups are concluded to take up one non-planar conformation. These findings are supported by the shape of the hydroxyl and carbonyl bands, which are unsymmetrical in the former class, although they could not be reliably separated into bands pertinent to the individual conformers. This conclusion is at variance with the common interpretation which has invariably ascribed to these and similar ortho derivatives non-planar conformations with a variable torsional angle. © 1998 John Wiley & Sons, Ltd.
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  • 44
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    Digitale Medien
    Ab initio calculations on P - C bond cleavage in phosphoranyl radicals: implications for the biodegradation of organophosphonate derivatives (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 149-154 
    Details
    ISSN: 0894-3230
    Schlagwort(e): P - C bond cleavage ; phosphoranyl radicals ; organophosphonate biodegradation ; ab initio calculations ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Barrier heights for P - C bond homolysis in P-hydroxy-P-methyl-P,P-dioxophosphoranyl and P,P,P-trihydroxy-P-methylphosphoranyl were calculated using well correlated levels of electronic structure theory. The best estimate for the difference in barriers between the two indicates that homolysis is more facile for P,P,P-trihydroxy-P-methylphosphoranyl by roughly 9 kcal mol-1. This result suggests that bacterial pathways leading to P - C bond cleavage in organophosphonate derivatives will preferentially proceed via initial one-electron reduction of substrates rather than oxidation. © 1998 John Wiley & Sons, Ltd.
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  • 45
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    Digitale Medien
    Mechanism of cycloaddition to indolizines (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 201-208 
    Details
    ISSN: 0894-3230
    Schlagwort(e): indolizines ; cycloaddition ; reaction mechanism ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The peculiarities of [8+2] cycloaddition reactions of indolizines with dienophiles are reviewed. Quantum chemical SINDO1, AM1 and ab initio calculations of transition states were performed for [8+2] cycloaddition reactions of indolizine and 6-nitroindolizine with a series of alkenes with donor and acceptor groups. The calculations predict a dipolar cycloaddition mechanism (electrophilic addition and ring closure) for reactions of indolizine and 6-nitroindolizine with nitroethylene. For the reaction of 6-nitroindolizine with N,N-dimethylaminoethylene, the predicted mechanism corresponds to a previously unknown ‘inverse’ dipolar cycloaddition (nucleophilic addition and ring closure). © 1998 John Wiley & Sons, Ltd.
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  • 46
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    Digitale Medien
    Theoretical studies on the gas-phase nucleophilic substitution reactions of benzyl chlorides with phenoxides and thiophenoxides (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 115-124 
    Details
    ISSN: 0894-3230
    Schlagwort(e): cross-interaction constants ; gas-phase nucleophilic substitution reactions ; PM3 MO ; benzyl chlorides ; phenoxides ; thiophenoxides ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Gas-phase nucleophilic substitution reactions of Y-benzyl chlorides with X-phenoxide and X-thiophenoxide nucleophiles were investigated theoretically using the PM3 semi-empirical MO method. The Leffler-Grunwald rate-equilibrium and Brønsted correlations predict that the degree of bond formation in the transition state (TS) is approximately 45 and 40% on the reaction coordinate for the phenoxides and thiophenoxides, respectively. For a weaker nucleophile, a later TS is obtained with an increased bond making and breaking. The variation of the TS structure with substituents in the nucleophile is thermodynamically controlled and is well correlated by rate-equilibrium relationships. In contrast, the TS variation (a tighter TS) with substituent (for a stronger acceptor Y) in the substrate is dependent only on variations of the intrinsic barrier and so cannot be correlated by such thermodynamically based rate-equilibrium relationships. The gas phase ρX and ρY values are much greater in magnitude than those in solution. A similar gas-phase theoretical cross-interaction constant, ρXY (ca -0.60), is obtained for both phenoxides and thiophenoxides, which is in good agreement with the experimental value (-0.62) for the thiophenoxide reactions in MeOH at 20.0 °C. The oxy and sulfur anion bases lead to a similar TS structure, but a lower reactivity for the former is due to a greater endothermicity of the reaction. A relatively wide range variation of the reaction energies, ΔG°, can be ascribed to the loss of resonance stabilization of anion nucleophiles upon product formation. © 1998 John Wiley & Sons, Ltd.
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  • 47
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    Digitale Medien
    Osmium(VIII)/ruthenium(III) catalysis of periodate oxidation of acetaldehyde in aqueous alkaline medium (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 171-176 
    Details
    ISSN: 0894-3230
    Schlagwort(e): acetaldehyde ; oxidation ; periodate ; osmium (VIII) catalysis ; ruthenium (III) catalysis ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Os(VIII) and Ru(III) catalysis of the periodate oxidation of acetaldehyde in aqueous alkaline medium was investigated. The catalytic efficiency is Ru(III)〈Os(VIII). The product of oxidation in both cases is acetate and IO3-. The stoichiometry is the same in both catalyzed reactions, i.e. [IO4-]:[CH3CHO] = 1:1. Probable mechanisms are proposed and discussed. The reaction constants involved in the mechanisms are derived. © 1998 John Wiley & Sons, Ltd.
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  • 48
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    Digitale Medien
    Solvolysis of benzylic chlorides with extended charge delocalization. α-tert-butyl(2-naphthyl)methyl, 9-fluorenyl and monosubstituted benzhydryl chlorides (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 223-229 
    Details
    ISSN: 0894-3230
    Schlagwort(e): Solvolysis ; Grunwald-Winstein-type analysis ; Hammett-type analysis ; YxBnCl scale ; 1-aryl-1-phenylmethyl cations ; Mulliken population analysis ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Rate constants of solvolysis of α-tert-butyl(2-naphthyl)methyl chloride (1), 9-fluorenyl chloride (2) and a series of monosubstituted benzhydryl chlorides (3) in a wide range of solvents were measured. Grunwald-Winstein-type correlation analysis of log k for 2 and 3 against YBnCl, with or without nucleophilicity N, yielded less satisfactory linear correlations than that against log k(1). A new scale of solvent ionizing power, YxBnCl, for the correlation of solvolytic reactivities of benzylic chlorides with extended charge delocalization based on log k(1) was developed. Application to the mechanistic study suggested the solvolysis of 2 and 4-nitrobenzhydryl chloride were non-limiting. Hammett plots against σ+ constants exhibited more negative ρ values in less nucleophilic solvents. In a benzhydryl chloride containing a strong deactivating substituent, such as 4-nitro, the positive charge delocalizes mainly over the unsubstituted ring in the cationic transition state. The uneven charge distribution was also confirmed by Mulliken population analysis at the level of the RHF/6-31G*//RHF/3-21G(*) basis set for cations. Comparison of the results of correlation analysis using the equation log(k/k0) = mY vs the equation log(k/k0) = mY+hI, and using the equation log(k/k0) = mY+lN vs the equation log(k/k0) = mY+lN+hI indicated the use of YBnCl or YxBnCl could give a better understanding of solvolytic mechanisms than the combinatorial use of YCl and I. © 1998 John Wiley & Sons, Ltd.
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  • 49
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    Digitale Medien
    Empirical treatment of solvent-solute interactions: medium effects on the electronic absorption spectrum of β-carotene (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 193-200 
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    ISSN: 0894-3230
    Schlagwort(e): Solvent-solute interactions ; β-carotene ; electronic absorption spectrum ; solvent effects ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Solvent effects on the wavenumber of the maximum of the longest wavelength electronic absorption band of all-trans-β-carotene were determined in 34 solvents. Together with results from previous studies, a data set for 51 solvents, mostly non-hydrogen bond donors, was constructed. This information was analyzed in terms of reaction field models and also showed its value for correlation purposes when used either alone or in combination with standard empirical solvent polarity-polarizability scales. © 1998 John Wiley & Sons, Ltd.
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  • 50
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    Digitale Medien
    Theoretical study on pyrolysis and sensitivity of energetic compounds. Part 4. Nitro derivatives of phenols (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 177-184 
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    ISSN: 0894-3230
    Schlagwort(e): nitro derivatives of phenols ; pyrolysis initiation reactions ; impact sensitivity ; UHF-SCF-AM1 MO method ; activation energy. ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The UHF-SCF-AM1 MO method was applied to the study of two kinds of pyrolysis reactions of six nitro derivatives of phenols (homolysis reaction by rupture of the C - NO2 bond into radicals and isomerization reaction involving phenolic hydrogen transferring to oxygen on the NO2 group). The molecular geometries of reactants, transition states and products were fully optimized. The potential energy curves and activation energies were first obtained. The results show that this category of compounds is more easily initiated via isomerization reactions than by homolysis reactions. The parallel relationship among the Wiberg bond order of the pyrolysis-initiation H - O bond in the molecule of a reactant, the activation energy of the isomerization reaction breaking the H - O bond and impact sensitivity of the reactant gives ‘the principle of the smallest bond order’ (PSBO) powerful support. The sensitizing effect of a phenol group was elucidated based on calculation results. The different influences of OH and NO2 groups on the heat of formation of a molecule are discussed. © 1998 John Wiley & Sons, Ltd.
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    Digitale Medien
    Unexpected rate enhancement in the intramolecular carboxylic acid-catalyzed cleavage of o-carboxybenzohydroxamic acid (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 216-222 
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    ISSN: 0894-3230
    Schlagwort(e): o-carboxybenzohydroxamic acid ; phthalic anhydride ; phthalic acid ; hydrolysis ; intramolecular acid catalysis ; kinetics ; activation parameters ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Phthalic anhydride was detected spectrophotometrically in the hydrolysis of o-carboxybenzohydroxamic acid (OCBA) in CH3CN-H2O solvent containing 0.03 mol dm-3 HCl. Pseudo-first-order rate constants (k1) for hydrolysis of OCBA are almost independent of the change in CH3CN content from 10 to 80% (v/v) in mixed aqueous solvents. The rate constants k1 are more than 10-fold larger than the corresponding rate constants for hydrolysis of phthalamic acid. These observations are explained in terms of a mechanism slightly different from the mechanism for hydrolysis of phthalamic acid. The activation parameters, ΔH* and ΔS*, are not affected appreciably by an increase in CH3CN content from 10 to 80% in mixed aqueous solvents. © 1998 John Wiley & Sons, Ltd.
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  • 52
    Digitale Medien
    Digitale Medien
    Solute-solvent and solvent-solvent interactions in binary solvent mixtures. Part 7. Comparison of the enhancement of the water structure in alcohol-water mixtures measured by solvatochromic indicators (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 185-192 
    Details
    ISSN: 0894-3230
    Schlagwort(e): solute-solvent interactions ; solvent-solvent interactions ; binary solvents ; alcohol-water mixtures ; water structure ; solvatochromic indicators ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: A preferential solvation model that takes into account the enhancement of the structure of water when small amounts of alcohol are added was applied to solvatochromic data for binary mixtures of water with 2-methylpropan-2-ol, propan-2-ol, ethanol and methanol. Application of the model allows the calculation of the effect of the enhancement of the water structure on solvatochromic solvent properties. It is demonstrated that the enhancement of water structure increases the solvent dipolarity/polarizability and hydrogen-bond donor acidity and decreases the solvent hydrogen-bond acceptor basicity. The effect decreases in the order 2-methylpropan-2-ol-water, propan-2-ol-water, ethanol-water and methanol-water. © 1998 John Wiley & Sons, Ltd.
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  • 53
    Digitale Medien
    Digitale Medien
    Structural dependence of heterolytic bond dissociation energy of carbon-carbon σ bonds in hydrocarbons (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 157-170 
    Details
    ISSN: 0894-3230
    Schlagwort(e): heterolytic bond dissociation energy ; carbon-carbon σ bonds ; hydrocarbons ; structural dependence ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The coordination of a resonance-stabilized hydrocarbon cation and anion yielded a series of unprecedented hydrocarbons, which are susceptible to thermal heterolytic cleavage of carbon-carbon σ bonds in polar media, generating the original ions under reversible conditions. When the component ions were sufficiently stabilized, some ion pairs were even isolated as solids, thus providing the first examples of hydrocarbon salts. The direct observation of the heterolysis by means of spectroscopy permitted reliable thermodynamic treatments of the observed degree of ionic dissociation. Correlation of the free energy of heterolysis with solvent dielectric constants and parameters of ion stabilities such as pKHA, pKR+ and redox potentials revealed the importance of the thermodynamic stabilities of ions, the degree of solvation and steric congestion in the starting molecule as major controlling factors in the heterolysis. © 1998 John Wiley & Sons, Ltd.
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  • 54
    Digitale Medien
    Digitale Medien
    Correlation of the rates of solvolysis of the N,N-diphenylcarbamoylpyridinium ion (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 273-276 
    Details
    ISSN: 0894-3230
    Schlagwort(e): N,N-diphenylcarbamoylpyridinium ion ; solvolysis ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Solvolyses of the N,N-diphenylcarbamoylpyridinium ion are subject to specific and/or general base catalysis, which can be eliminated by addition of perchloric acid or increased, especially in fluoroalcohol-containing solvents, by addition of pyridine. The uncatalyzed solvolyses in aqueous methanol and aqueous ethanol involve a weakly nucleophilically assisted (l  =  0.22) heterolysis and the solvolyses in the pure alcohols are anomalously slow. © 1998 John Wiley & Sons, Ltd.
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  • 55
    Digitale Medien
    Digitale Medien
    Effects of ionic and non-ionic micelles on rate of hydroxide ion-catalyzed hydrolysis of securinine (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 209-215 
    Details
    ISSN: 0894-3230
    Schlagwort(e): securinine ; cationic micelles ; anionic micelle ; non-ionic micelle ; alkaline hydrolysis ; kinetics ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The effects of micelles of cetyltrimethylammonium bromide (CTABr), tetradecyltrimethylammonium bromide (TTABr), sodium dodecyl sulfate (SDS) and polyoxyethylene 10 lauryl ether (C12E10) on the rates of alkaline hydrolysis of securinine were studied at a constant [-OH] (0.05 M). An increase in the total concentrations of CTABr, TTABr, SDS and C12E10 from 0.0 to 0.2 M causes a decrease in the observed pseudo-first-order rate constants (kobs) by factors of ca 2.5, 3, 7 and 4, respectively. The observed data are explained in terms of pseudophase and pseudophase ion-exchange (PIE) models of micelle. The binding constants, KS, of securinine with SDS, C12E10, CTABr and TTABr micelles are 32.4, 14.8, 22.1, and 9.1 M-1, respectively. The magnitudes of the second-order rate constants, kM, for the reactions in the micellar pseudophase are negligible compared with the corresponding rate constant, kW, for the reaction occurring in the aqueous pseudophase for CTABr, TTABr, SDS and C12E10. © 1998 John Wiley & Sons, Ltd.
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  • 56
    Digitale Medien
    Digitale Medien
    Colloidal CdS-induced photocatalytic reaction of 2-methylindole - mechanistic analysis of oxidation of indoles (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 277-282 
    Details
    ISSN: 0894-3230
    Schlagwort(e): colloidal CdS-induced photochemical reactions ; photocatalytic reactions ; photooxidation ; 2-methylindole ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: 2-Methylindole (2-MI) is adsorbed on the surface of colloidal CdS particles with an adsorption intensity of 0.6 × 103 dm3 mol-1. A new emission band at 530 nm is produced by forming an exciplex between excited CdS and 2-MI and the red emission due to CdS is simultaneously quenched. The emission maxima of green bands for different indoles increase in the order indole 〈 tryptophan 〈 2-MI 〈 3-MI and are observed at 508, 520, 530 and 540 nm, respectively. The shift in emission maxima is related to the oxidation potential of these substrates. The irradiation of an aerated reaction mixture containing CdS and 2-MI with visible light induces the oxidation of adsorbed 2-MI by photogenerated holes to produce 2-methyl-3-indolinone and 2-acetamidobenzaldehyde. The latter product is formed due to oxidative C - C bond cleavage of the pyrrole ring. The reactivity of trapped holes towards the adsorbed 2-MI is evidenced by a decrease in the lifetime of the red emission of CdS in the presence of 2-MI. In this reaction the possibility of the participation of singlet oxygen is ruled out. A general mechanism of CdS-induced oxidation of indoles is discussed. © 1998 John Wiley & Sons, Ltd.
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  • 57
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    Digitale Medien
    Conformational analysis of a 12-membered crown dithioether in the solid state and in solution (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 241-253 
    Details
    ISSN: 0894-3230
    Schlagwort(e): crown dithioether ; conformational analysis ; multicomponent equilibrium ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The solid-state molecular structure and the conformational behaviour in solution of the 12-membered crown dithioether 8-methyl-1,4-dioxa-7,10-dithiacyclododecane-5,12-dione were studied by x-ray crystallography, 1H and 13C NMR spectroscopy and molecular mechanics. The conformational rigidity of some constituent structural fragments allowed a detailed analysis of the structure and distribution of the conformers. A protocol for studies of multiconformational equilibrium was developed by means of the combined use of structure calculations and dynamic NMR measurements. © 1998 John Wiley & Sons, Ltd.
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  • 58
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    Digitale Medien
    Metal-catalyzed carbenoid reactions with iodonium and sulfonium ylides (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 321-333 
    Details
    ISSN: 0894-3230
    Schlagwort(e): metal-catalyzed carbenoid reactions ; iodonium ylides ; sulfonium ylides ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Transition metal-catalyzed decomposition of phenyliodonium and diphenylsulfonium ylides was investigated with regard to application in asymmetric carbenoid reactions. Phenyliodonium ylides react in the presence of Rh(II) catalysts with the same selectivity in inter- and intramolecular cyclopropanations as the corresponding diazo compounds, and intramolecular CH insertions proceed with identical enantioselectivities. With diphenylsulfonium ethoxycarbonylmethylide the Cu(I)-catalyzed cyclopropanation of olefins affords trans/cis ratios and asymmetric inductions identical with those of diazo compounds, but with Rh(II) catalysts some small, although significant, selectivity variations occur, which are ascribed to coordination of diphenyl sulfide to one of the coordination sites of the catalyst. © 1998 John Wiley & Sons, Ltd.
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  • 59
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    Digitale Medien
    Principles of physical organic chemistry for molecular architecture and functions (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 299-304 
    Details
    ISSN: 0894-3230
    Schlagwort(e): molecular architecture ; molecule-based magnets ; biopolymers ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Application of the principles of physical organic chemistry to the construction of molecule-based magnets is discussed. Not only the magnetic structures of conventional magnets but also secondary and tertiary structures of biopolymers are instrumental in the molecular design. © 1998 John Wiley & Sons, Ltd.
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  • 60
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    Digitale Medien
    Studies of reactive intermediates using matrix and gas-phase techniques (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 350-355 
    Details
    ISSN: 0894-3230
    Schlagwort(e): 1,3-dipoles ; cumulenes ; infrared spectroscopy ; mass spectrometry ; flash vacuum thermolysis ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: In the direct investigation of reactive intermediates it is particularly valuable to use a combination of several spectroscopic techniques. This commentary highlights recent examples, using primarily flash vacuum thermolysis for the generation of the intermediates, and matrix IR spectroscopy in conjunction with gas-phase mass spectrometric methods for their identification. The examples include nitrile imines, nitrile ylides, nitrile sulfides and selenides, dinitrogen sulfide and several novel cumulenes (X=C=C=Y, RN=C=C=C=X). © 1998 John Wiley & Sons, Ltd.
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  • 61
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    Digitale Medien
    Non-cross-linked and cross-linked poly(alkylmethyldiallylammonium halides): synthesis and aggregation behavior (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 305-320 
    Details
    ISSN: 0894-3230
    Schlagwort(e): poly(alkylmethyldiallylammonium halides) ; aggregation behavior ; polyelectrolytes ; polysoaps ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Hydrophobically modified polyelectrolytes (polysoaps) are a unique class of water-soluble polymers containing distinct hydrophobic and hydrophilic regions. Above a certain concentration, polysoaps form intramolecular and intermolecular aggregates in aqueous solution. They have attracted much attention not only for their ability to mimic some functions demonstrated by biopolymers but also for their important industrial applications. This review highlights some interesting features of novel non-cross-linked and cross-linked poly(alkylmethyldiallylammonium halides) that have been described in recent years. © 1998 John Wiley & Sons, Ltd.
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  • 62
    Digitale Medien
    Digitale Medien
    Is the brain ready for physical organic chemistry? (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 334-340 
    Details
    ISSN: 0894-3230
    Schlagwort(e): brain chemistry ; membrane proteins ; molecular neurobiology ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Efforts to developed detailed insights into the structure and function of the molecules of memory, thought and sensory perception - physical organic chemistry on the brain - are described. By combining more conventional chemical tools with a number of techniques adapted from modern biology, it is now possible to perform systematic structure-function studies on the integral membrane proteins that play a central role in molecular neurobiology. There are substantial challenges associated with such studies, but we believe the potential payoff is considerable. © 1998 John Wiley & Sons, Ltd.
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  • 63
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    Digitale Medien
    Vicarious nucleophilic substitution of hydrogen. Mechanism and orientation (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 341-349 
    Details
    ISSN: 0894-3230
    Schlagwort(e): vicarious nucleophilic substitution ; hydrogen ; mechanism ; orientation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Hydrogens located at activated positions in electrophilic arenes, e.g. ortho and para hydrogens in nitrobenzenes, can be replaced with a nucleophile moiety provided there is at least one nucleofuge X connected to the nucleophilic centre. As the group really leaving in this hydrogen substitution process is not the hydride anion but X, the reaction has been named vicarious nucleophilic substitution of hydrogen (VNS). The concepts on the mechanism of the reaction and their experimental background are presented. Reactivity and orientation - the fundamental questions concerning synthetical applications of VNS - are discussed in light of the supposed mechanistic picture.
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    Preface (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 291-291 
    Details
    ISSN: 0894-3230
    Schlagwort(e): Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: No Abstract
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  • 65
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    Digitale Medien
    Unusual reactivity of highly strained cyclophanesDedicated to Prof. Dr Maitland Jones, Jr, on the occasion of his 60th birthday (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 362-376 
    Details
    ISSN: 0894-3230
    Schlagwort(e): highly strained cyclophanes ; reactivity ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: An essential feature of the concept of aromaticity has been the stability and lack of reactivity of aromatics relative to that of other unsaturated compounds. Contrary to this general experience, high and unusual reactivity is encountered when simple, monocyclic benzene rings are bent by short bridges into a boat-shaped conformation, as is the case in small [n] paracyclophanes (n ≤ 8) and [n]metacyclophanes (n ≤ 7). This is illustrated, mostly with examples taken from the authors' own work, for thermal and photochemical behavior and reactions with electrophiles, nucleophiles and dienophiles. © 1998 John Wiley & Sons, Ltd.
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  • 66
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    Digitale Medien
    Heterogeneous catalysis of organic reactions (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 356-361 
    Details
    ISSN: 0894-3230
    Schlagwort(e): heterogeneous catalysis ; organic reactions ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: A decade's worth of work is reviewed: building on the demonstrated prowess of heterogeneous catalysis in industrial organic chemistry, the author's laboratory devised efficient catalysts for a number of organic reactions, based on aluminosilicates such as clays and zeolites. This review also spells out, at a time when creative research is overshadowed by imitative research, some of the methodological musts that also happen to characterize physical organic chemistry: the devising of experiments so that accurate numerical data can be obtained; the importance of remote, interdisciplinary connections; the need for estrangement from stereotypic preconceptions that may obscure the true explanations for the phenomena; and the over-riding need for concerning oneself only, or at least predominantly, with the important facts that nature tells, if one cares to listen. © 1998 John Wiley & Sons, Ltd.
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  • 67
    Digitale Medien
    Digitale Medien
    Flash photolytic generation and investigation of short-lived reaction intermediates: a case study (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 292-298 
    Details
    ISSN: 0894-3230
    Schlagwort(e): flash-photolysis ; short-lived reaction intermediates ; mandelic acid ; keto-enol tautomers ; enzyme-catalyzed racemization ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The advantage of adding more structure-diagnostic information to the simple detection of flash photolytically generated transient species by changes in UV-visible light absorbance is illustrated by a case study involving the mandelic acid keto-enol system. An early report based on preliminary evidence proposed that flash photolysis of phenyldiazoacetic acid produces the enol of mandelic acid by hydration of phenylhydroxyketene, itself generated by a photo-Wolff reaction of the diazo acid. Further examination, however, shows that this is only a minor route, and that the major pathway is a new enol-forming reaction involving what appears to be hydration of a carboxycarbene formed by dediazotization of the diazo compound. Hydration of phenylhydroxyketene is nevertheless the reaction by which mandelic acid enol is generated when esters of benzoylformic acid are the flash photolysis substrates. These mechanisms, and also identification of the enol as a tranisent species, are supported by detailed arguments involving acid-base catalysis, solvent isotope effects, and the use of oxygen-18 as a tracer. The work produces a keto-enol equilibrium constant for the mandelic acid system, pKE  =  16.19, and also acidity constants of the enol ionizing as an oxygen acid, pKEa  =  6.39, and the keto isomer ionizing as a carbon acid, pKKa  =  22.57. The bearing of these results on the enzyme-catalyzed racemization of mandelic acid is discussed. © 1998 John Wiley & Sons, Ltd.
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  • 68
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    Novel one-pot polymer syntheses based on dithiols obtained from a five-membered cyclic dithiocarbonate and diamines (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 1909-1915 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Novel one-pot syntheses of sulfur-containing polymers from a five-membered cyclic dithiocarbonate (5-phenoxymethyl-1,3-oxathiolane-2-thione (1)) and diamines were examined. Polydisulfides with M̄n 5400 ∼ 15800 were obtained in the oxidation polymerization of dithiols 2, which were obtained in situ by the reactions of 1 and diamines. When 1,12-dodecanediamine was employed, a polydisulfide with relatively high M̄n was obtained, while in the case of m-xylylenediamine, the polymerization was accompanied by intermolecular cyclization to give a polydisulfide with low M̄n. Polythiourethanes with M̄n 6500 ∼ 17200 were obtained in the polyaddition of 2 with 4,4′-methylenebis(phenyl isocyanate). The yields and M̄n values increase with increasing methylene chain length of 2. Polythioethers with M̄n 2600 ∼ 19900 were obtained by the polycondensations of 2 with α,α′-dibromo-p-xylene. The glass transition temperature (Tg) of the polymer decreases with increasing methylene chain length in the main chain.
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  • 69
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    Synthesis of novel oligomeric peroxides via condensation of dihydroperoxides with aromatic diols (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 1927-1933 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Novel oligoperoxides α-hydro-ω-hydroperoxyoligo(dioxydimethylmethylene-1,4-phenylene-dimethylmethylene) (BPB) (Tm = 143°C, Td = 176°C), α-hydro-ω-hydroxyoligo(dioxydimethylmethylene-1,4-phenylenedimethylmethylenedioxydimethylmethyleneethy lenedimethylmethylene) (EPB) (Tm = 100°C, Td = 175°C) and α-hydro-ω-hydroperoxyoligo(dimethylmethylene-2,6-naphthylenedimethylmethylene (NPN) (Tm = 137°C, Td = 177°C)), the products of condensation of tertiary diols with tertiary dihydroperoxides, were obtained. The structure of the oligoperoxides was confirmed by spectroscopic methods (IR, NMR). Their thermal properties were studied by differential scanning calorimetry (DSC), and the mechanism of the thermal decomposition of the oligomeric peroxides was proposed basing on the GC-MS analysis of the degradation products.
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  • 70
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    Liquid crystalline thermosets from triaromatic azomethine group containing diepoxides and 4,4′-methylenedianiline (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 1935-1938 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Five triaromatic azomethine-linked diepoxides have been cured by reaction with 4,4′-methyl-enedianiline (MDA) in a molar ratio epoxide/amine of 1:1 and 2:1. The influence of the structure of monomers, of the molar ratio epoxide/amine, and of the curing conditions on the formation of liquid crystalline networks was studied. Initially a decrease of the clearing temperature was caused by mixing with the nonmesogenic diamine. The clearing point of the mixture increased with conversion, and thermosets with a liquid crystal order of the monomers were obtained from the 2:1 combinations. Curing occurs in two stages as revealed by DSC: first epoxy/amine reaction occurs, and at higher temperature homopolymerisation of residual oxirane takes place. This is due to non-homogeneous melting of the diepoxides in dynamic curing.
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  • 71
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    Vinylic polymerization of bicyclo[2.2.1]hept-2-ene by Co(II)-catalysis (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 1951-1956 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The vinylic polymerization of bicyclo[2.2.1]hept-2-ene (norbornene) with Co(II) compounds, such as Co(II) stearate, substituted bis(1,3-diketo)cobalt(II), Co(dppe)Cl2, and the metallocene [η5-(C5Me5)Co-η2-Cl]2, in chlorobenzene activated with methylaluminoxane (MAO) is reported. MAO* synthesized by the hydrolysis of trimethylaluminium in chlorobenzene instead of toluene increases the catalytical activity strongly, and a turn over of 2.7 tons of poly(2,3-bicyclo[2.2.1]hept-2-ene) per mol cobalt per hour was achieved. The polymers obtained are amorphous (WAXS). They show weight-average molecular weights up to M̄w = 1.5 · 106 and are soluble in chlorobenzene, 1,2-dichlorobenzene, cyclohexane, and decahydronaphthalene.
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  • 72
    Digitale Medien
    Digitale Medien
    Influence of batch reaction conditions on norbornene/ethylene copolymers made using C2v- and Cs-symmetric metallocene/MAO catalysts (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 1939-1942 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The norbornene/ethylene copolymerization was studied using C2v- and Cs-symmetric metallocene catalysts in the presence of methylaluminoxane as a cocatalyst. The batch reactions were carried out at four different temperatures copolymerizing a constant amount of ethylene with the appropriate amount of norbornene. The activity decreases for the Cp2ZrCl2 catalyst and increases to a constant level for the [Ph2C(Flu)(Cp)]ZrCl2 catalyst when the norbornene/ethylene ratio in the reaction increased. The highest activities and the highest norbornene contents in the copolymers were achieved using the [Ph2C(Flu)(Cp)]ZrCl2. The microstructure of the copolymer determined with 13C NMR was observed to depend on catalyst, temperature and monomer ratio. Cp2ZrCl2 produces more norbornene blocks as a function of norbornene content in the copolymer and longer norbornene sequences than [Ph2C(Flu)(Cp)]ZrCl2. The correlation between the increasing norbornene content in the copolymer and the increasing glass transition temperature is more obvious for [Ph2C(Flu)(Cp)]ZrCl2.
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  • 73
    Digitale Medien
    Digitale Medien
    New azo-dye containing side group copolymethacrylates (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 1943-1949 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: A new type of side group copolymethacrylates was synthesized, characterized and tested with respect to its behavior on irradiation with an Ar+-laser beam. The side groups consist of methylene spacer units with different length and of three-ring mesogens with an ester group between the first and the second aromatic ring and on the other hand of two-ring mesogens with an azo-structure between the aromatic rings, respectively. The azochromophoric structure contains different substituents in the terminal aromatic ring. The homopolymers of the azo-dye containing monomers do not display liquid crystalline (lc) mesophases, but the three-ring mesogens induce lc-phases in opposition to the equimolaric copolymers with two-ring ester group mesogens. The lc phases are important for the stability of the recorded change of birefringence (photorecording process) which is influenced by irradiation of the polymers.
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  • 74
    Digitale Medien
    Digitale Medien
    Miscibility of functionalized polyolefins with polyamide-6 as detected by solid-state NMR (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 1957-1963 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Solid-state NMR was used to investigate the miscibility of LLDPE/polyamide-6 blends. Two different blends B1 and B2, obtained with LLDPE only and with LLDPE functionalized with diethyl succinate, respectively, as well as their individual components, were characterized by means of 13C selective techniques. Measurement of 1H spin-lattice relaxation times in both laboratory and rotating frames allowed the miscibility at a molecular level to be investigated for both blends: in B1 the domains of the different components are on average larger than 100-200 Å, whereas in B2, where two different fractions could be separated on the basis of their solubility in heptane, the average linear dimensions of the insoluble fraction are between 20 and 200 Å.
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  • 75
    Digitale Medien
    Digitale Medien
    Tetrahexylsexithiophene: crystal structure and molecular mechanics calculations (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 1973-1979 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The synthesis, crystal structure and detatiled molecular mechanics calculations, including crystal packing interactions, of tetrahexylsubstituted sexithienylene are presented. Unexpectedly the molecule, which arranges in the P-1 space group, displays no herringbone arrangement, thienylenic rings are far from coplanarity and the alkylic side chains present different conformations. Molecular mechanics fully accounts for these findings. From crystal packing computations it is derived that different arrangements are unable to lower the packing energy, due to the closeness of two alkyl chains on the same side of the thienylenic backbone, which prevents interspersion of non-parallel molecules.
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  • 76
    Digitale Medien
    Digitale Medien
    Copolymerization of ethene with 1-hexene or 1-hexadecene over ethylene, dimethylsilylene and 1,4-butanediylsilylene bridged bis(indenyl) and bis(tetrahydroindenyl)zirconium dichlorides (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 1965-1972 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Ethene was copolymerized with 1-hexene and 1-hexadecene over five different homogeneous methylaluminoxane (MAO) activated racemic ansa-metallocene catalysts in order to study both the influence of the ligand (indenyl and tetrahydroindenyl) and the influence of the interannular bridge (ethylene, dimethylsilylene and 1,4-butanediylsilylene) on the copolymerization behaviour. Hydrogenation of the indenyl ligands was found to decrease comonomer content and molar mass of the copolymers. A similar tendency was observed for the ethylene bridge compared to the silylene bridges. The reasons for this behaviour are discussed and related to structural differences of the catalyst precursors. In addition a segregation fractionation technique was applied to study the chemical composition distribution (CCD) of the produced copolymers. Batchwise feed of comonomer resulted in a broad CCD. More homogeneous copolymers could be produced using partly continuous feed of comonomer.
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  • 77
    Digitale Medien
    Digitale Medien
    4-[Diphenyl(trimethylsilyl)methyl]benzophenone as initiator in the photopolymerization of methyl methacrylate and styrenePartly presented at the 4th Greek Polymer Chemical Congress, Patras, November 1997, Greece. (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 1981-1987 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The polymerization of methyl methacrylate (MMA) and styrene (St) has been studied using 4-[diphenyl(trimethylsilyl)methyl]benzophenone 1 as photoinitiator. The polymerization follows a free radical mechanism; the polymerization rate increases linearly with the monomer concentration and was found to be proportional to the 0.33 and 1.40 power of the photoinitiator and the monomer (MMA) concentration, respectively. The overall activation energy in the case of MMA photopolymerization was calculated to be 25.0 kJ/mol. From 1H NMR studies it is concluded that the obtained polymers contain two different trimethylsilyl moieties, one at the head and the other at the tail of the polymer chain, showing primary termination reactions even at low initiator concentrations. The p-benzoyltrityl radical 1· is incorporated into the polymer chain to a very small extent, acting as a scavenger. This is also concluded by laser flash photolysis (LFP) and ESR spectroscopy measurements. A “living” character of the polymerization was observed only at very low initiator concentrations. The triplet state (31*) of the initiator was quenched by styrene, reducing its efficiency. The rate constant kq of the quenching process of 31* was measured by LFP (kq = 3.1 · 109 M-1 · S-1). The triplet state and the photodissociation efficiency of the initiator is unaffected by MMA at various concentrations.
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  • 78
    Digitale Medien
    Digitale Medien
    Reactivity ratios and sequence determination of acrylonitrile/hexyl methacrylate copolymers by one and two dimensional NMR spectroscopy (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 2005-2015 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Acrylonitrile/hexyl methacrylate copolymers of different monomer composition were prepared by photopolymerization using uranyl ions as photosensitizer. The copolymer composition data were used to calculate the comonomer reactivity ratios according to the Kelen Tüdős (KT) and the error-in-variables model (EVM) method. The reactivity ratios obtained from the EVM method are rA = 0.14 ± 0.03 and rH = 1.32 ± 0.17. The microstructure was obtained in terms of distribution of A and H centred triad sequences from 13C{1H} NMR spectra of the copolymers. The copolymerization mechanism was determined by using Cheng's methodology. It was found that the Markov second order model gave a good fit to the experimental data. The complex and overlapping proton and carbon signals of A/H copolymers were assigned to various compositional and configurational sequences with the help of inverse HETCOR experiments.
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  • 79
    Digitale Medien
    Digitale Medien
    The effect of temperature on the polymerization of propene with dimethylsilylbis(1-indenyl)zirconium dichloride/methylaluminoxane and dimethylsilylbis(2-methyl-1-indenyl)zirconium dichloride/methylaluminoxane. Modeling of kinetics (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 1989-2004 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Propene polymerizations were performed using the two ansa-zirconocene catalyst systems dimethylsilylbis(1-indenyl)zirconium dichloride/methylaluminoxane and dimethylsilylbis(2-methyl-1-indenyl)zirconium dichloride/methylaluminoxane. The polymerization rate was observed by continuously monitoring the monomer consumption. Reaction rate profiles were obtained in the temperature range from 40°C to 130°C at pressures between 1 and 2.5 bar and catalyst concentrations from 4.6 · 10-6 M to 4.2 · 10-5 M. Isotacticity, as measured by NMR, melting point and molecular weight decreases markedly at higher temperatures. Small amounts of 1,3-inserted monomer (〈1 mol-%) was observed at polymerization temperatures above 80°C. No 2,1-inserted monomer was detected. A kinetic model was developed that describes the polymerization rate for Me2Si(Ind)2ZrCl2 as the catalyst over the entire observed temperature range, and the polymerization rate for Me2Si(2-Me-Ind)2ZrCl2 in a limited temperature range. The model includes an activation reaction, latent sites that may revert to active sites and a permanent deactivation that is second order with respect to the active sites. The activation energy for the propagation reaction was found to be 37 kJ/mol for Me2Si(Ind)2ZrCl2 and 32 kJ/mol for Me2Si(2-Me-Ind)2ZrCl2/MAO. Several kinetic models are compared and discussed.
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  • 80
    Digitale Medien
    Digitale Medien
    Preparation of poly(lactic acid)-grafted amylose through the trimethylsilyl protection method and its biodegradation (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 2017-2022 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Biodegradable poly(lactic acid)-grafted amylose was synthesized using a trimethylsilyl (TMS) protection method. Tetrahydrofuran soluble, mostly trimethylsilyl protected amylose was prepared and reacted with potassium tert-butoxide to give the corresponding alkoxide. Poly(lactic acid)-grafted amyloses were obtained by ring-opening anionic polymerization of lactide using the polymeric alkoxides as initiators and subsequent removal of the TMS groups. The obtained graft copolymers show biodegradability and a microphase-separated morphology.
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  • 81
    Digitale Medien
    Digitale Medien
    On the multiple melting behavior of polymorphic syndiotactic polystyrene and its behavior in a miscible state (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 2041-2049 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Differential scanning calorimetry (DSC) and X-ray diffraction analysis were performed to study the multiple melting behavior and crystal forms of neat syndiotactic polystyrene (s-PS) in comparison with its miscible blends with amorphous atactic polystyrene (a-PS) or poly(2,6-dimethyl-p-phenylene oxide) (PPO). For neat s-PS annealed at 242 to 250°C, three melting peaks of changing intensity were observed (labeled I, II, and III from low to high temperature), while if annealed at high temperatures (254°C or higher) only two melting peaks were left. The missing peak was determined to be Peak-III for s-PS annealed at high temperatures. For the s-PS/a-PS or s-PS/PPO blends, two melting peaks were observed, and the missing peak is Peak-II. The missing Peak-II in the blend systems is attributed to the β-modification. Apparently, the condition of s-PS being compatible with a-PS or PPO is unfavorable for generating the β-modification upon melt crystallization. This study also showed that Peak-I is associated with the β-modification. Peak-III may be associated with the less stable β′ form, which can be transformed to the β form upon annealing at high temperatures. For the s-PS/a-PS or s-PS/PPO blends upon annealing at higher temperatures, Peak-III quickly decreases, leaving only a single melting peak (Peak-I) for the blends. The phenomenon of the decrease of Peak-III crystals in the blend is similar to that for neat s-PS annealed at high temperatures.
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  • 82
    Digitale Medien
    Digitale Medien
    Rheological behavior of cured acrylate-terminated unsaturated copolyesters (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 2029-2039 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The viscoelastic properties of cured acrylate-terminated copolyesters were studied by means of dynamic mechanical analysis. The influence of terminal double bonds, as well as of the cis-trans configuration and distribution along the main chain, on the transition temperatures and rheological properties of the cured polyesters are presented. The presence of double bonds at both chain ends allows the formation of a regular network by crosslinking copolymerization with styrene. The present investigation was carried out on three series of acrylate-terminated copolyesters: poly[(tetramethylene maleate)-co-(tetramethylene phthalate)] (BMPA), poly[(tetramethylene fumarate)-co-(tetramethylene phthalate)] (BFPA) and poly[(tetramethylene phthalate)-co-(tetramethylene succinate)] (BPSA). The results of the mechanical spectrometric analysis are presented as master curves by applying the time-temperature superposition principle. The WLF parameters of the cured unsaturated polyesters were determined above the glass transition temperatures. The glass transition temperatures shift to higher temperatures with increasing degree of crosslinking. The equilibrium shear storage modulus plateau changes as a consequence of the different configuration of the double bonds and the degree of crosslinking. The degree of crosslinking, nc, determined from the equilibrium shear modulus G′e, was higher than those determined from equilibrium swelling measurements, which is apparently connected with the contribution of physical entanglements.
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  • 83
    Digitale Medien
    Digitale Medien
    Preparation of poly(methyl methacrylate) microspheres modified with amino acid moieties (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 2023-2028 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Poly(methyl methacrylate) microspheres [poly(SerMA-co-MMA), poly(AlaMAm-co-MMA), poly(AlaEMA-co-MMA), and poly(AlaMAm/AlaEMA-co-MMA)] modified with O-methacryloyl-L-serine (SerMA), N-methacryloyl-L-alanine (AlaMAm) and L-alanine 2-methacryloyloxyethyl ester (AlaEMA) were prepared by the emulsifier-free emulsion copolymerization of methyl methacrylate (MMA) with SerMA, AlaMAm and AlaEMA, respectively, initiated with 2,2′-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (ABIP). The series of polymer microspheres showed unimodal distribution of the particle size (290-800 nm) in water. From X-ray photoelectron spectroscopy it was concluded that the amino acid moieties are located on the surface of the particles in all cases. Poly(SerMA-co-MMA) showed the most effective suppression of adsorption of proteins such as albumin (Alb), γ-globulin (Glo) and fibrinogen (Fib) among the examined poly(methylmethacrylate) microspheres.
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  • 84
    Digitale Medien
    Digitale Medien
    Thermotropic liquid crystalline poly(vinyl ether)s with pendant cellobiose residues: Synthesis and mesophase structure (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 2071-2077 
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    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: This study concerns the synthesis of poly(vinyl ether)s (VEs) carrying pendant cellobiose heptadecanoate (CHD) residues and their mesomorphic properties examined by DSC, polarization microscopy and X-ray diffraction. The poly(VE)s were synthesized through cationic polymerization of a CHD-substituted VE, i.e., 10-(vinyloxy)decyl-2,2′,3,3′,4′,6,6′-hepta-O-decanoyl-β-D-cellobioside. From X-ray diffraction analyses, the mesophase of the poly(VE) proved to be closely similar to that of the star-shaped triplet derivative. The mesophase is characterized by the following features; (i) it consists of discotic columns built up by a regular stacking of the pendant CHD residues, (ii) each polymer main chain is presumed to have an extended conformation due to the periodic stacking of the pendant CHD moieties into a columnar structure, and (iii) each polymer molecule independently forms three discotic columns, which surround the main chain, without any intercalation of the CHD pendants originating from different polymer molecules.
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  • 85
    Digitale Medien
    Digitale Medien
    Effects of mixed surfactants on the styrene miniemulsion polymerization in the presence of an alkyl methacrylate (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 2051-2061 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The influence of the mixed surfactants sodium dodecyl sulfate (SDS) and nonylphenol polyethoxylate with an average of 40 ethylene oxide units per molecule (NP-40) on the styrene (ST) miniemulsion polymerization in the presence of dodecyl methacrylate (DMA) and stearyl methacrylate (SMA) was investigated. Both Ostwald ripening and creaming could not be neglected for the miniemulsions stabilized by SDS/NP-40 in combination with DMA upon aging at 35°C, whereas no appreciable Ostwald ripening and creaming were detected for the SMA containing miniemulsions. For both the DMA and SMA containing polymerizations at 80°C, the rate of polymerization (Rp) decreases with increasing NP-40 concentration ([NP-40]). Incorporation of a small quantity of the extremely water-insoluble blue dye into the reaction system was applied to probe the particle nucleation loci. For the DMA containing polymerizations with [NP-40] = 0, 1.25, and 2.50 mM and the SMA polymerization with [NP-40] = 0 mM, homogeneous or micellar nucleation can not be ignored. On the other hand, the more hydrophobic SMA in combination with SDS/NP-40 effectively retards the particle nucleation occurring in the aqueous or micellar phase.
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  • 86
    Digitale Medien
    Digitale Medien
    Crystallization kinetics and melting behavior of poly(butylene adipate), poly(butylene isophthalate) and their copolymers (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 2063-2070 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The melting behavior and the isothermal crystallization kinetics of poly(butylene adipate), poly(butylene isophthalate) and their random copolymers were investigated by means of differential scanning calorimetry. Multiple endotherms, commonly observed on the melting polyesters, were found to be influenced by crystallization temperature and composition. By applying the Hoffman-Weeks method to the isothermally crystallized samples, the equilibrium melting temperatures of the homopolymers were obtained. From calorimetric results on samples with different degree of crystallinity, the equilibrium melting enthalpy of poly(butylene isophthalate) was calculated and the presence of a crystal-amorphous interphase in copolymers was evidenced. Isothermal melt crystallization kinetics was analyzed according to the Avrami equation. As expected, the introduction of a comonomer was found to decrease the overall crystallization rate of the polymers. Values of Avrami exponent close to three were obtained for all the samples, independently of composition and crystallization temperature, in agreement with a crystallization process originating from predetermined nuclei and characterized by three-dimensional spherulitic growth. In the case of poly(butylene isophthalate), the dependence of the rate of crystallization on Tc shows a maximum at an undercooling of approximately 60°C.
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  • 87
    Digitale Medien
    Digitale Medien
    Preparation, properties and biodegradation of stereoregular copolymers of (R,S)-3-butyrolactone and 4-butyrolactone (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 2079-2085 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Copolymers obtained from the copolymerization of (R,S)-3-butyrolactone ((R,S)-3-BL) and 4-butyrolactone (4-BL) catalyzed by isobutylaluminoxane were compared in structure and properties to copolymers having the same types of repeating units produced by Alcaligenes eutrophus when grown with an appropriate substrate or substrate or substrate mixtures. Copolymers with 4-BL contents up to 33 mol-% were prepared and characterized by 1H NMR, 13C NMR, gel permeation chromatography (GPC) and differential scanning calorimetry (DSC). The copolymers had a sequence distribution of 3-hydroxybutyrate (3-HB) and 4-hydroxybutyrate (4-HB) units which were non-random in that very few, if any, sequences of more than two successive 4-HB units were present. The 3-HB units are believed to be separated into isotactic (R) and (S) blocks, but thermal analysis by DSC showed that both the glass transition temperatures and melting temperatures decreased in a regular manner with increasing contents of 4-HB units. Number average molecular weights of the copolymers (Mn) ranged from 7000 to 15000. Biodegradation tests with a bacterial hydrolase showed that the biodegradation rates increased with increasing contents of 4-HB units, as is the case for the equivalent bacterial copolymers.
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  • 88
    Digitale Medien
    Digitale Medien
    Preparation of side-chain liquid-crystalline polymers via pyridylphenol hydrogen bonding (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 2101-2105 
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    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: A series of side-chain liquid-crystalline polymers via pyridyl-phenol hydrogen bonding complexation was prepared by using non-liquid-crystalline precursors, such as poly[(4-vinylpyridine)-co-(butyl acrylate)] and 4-(4′-nitrophenylazo)phenol, 4-(4′-methoxyphenylazo)phenol or 4-(phenylazo)phenol. The association by hydrogen bonding was confirmed by means of FTIR. While the hydrogen bonds are weakened, and some dissociation occurs at the isotropization temperature, complete dissociation needs much higher temperature. The hydrogen bonds re-form as the complex is cooled to room temperature. The nematic liquid crystallinity of the polymer complexes was shown by means of wide-angle X-ray diffraction (WXRD) and polarized optical microscopy (POM). The different substitution groups of azophenol give isotropization temperatures of the polymer complexes in the following sequence: —NO2 〉 —H 〉 —OCH3.
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  • 89
    Digitale Medien
    Digitale Medien
    Tailored rigid-flexible block copolymers, 4Part 3: cf. ref.3. Synthesis and properties of diblocks of poly(p-benzamide) and poly(m-benzamide) (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 2087-2094 
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    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Copolymers were prepared by the condensation of rigid benzoyl-terminated poly(p-benzamide) and flexible anilino-terminated poly(m-benzamide) prepolymers. The use of two monomers of the AB type and of end-capped prepolymers resulted in the formation of diblock molecules uncontaminated by triblock or multiblock sequences. Moreover, selective extraction techniques in N,N-dimethylacetamide (DMAc) with varying amounts of LiCl resulted in the complete elimination of unreacted prepolymers, as evidenced from material balance and 1H NMR data. The above extraction technique also allowed a fractionation of the copolymers in terms of their rigid/flexible compositional distribution ratio. The molecular weight-intrinsic viscosity dependence of poly(m-benzamide) determined in a nonaggregating solvent using light scattering yielded a persistence length of 8 Å, suggesting a relatively large chain flexibility. A tailored copolymer with a fraction β of flexible residues = 0.70 was fractionated, determining the solubility, the viscosity, and the critical composition for mesophase formation in DMAc/LiCl and in 96% H2SO4 solutions. The results, in line with those previously reported for a related system, reveal that the addition of the flexible segment has a little effect on the critical concentration of poly(p-benzamide), whereas the biphasic gap is strongly reduced. Moreover, the solubility of the copolymer is substantially increased over that of the rigid homopolymer. The net result is an unexpected widening of the range of stability of the pure mesophase, with significant implications from both a fundamental and an applied standpoint.
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  • 90
    Digitale Medien
    Digitale Medien
    Synthesis and characterization of a new polymer containing an electron accepting perfluoro group (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 2107-2112 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Poly{oxy-4,4′-octafluorobiphenylyleneoxy-alt-[1,4-phenylenevinylene(3-trimethylsilyl-1,4-phenylene)vinylene-1,4-phenylene]} (PFSi) was synthesized and characterized to investigate its thermal, optical, electrical and xerographic properties. An LED (light emitting device) using PFSi as the light emitting layer exhibits maximum emission for blue light, but a high turn-on voltage. The origin of the low device performance was examined by investigating the photoconducting behavior. PFSi shows a high photoconductivity in the presence of electron acceptors such as 5-nitroanthranilonitrile (5NAN), and the photoconductivity of PFSi is improved by introducing charge transport materials such as triphenylamine (TPA).
    Zusätzliches Material: 5 Ill.
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  • 91
    Digitale Medien
    Digitale Medien
    Synthesis and phase behavior of side-chain liquid crystalline polymers with pendant quinoline rings bearing either a polar or a non-polar end group (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 2095-2099 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Four methacrylate monomers, 2a-2d, containing a quinoline ring with a NO2 or a OCH3 group at the end of the mesogen were synthesized and polymerized. The copolymers with six methylene units as spacer, 3b-3d, showed a smectic phase induced by electron donor-acceptor interactions of the side groups. The homopolymer with eleven methylene units as spacer and methoxy end group, 3f, showed a smectic phase induced by the large length of the spacer. The copolymers with eleven methylene units, 3g-3i, showed a smectic phase induced by both the electron donor-acceptor interactions of the side groups and the large length of the spacer.
    Zusätzliches Material: 5 Ill.
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  • 92
    Digitale Medien
    Digitale Medien
    Molecular character of sharkskin phenomenon in metallocene linear low density polyethylenes (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 2113-2118 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The effect of molecular weight on sharkskin dynamics is systematically studied using three metallocene-catalyzed linear low density polyethylenes (mLLDPE) of different molecular weights. A new experimental method is developed to enable in-situ characterization of the time scale τ on which sharkskin forms on the extrudate. This laser scattering technique directly measures the periodic variation of the refracted light of the rough sharkskin-like extrudate surface. It is found that τ increases with molecular weight as strongly as the overall molecular chain relaxation time, τ*, determined from oscillatory shear measurements. The observed molecular weight dependence provides additional support for the recently proposed interfacial stress growth/relaxation mechanism for sharkskin formation.
    Zusätzliches Material: 7 Ill.
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  • 93
    Digitale Medien
    Digitale Medien
    Synthesis and properties of poly(dimethyldiphenylsilylenemethylene) (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 2119-2127 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The novel polycarbosilane poly(dimethyldiphenylsilylenemethylene) (PDMDPSM) (2) was prepared by H2PtCl6-catalyzed ring-opening polymerization (ROP) of 1,1-dimethyl-3,3-diphenyl-1,3-disilacyclobutane (1). Polymerization in bulk at elevated temperature afforded high molecular weight PDMDPSM (Mw = 577000) with monomodal molecular weight distribution and regularly alternating SiR2/CH2 backbone structure. NMR spectroscopy revealed that a minor fraction of the monomer units is incorporated irregularly during polymerization in bulk, leading to structural units of the type —SiPh2—CH2—SiPh2— and —SiMe2—CH2—SiMe2—. The amount of regularly alternating dimethyl- and diphenylsilylenemethylene units increases when the polymerization is performed in solution at lower temperatures. PDMDPSM shows a glass transition temperature (Tg) at 50°C, which lies between the TgS of poly(methylphenylsilylenemethylene) (PMPSM) and poly(diphenylsilylenemethylene) (PDPSM), demonstrating the stiffening effect of the SiPh2 groups. Thermogravimetric analysis (TGA) evidenced nearly identical thermal stability of PDMDPSM and PMPSM.
    Zusätzliches Material: 2 Ill.
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  • 94
    Digitale Medien
    Digitale Medien
    Supramolecular structures of poly(p-phenylenes) with oxyethylene side chains and their mixtures with lithium salts (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 2129-2140 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: A series of poly(p-phenylenes) with linear and branched oxyethylene side chains was synthesized. Microphase separation in the solid state causes polymers with linear side chains to form supramolecular structures, which consist of main chain layers separated by an amorphous side chain matrix. If mixed with lithium salts such as lithium triflate (LiOTf) or lithium bis(trifluoromethanesulfone)imide (LiNTf), the polymers form solid solutions. The experimental data suggest that the layered structures undergo a one-dimensional swelling in the direction perpendicular to the main chain layers when blended with LiOTf and that this salt is only incorporated into the side chain matrix. The observed melting point depression in polymer-LiNTf mixtures is explained by the assumption that the salt species are incorporated both into the side chain and the main chain microphase.
    Zusätzliches Material: 10 Ill.
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  • 95
    Digitale Medien
    Digitale Medien
    Dielectric relaxation and relationships of local molecular mobility in linear and comb-like thermotropic polymers (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 2147-2152 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The dielectric behavior of several thermotropic linear and comb-like polymers with flexible spacers and mesogenic fragments with different structure has been studied in a frequency range from 60 Hz to 1 MHz in the bulk state (below the glass transition temperature). Two regions of dipole relaxation caused by local mobility were detected. It was shown that the relaxation parameters of the first process virtually do not change with increasing spacer length and have similar values in polymers with different structure. In the case of the second process, in increase in spacer length considerably increases the mobility of kinetic elements.
    Zusätzliches Material: 6 Ill.
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  • 96
    Digitale Medien
    Digitale Medien
    Sequential reordering in condensation polymers, 5Parts 1-4: refs.4-7. Preparation and characterization of copolymers via transesterification of a polycaprolactonepoly(2,2-dimethyltrimethylene carbonate) blend (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 2153-2164 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: A blend of polycaprolactone (PCL) and poly(2,2-dimethyltrimethylene carbonate) (PDTC) containing catalytic amounts of dibutyltin dimethoxide was prepared from solution. The blend was then subjected to melt-mixing at 200°C and samples were taken after different times. According to thermal analysis data, with the increase of reaction time a gradual loss of the crystallizability of the blend components is observed, and after 70 min at 200°C the system becomes completely amorphous. 13C NMR data suggest that the loss of crystallizability of the blend is accompanied by an abrupt decrease in PCL and PDTC sequence lengths reaching after 70 min at 200°C an average length of about two repeating units each. These effects are explained by transesterification reactions between the lactone and carbonate units.
    Zusätzliches Material: 7 Ill.
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  • 97
    Digitale Medien
    Digitale Medien
    Crystallization kinetics and morphology of poly(ferrocenyldimethylsilane) (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 2141-2145 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: A series of poly(ferrocenyldimethylsilanes) were prepared via anionic ring-opening polymerization. The isothermal crystallization kinetics were investigated by means of differential scanning calorimetry and analyzed in terms of the Avrami equation. The value of the Avrami exponent is approximately 3 for the majority of samples and crystallization temperatures. This suggests a three-dimensional spherulitic growth and an instantaneous nucleation mechanism. The morphology of melt-crystallized samples was studied by atomic force microscopy. Hedrites (immature spherulites) were formed at low undercoolings whereas at high levels of undercooling mature, well developed spherulites were observed with small hedritic features in their center. The melting enthalpy for a 100% crystalline polymer was estimated to be 36 J/g from differential scanning calorimetry and X-ray diffraction data obtained by using specimens with different degrees of crystallinity.
    Zusätzliches Material: 7 Ill.
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  • 98
    Digitale Medien
    Digitale Medien
    Reactive polymers incorporating silyl enol ether groups, 3Part 2: cf. ref.1. Incorporation of α,β-bis(siloxy)vinylene units into the backbone of vinyl polymers by free-radical (co)polymerization of 2,3-bis(trialkylsiloxy)butadienes (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 2173-2178 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: 2,3-Bis(trimethylsiloxy)butadiene (1) and 2,3-bis(tert-butyldimethylsiloxy)butadiene (2) have been homopolymerized and copolymerized with styrene (St) and methyl methacrylate (MMA) under freeradical conditions. Comparison of initial rates and molecular weights with polymerizations conducted under identical experimental conditions on monosubstituted analogs, 2-trimethysiloxybutadiene (3) and 2-(tert-butyldimethylsiloxy)butadiene (4), demonstrated that the second substituent does not decrease the polymerizability significantly despite the additional steric hindrance introduced in the monomer. Extensive 1H and 13C NMR characterization of the polymers demonstrated that the main incorporation mode is of the 1,4-type, allowing more than 95% of the butadienyl subunit to be incorporated in the main chain as an α,β-bis(trialkylsiloxy)vinylene functional group. Monomer reactivity ratios r1 and r2 at 60°C were determined from copolymer composition curves obtained at low conversions and are compared with values obtained for other 1,2-butadienyl systems.
    Zusätzliches Material: 3 Ill.
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  • 99
    Digitale Medien
    Digitale Medien
    Radical polyaddition-isomerization of bifunctional vinylcyclopropanes with dithiols (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 2165-2172 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Synthesis and radical polyaddition-isomerization of novel bifunctional vinylcyclopropane derivatives, 1,4-xylylene bis(2-isopropenyl-2-methylcyclopropanecarboxylate) (1a), ethylene bis(2-isopropenyl-2-methylcyclopropanecarboxylamide) (1b), and bis(2-isopropenyl-2-methylcyclopropanemethyl) terephthalate (1c) with dithiols were carried out. Polymers with molecular weights and polydispersities of 1100-11000 and 1.18-7.35, respectively, were obtained. The radical polyadditions of 1a with dithiols afford polymers containing a homopolymer of 1a, while those of 1b and 1c with dithiols afford polymers with a content of dithiol units of exactly 50 mol-%.
    Zusätzliches Material: 1 Ill.
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  • 100
    Digitale Medien
    Digitale Medien
    Ge- and sn-containing poly(p-xylylene): synthesis, structure and thermal behavior (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 2179-2184 
    Details
    ISSN: 1022-1352
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Block copolymers of unsubstituted poly(p-xylylene)(PPX) and Ge- or Sn-organic bridged PPX (GePPX and SnPPX) were prepared by pyrolysis of specially synthesized organometallic p-cyclophane precursors followed by deposition and polymerization of the thus produced p-xylylene monomers. The copolymer structure and thermal behavior were investigated depending on deposition temperature (10 and -196°C). The copolymer PPX-GePPX obtained from solid monomers deposited at -196°C (2a) consists of long quasi-independent PPX and GePPX blocks and has paracrystalline structure. Thermal treatment of 2a near 160°C yields crystalline regions of PPX along with paracrystalline aggregates of GePPX. Pyrolysis of this system near 300°C in an inert atmosphere results in the formation of Ge crystals in PPX matrix. In contrast, the copolymer PPX-GePPX produced by simultaneous deposition and polymerization at 10°C (2a′) contains shorter blocks of PPX and GePPX than the copolymer deposited at -196°C. Copolymer 2a′ turns to the amorphous state during thermal treatment, and its pyrolysis does not lead to Ge-crystal formation; hence the supramolecular structure of the polymer plays an important role in inorganic phase formation. The copolymer PPX-SnPPX (2b) is formed only at a deposition temperature of -196°C; deposition at 10°C yields oligomeric resins. The structure of 2b is nearly the same as that of 2a; pyrolysis of 2b in air results in SnO2-crystal formation in a PPX matrix.
    Zusätzliches Material: 3 Ill.
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