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  • Computational Chemistry and Molecular Modeling  (6)
  • crystal structure  (4)
  • 1995-1999  (10)
  • 1
    Electronic Resource
    Electronic Resource
    Utopia dielectrica (1995)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 56 (1995), S. 523-531 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The dielectric constant of a material is a macroscopic property that measures the reduction of the electrostatic forces between charged plates separated by the material, compared to a vacuum as intermediate material. It is next encountered as a scaling parameter in Coulomb's law for interacting charges, not only in the force, but also in the energy. In deriving the theory for dielectrics, the macroscopic nature is essential: Only then is the basic assumption that the dielectric material is homogeneous and isotropic a valid one. The appearance of the dielectric constant as a simple scaling factor in Coulomb's law has tempted many computational chemists to forget about the macroscopic nature of the dielectric and to apply the screened Coulomb's law between charges, supposedly in a low-dielectric medium such as proteins, in microscopic force fields. Optimization of force fields even led to distance-dependent “dielectric constants.” Another use of the dielectric constant appears in the dielectric continuum reaction field approaches for the computations of solvation energies and solvent effects. The solute is embedded in a cavity surrounded by the dielectric. Specific interactions between solvent molecules and solute are thus neglected. The cavity size and dielectric constants of interior and exterior are optimized for the model. The aim of this article is to show, by means of calculations on interacting point charges embedded in cavities surrounded by dielectrics and microscopic models of “low-dielectric” materials by explicit polarizabilities, that as far as the dielectric “constant” is concerned anything can happen, depending on the nature of the charges, the distance to the cavity boundary, the spatial arrangement of charges, and polarizabilities. Thus, a warning is issued to injudicious use of dielectric models in microscopic calculations. © 1995 John Wiley & Sons, Inc.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560560855
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  • 2
    Electronic Resource
    Electronic Resource
    Implementation of reaction field methods in quantum chemistry computer codes (1995)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 16 (1995), S. 1445-1446 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540161113
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  • 3
    Electronic Resource
    Electronic Resource
    Implementation of reaction field methods in quantum chemistry computer codes (1995)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 16 (1995), S. 37-55 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: The embedding of a quantum mechanically described subsystem by classical representations of its surroundings is reviewed. The choices for a distributed monopole representation and a distributed (group) polarizability representation, as well as the continuum approach to model bulk effects, are discussed. Focus is on the practical implementation of the classical description in quantum chemistry codes (in particular, HONDO8.1). Expressions are given for the self-consistent coupling between the classical partitions (dipole polarizabilities and boundary surface dipoles and charges) and for the coupling between classical and quantum partitions. The latter is mediated through expanded, rather than exact, potentials and fields. In this way, the computation of only a limited number of formal interactions between unit charge distributions located at the expansion centers suffices to evaluate the reaction field contributions. The electronic part of the coupling can be included in the Hamiltonian via the Fock matrix. The field operators, as well as the one- and two-electron matrix elements over the basis functions, are simple. The expressions for these are given explicitly.Nonequilibrium potentials and Monte Carlo sampling over classical degrees of freedom have been added to better mimic experimental conditions. © 1995 by John Wiley & Sons, Inc.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540160105
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  • 4
    Electronic Resource
    Electronic Resource
    Strukturverfeinerung und magnetische Messungen an Mn2SnS4 (1995)
    Weinheim : Wiley-Blackwell
    Zeitschrift für anorganische Chemie 621 (1995), S. 1600-1604 
    Details
    ISSN: 0044-2313
    Keywords: Dimanganese tin tetrasulfide ; crystal structure ; magnetism ; IR ; Raman ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Structure Redetermination and Magnetic Studies on Mn2SnS4The crystal structure of Mn2SnS4 was redetermined by single crystal and powder X-ray studies. It has a deficient NaCl superstructure crystallizing in the orthorhombic space group Cmmm proposed by Wintenberger and Jumas in 1980 (Z = 2, a = 740.7(1), b = 1047.5(1) and c = 366.7(2) pm, Rf = 1.4% for 266 unique reflections with I 〉 0σ1). Some additional reflections, which are not compatible with this cell, can be refined assuming formation of twinned trilling crystals. Mn2SnS4 undergoes antiferromagnetic ordering below 160 K. The effective magnetic moment μeff of Mn2+ is 5.92 B.M. The IR and Raman spectra display 5 and 3 bands in the range 150-320 cm-1, respectively.
    Notes: Die Kristallstruktur von Mn2SnS4 wurde mittels Röntgeneinkristall- und Röntgenpulver-Messungen nachbestimmt. Mn2SnS4 besitzt eine defiziente NaCl-Überstruktur und kristallisiert, wie von Wintenberger und Jumas [6] vorgeschlagen, in der orthorhombischen Raumgruppe Cmmm (Rf = 1,4% für 266 unabhängige Reflexe mit I 〉 0σ1). Die in dieser Zelle nicht indizierbaren Reflexe können unter der Annahme einer Mehrlingsbildung verfeinert werden. Das effektive magnetische Moment μeff von Mn2+ beträgt 5,92 B.M. Unterhalb 160 K liegt eine antiferromagnetische Ordnung vor. Die IR-und Raman-Spektren zeigen 5 bzw. 3 Banden im Bereich von 150-320 cm-1.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/zaac.19956210927
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  • 5
    Electronic Resource
    Electronic Resource
    Calculations of molecules, clusters, and solids with a simplified LCAO-DFT-LDA scheme (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 185-192 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A simplified LCAO-DFT-LDA scheme for calculations of structure and electronic structure of large molecules, clusters, and solids is presented. Forces on the atoms are calculated in a semiempirical way considering the electronic states. The small computational effort of this treatment allows one to perform molecular dynamics (MD) simulations of molecules and clusters up to a few hundred atoms as well as corresponding simulations of condensed systems within the Born-Oppenheimer approximation. The accuracy of the method is illustrated by the results of calculations for a series of small molecules and clusters. © 1996 John Wiley & Sons, Inc.
    Additional Material: 3 Ill.
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  • 6
    Electronic Resource
    Electronic Resource
    Direct reaction field force field: A consistent way to connect and combine quantum-chemical and classical descriptions of molecules (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1111-1132 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The direct reaction field (DRF) force field gives a classical description of intermolecular interactions based on ab initio quantum-chemical descriptions of matter. The parameters of the DRF force field model molecular electrostatic and response properties, which are represented by distributed charges and dipole polarizabilities. The advantage of the DRF force field is that it can be combined transparently with quantum-chemical descriptions of a part of a large system, such as a molecule in solution or an active site in a protein. In this study, the theoretical basis for the derivation of the parameters is reviewed, paying special attention to the four interaction components: electrostatic, induction, dispersion, and repulsion. The ability of the force field to provide reliable intermolecular interactions is assessed, both in its mixed quantum-chemical-classical and fully classical usage. Specifically, the description of the water dimer and the solvation of water in water is scrutinized and seen to perform well. The force field is also applied to systems of a very different nature, viz. the benzene dimer and substituted-benzene dimers, as well as the acetonitrile and tetrachloromethane dimers. Finally, the solvation of a number of polar solutes in water is investigated. It is found that as far as the interaction energy is concerned, the DRF force field provides a reliable embedding scheme for molecular environments. The calculation of thermodynamic properties, such as solvation energy, requires better sampling of phase space than applied here. © 1996 John Wiley & Sons, Inc.
    Additional Material: 11 Ill.
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  • 7
    Electronic Resource
    Electronic Resource
    Solvatochromism of the π* ← n transition of acetone by combined quantum mechanical - classical mechanical calculations (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 1067-1076 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The solvent shift of the π* ← n transition of acetone in water, acetonitrile, and tetrachloromethane was calculated in a combined quantum mechanical - classical mechanical approach, using both dielectric continuum and explicit, polarizable molecular solvent models. The explicit modeling of solvent polarizability allows for a separate analysis of electrostatic, induction, and dispersion contributions to the shifts. The calculations confirm the qualitative theories about the mechanisms behind the blue shift in polar solvents and the red shift in nonpolar solvents, the solvation of the ground state due to electrostatic interactions being preferential in the former, and favorable dispersion interaction with the excited state, in the latter case. Good quantitative agreement for the solvent shift between experiment (+1,700, +400, and -350 cm-1 in water, acetonitrile, and tetrachloromethane, respectively) and the explicit solvent model (+1,821, +922, and -381 cm-1) was reached through a modest Monte Carlo sampling of the solvent degrees of freedom. A consistent treatment of the solvent could only be realized in the molecular solvent model. The dielectric-only model needs reparameterization for each solvent. © 1996 John Wiley & Sons, Inc.
    Additional Material: 1 Ill.
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  • 8
    Electronic Resource
    Electronic Resource
    Ein neuer Strukturtyp am Lanthanoid-Oxowolframat FeCe(WO4)W2O8 = FeCe(WO4)3Professor Gerhard Thiele zum 60. Geburtstag gewidmet (1996)
    Weinheim : Wiley-Blackwell
    Zeitschrift für anorganische Chemie 622 (1996), S. 254-258 
    Details
    ISSN: 0044-2313
    Keywords: Iron cerium oxotungstate, FeCe(WO4)3 ; crystal structure ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: A New Structure Type for the Rare Earth Oxotungstate FeCe(WO4)W2O8 = FeCe(WO4)3Single crystals of the hitherto unknown compound FeCe(WO4)3 have been prepared by crystallization from melts of Fe2O3, CeO2 and WO3. It crystallizes with triclinic symmetry, space group P1, a = 7,486(3); b = 7,528(1); c = 16,502(4) Å, α = 101,00(2); β = 96,62(3); γ = 98,62°; Z = 2. Tungsten shows octahedral and tetrahedral coordination by oxygen. The crystal structure is characterized by layers related to the Scheelite and Wolframite type. Thermogravimetric measurements led to a lost of oxygen during reaction. It results in a decrease of the oxidation states of Fe3+ and Ce4+ respectively, as will be discussed using magnetic measurements and calculations of the Coulomb terms of lattice energy. The structure contains a one-fold coordinated oxygen.
    Notes: Einkristalle der bisher unbekannten Verbindung FeCe(WO4)3 wurden aus Schmelzen von Fe2O3, CeO2 und WO3 erhalten und röntgenographisch untersucht. Sie kristallisieren mit trikliner Symmetrie, Raumgruppe P1, mit a = 7,486(3); b = 7,528(1); c = 16,502(4) Å, α = 101,00(2); β = 96,62(3); γ = 98,62(2)°, Z = 2. Wolfram ist oktaedrisch und tetraedrisch von O2- koordiniert. Im Kristallverband sind Polyederschichten zu erkennen, die sich vom Scheelit und Wolframit ableiten. Thermogravimetrische Messungen zeigen Sauerstoffabgabe während der Reaktion. Die damit verbundene Erniedrigung der Oxidationszahl von Fe3+ bzw. Ce4+ wird anhand magnetischer Messungen und Berechnungen der Coulomb-Terme der Gitterenergie diskutiert. Die Kristallstruktur weist einfach koordinierten Sauerstoff auf.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/zaac.19966220209
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  • 9
    Electronic Resource
    Electronic Resource
    Refinement of the Crystal Structure of Sr3Hg2Dedicated to Professor John D. Corbett on the Occasion of his 70th Birthday (1996)
    Weinheim : Wiley-Blackwell
    Zeitschrift für anorganische Chemie 622 (1996), S. 401-404 
    Details
    ISSN: 0044-2313
    Keywords: Strontium amalgam, Sr3Hg2 ; crystal structure ; U3Si2 type ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Die Verfeinerung der Kristallstruktur des Sr3Hg2Aus Mischungen von Strontium und Quecksilber im Molverhältnis 7:3 kann man durch Tempern bei 520°C (20 Tage) phasenreine Proben von Sr3Hg2 erhalten, aus denen Einkristalle ausgelesen werden können. Sr3Hg2 kristallisiert im U3Si2-Typ (Raumgruppe P4/mbm) mit a = 8,883(2) Å und c = 4,553(1) Å. Alle Hg-Atome sind in Hanteln eingebunden (dHg—Hg = 3,41 Å).
    Notes: Mixtures of strontium and mercury in molar ratios of 7:3 have been annealed for 20 days at 520°C. From the pure product Sr3Hg2 single crystals have been obtained. Sr3Hg2 crystallizes in the U3Si2 type of structure (space group P4/mbm); the cell constants are a = 8.883 (2) Å and c = 4.553(1) Å. All of the Hg atoms are involved in Hg2 dumbbells with Hg—Hg distances of 3.41 Å.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/zaac.19966220304
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  • 10
    Electronic Resource
    Electronic Resource
    Synthesis, Crystal Structure, and IR Investigations of [(NH3)5V—O—V(NH3)5]I4 · NH3 and [(NH3)5Ti—O—Ti(NH3)5]I4 · NH3 (1996)
    Weinheim : Wiley-Blackwell
    Zeitschrift für anorganische Chemie 622 (1996), S. 2074-2078 
    Details
    ISSN: 0044-2313
    Keywords: μ-oxo bis [pentaammine vanadium(III)] iodide monoammoniate ; μ-oxo bis [pentaammine titanium(III)] iodide monoammoniate ; preparation ; crystal structure ; IR spectroscopy ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Darstellung, Kristallstruktur und IR-spektroskopische Untersuchungen an [(NH3)5V—O—V(NH3)5]I4 · NH3 und [(NH3)5Ti—O—Ti(NH3)5]I4 · NH3Reaktionen von VI2 bzw. TiI3 mit Ammoniak in Gegenwart von Wasser- bzw. Sauerstoffspuren führen zu [(NH3)5M—O—M(NH3)5]I4 · NH3 mit M = V, Ti. Ihre Strukturen wurden röntgenographisch an Einkristallen bestimmt:Pbca (Nr. 61), Z = 4, M = V: a = 12,482(4) Å, b = 14,819(6) Å, c = 13,286(5) Å, Z(Fo2 ≤ 3σFo2) = 983, Z(Variablen) = 88, R/Rw = 0,053/0,063, M = Ti: a = 12,628(4) Å, b = 14,970(4) Å, c = 13,359(3) Å, Z(Fo2 ≤ 3σFo2) = 1188, Z(Variablen) = 88, R/Rw = 0,043/0,047Die Strukturen bauen sich aus Oktaederdoppeleinheiten [(NH3)5M—O—M(NH3)5]4+ mit ekliptischer Konformation auf, deren verbrückende Sauerstoffatome nach dem Motiv einer verzerrten, kubisch flächenzentrierten Packung angeordnet sind. IR-spektroskopische Daten von undeuterierten und zu 5% deuterierten Proben der Vanadiumverbindung weisen auf N—H … I-Wasserstoffbrückenbindungen und π-Bindungsanteile zwischen dem jeweiligen Übergangsmetall und dem verbrückenden Sauerstoffatom hin.
    Notes: The reaction of VI2 or TiI3, respectively, with ammonia in the presence of traces of water or oxygen, respectively, leads to [(NH3)5M—O—M(NH3)5]I4 · NH3 with M = V, Ti. Their structures were solved by X-ray single crystal data:Pbca (No. 61), Z = 4, M = V: a = 12.482(4) Å, b = 14.819(6) Å, c = 13.286(5) Å, N(Fo2 ≤ 3σFo2) = 983, N(variables) = 88, R/Rw = 0.053/0.063, M = Ti: a = 12.628(4) Å, b = 14.970(4) Å, c = 13.359(3) Å, N(Fo2 ≤ 3σFo2) = 1188, N(variables) = 88, R/Rw = 0.043/0.047.The structures consist of corner sharing octahedra double units [(NH3)5M—O—M(NH3)5]4+ with eclipsed conformation which are stacked together according to the motif of a distorted cubic face centered arrangement for the bridging oxygen atoms. IR spectroscopic investigations of the undeuterated vanadium compound and of 5% deuterated samples hint to N—H … I hydrogen bridge bonds and to remarkable π-bonding between the transition metal and the bridging oxygen atoms.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/zaac.19966221213
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