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  • Chemistry  (167)
  • Organic Chemistry
  • 1995-1999  (167)
  • 1998  (167)
  • 1
    Electronic Resource
    Electronic Resource
    Photoreduction of IrCl62- complex in alcohol solutions and its reaction with hydroxyalkyl radicals (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Chemical Kinetics 30 (1998), S. 711-719 
    Details
    ISSN: 0538-8066
    Keywords: Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The photochemistry of IrCl62- complex in simple alcohols have been studied using laser-flash photolysis. Single electron transfer from the solvent molecule to the light-excited complex has been shown to be the primary photochemical process. Quantum yields of the photoreduction of IrCl62- complex and the rate constants of its reaction with hydroxyalkyl radicals were determined at 200-330 K. Deviations of the rate constants from Debye-Smoluchowski equation for diffusion-controlled reactions are discussed. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 711-719, 1998
    Additional Material: 5 Ill.
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  • 2
    Electronic Resource
    Electronic Resource
    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
    Keywords: crown dithioether ; conformational analysis ; multicomponent equilibrium ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: 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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  • 3
    Electronic Resource
    Electronic Resource
    Electron propagator theory of conformational effects on anisole and thioanisole photoelectron spectra (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 1037-1043 
    Details
    ISSN: 0020-7608
    Keywords: anisole ; thioanisole ; conformations ; propagator theory ; photoelectron spectra ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Electron propagator calculations in two diagonal self-energy approximations, partial third order and the outer valence Green/s function, are employed in interpretation of the photoelectron spectra of anisole and thioanisole. Major features of the spectra are accurately predicted and are associated to canonical molecular orbitals. Contributions from planar and orthogonal rotational structures are responsible for minor features of the spectra that are dependent on temperature. Correlation effects are large and change the order of final states in thioanisole. Partial third-order calculations are more accurate than outer valence Green/s function results.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1037-1043, 1998
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  • 4
    Electronic Resource
    Electronic Resource
    Polyurethanes crosslinked by metals and their complexes with crown ether (1998)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 36 (1998), S. 1379-1386 
    Details
    ISSN: 0887-624X
    Keywords: polyrethanes ; crown ether ; metal complexes ; X-ray scattering ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two types of crosslinked polyurethanes (PU) have been synthesized: (a) PU crosslinked by metal ions (Cu2+ and Co2+), and (b) PU crosslinked by the complexes of metals with crown ether. Using X-ray scattering under small and wide angles the peculiarities of the structure of networks have been investigated. It was found that PU crosslinked by the metal crown ether complexes have a looser structure because of the bulky crosslink and diminished molecular mobility of the chains between two crosslinks, as was proven by the dielectric spectroscopy method. The X-ray and IR data have allowed proposing a scheme of the structure of the crosslinked PU with various types of crosslinks. The structures discovered may be considered as similar to metal catenandes. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1379-1386, 1998
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  • 5
    Electronic Resource
    Electronic Resource
    Copolymerization of divinyl monomers with maleic and fumaric acid derivatives (1998)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 36 (1998), S. 371-378 
    Details
    ISSN: 0887-624X
    Keywords: copolymerization ; divinyl monomers ; diethyl fumarate ; maleic anhydride ; divinylbenzene ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The process of formation of reticular copolymer molecular structures produced in free radical copolymerization of divinyl monomers (divinyl ethers of diethylene glycol and hydroquinone, divinyl sulfide, p-divinylbenzene, etc.) with maleic and fumaric acid derivatives is studied. The basic factor that determines the features of molecular and network structures of copolymers is reactivity of the divinyl monomer in copolymerization with monovinyl monomer. The network of copolymers of maleic anhydride with the divinyl ether of hydroquinone is formed out of oligomer microgels. Divinyl sulfide in copolymerization with maleic acid is disposed to cyclocopolymerization; also crosslinking reactions occur. Formation of a network structure of copolymers of divinylbenzene with maleic and fumaric acid derivatives is shown to proceed via an alternating copolymerization mechanism. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 371-378, 1998
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  • 6
    Electronic Resource
    Electronic Resource
    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
    Keywords: calix[4]resorcinarenes ; organophosphorus compounds ; solvatophobic interactions ; host-guest complexation ; high-performance liquid chromatography ; x-ray analysis ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: 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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  • 7
    Electronic Resource
    Electronic Resource
    Mechanism of cycloaddition to indolizines (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 201-208 
    Details
    ISSN: 0894-3230
    Keywords: indolizines ; cycloaddition ; reaction mechanism ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: 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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  • 8
    Electronic Resource
    Electronic Resource
    SCF calculations with density-dependent local-exchange potential (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 591-600 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new model for the exchange potential in the framework of DFT is proposed. The potential is defined as 2αx∊xappr, where ∊xappr is the exchange-energy density and αx is not a constant but a functional αx[ρ] to be determined iteratively. The exact Fock expression and the LDA, GEA, and Becke88 approximations were used as ∊xappr. We provide results for atoms showing that this model potential yields total and exchange energies and other atomic properties that are in good agreement with Hartree-Fock values. In addition, total energies obtained by adding to the αx[ρ] approach correlation energy corrections computed via the WL and LYP functionals are in close accord with experimental values.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 591-600, 1998
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  • 9
    Electronic Resource
    Electronic Resource
    Studies of conformational isomerism in α-melanocyte stimulating hormone by design of cyclic analogues (1998)
    New York : Wiley-Blackwell
    Biopolymers 46 (1998), S. 155-167 
    Details
    ISSN: 0006-3525
    Keywords: α-MSH ; [D-Phe7]α-MSH ; isomerism ; cyclic analogues ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Results of energy calculations for α-MSH (α-melanocyte stimulating hormone, Ac-Ser1-Tyr2-Ser3-Met4-Glu5-His6-Phe7-Arg8-Trp9-Gly10-Lys11-Pro12-Val13-NH2) and [D-Phe7]α-MSH were used for design of cyclic peptides with the general aim to stabilize different conformational isomers of the parent compound. The minimal structural modifications of the conformationally flexible Gly10 residue, as substitutions for L-Ala, D-Ala, or Aib (replacing of hydrogen atoms by methyl groups), were applied to obtain octa- and heptapeptide analogues of α-MSH(4-11) and α-MSH(5-11), which were cyclized by lactam bridges between the side chains in positions 5 and 11. Some of these analogues, namely those with substitutions of the Gly10 residue with L-Ala or Aib, showed biological activity potencies on frog skin comparable to the potency of the parent tridecapeptide hormone. Additional energy calculations for designed cyclic analogues were used for further refinement of the model for the biologically active conformations of the His-Phe-Arg-Trp “message” sequence within the sequences of α-MSH and [D-Phe7]α-MSH. In such conformations the aromatic moieties of the side chains of the His6, L/D-Phe7, and Trp9 residues form a continuous hydrophobic “surface,” presumably interacting with a complementary receptor site. This feature is characteristic for low-energy conformers of active cyclic analogues, but it is absent in the case of inactive analogues. This particular spatial arrangement of functional groups involved in the message sequence is very close for α-MSH and [D-Phe7]α-MSH, as well as for biologically active cyclic analogues despite differences of dihedral angle values for corresponding low-energy conformations. © 1998 John Wiley & Sons, Inc. Biopoly 46: 155-167, 1998
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  • 10
    Electronic Resource
    Electronic Resource
    Ethanol-induced conformational transitions in holo-α-lactalbumin: Spectral and calorimetric studies (1998)
    New York : Wiley-Blackwell
    Biopolymers 46 (1998), S. 253-265 
    Details
    ISSN: 0006-3525
    Keywords: α-lactalbumin ; holo-form ; conformational transitions ; unfolding ; aromatic chromophores ; secondary structure ; ethanol ; fluorescence ; CD ; scanning calorimetry ; thermodynamics ; linear model of denaturation ; additive calculation scheme ; transfer of amino acid side chains from water to aqueous ethanol solutions ; changes in solvent accessibility of amino acid residues due to unfolding ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Conformational transitions of holo-α-lactalbumin in a hydro-ethanolic cosolvent system was studied by spectrofluorescence, CD in near- and far-uv regions, and high-sensitivity differential scanning calorimetry. Experimental results allow us to propose that in isothermal conditions α-lactalbumin undergoes a number of conformational transitions with increasing ethanol concentration: N ⇔ I ⇔ D ⇔ H. The existence of I-state was deduced from spectrofluorometric and near-uv CD data. In this state the aromatic chromophores of the amino acid side chains are more accessible to the solvent displaying higher local mobility. The H-state was detected from far-uv CD spectra as a state corresponding to the content of α-helices higher than originally found in native protein. However, calorimetric measurements provide data revealing only the two-state mechanism of α-lactalbumin unfolding in both water and in aqueous ethanol solutions. This indicates that the energy levels of N- and I-states as well as of D- and H-states are similar. Thermodynamics of the unfolding of α-lactalbumin in hydro-ethanolic solutions was analyzed with the help of the linear model of solvent denaturation. Unfolding increments of enthalpy, entropy, and Gibbs energy of transfer of the protein from a reference aqueous solution to hydro-ethanolic solutions of different concentrations were determined from the calorimetric data. They are linear functions of molar ethanol fraction. The slope of the unfolding increment of Gibbs energy of transfer was calculated from data on transfer of amino acid residues taking into account the average solvent accessibility of amino acid residues in the native structure of small globular proteins, using the additive group contribution method. © 1998 John Wiley & Sons, Inc. Biopoly 46: 253-265, 1998
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