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  • 1
    Digitale Medien
    Digitale Medien
    A chemically useful definition of electron difference densities (1986)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 909-914 
    Details
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Molecular difference densities (DD) are conventionally constructed using spherically averaged atomic densities at the appropriate positions. For atoms in degenerate ground states, this is an unphysical choice, and artifacts dominate the DD. We suggest the extraction of both the position and the orientation of an atom with an open valence shell from x-ray scattering or molecular density data. Subtracting the oriented atoms yields a uniquely defined, as well as chemically meaningful, DD. Covalent bonds to electronegative atoms such as O are no longer exceptional but show bond charges of normal magnitude. Lone pairs are characterized by a dipolar density shift from the bond to the back side of the atomic core.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/qua.560290428
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  • 2
    Digitale Medien
    Digitale Medien
    Some new aspects of bonding in cyclopropane (1986)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 1147-1154 
    Details
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The peculiar properties of the three-membered ring in cyclopropane are partially due to characteristic relaxation effects of the carbon orbitals in the molecule. AO contraction in the two strongly C—C bonding MO's of A'1 type is essential for the stability of the C3 ring, whereas AO expansion in the E'-type HOMO contributes to the π character of the peripheral C—C bonds.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/qua.560290513
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  • 3
    Digitale Medien
    Digitale Medien
    A chemically useful definition of electron difference densitiesBecause of several errors in the original printing, this article is being reprinted here in its entirety. (1986)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 30 (1986), S. 439-444 
    Details
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Molecular difference densities (DD) are conventionally constructed using spherically averaged atomic densities at the appropriate positions. For atoms in degenerate ground states, this is an unphysical choice, and artifacts dominate the DD. We suggest the extraction of both the position and the orientation of an atom with an open valence shell from x-ray scattering or molecular density data. Subtracting the oriented atoms yields a uniquely defined, as well as chemically meaningful, DD. Covalent bonds to electronegative atoms such as O are no longer exceptional but show bond charges of normal magnitude. Lone pairs are characterized by a dipolar density shift from the bond to the back side of the atomic core.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/qua.560300311
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  • 4
    Digitale Medien
    Digitale Medien
    Electron densities and chemical bonding in TiC, TiN, and TiO derived from energy band calculations (1983)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 23 (1983), S. 1535-1552 
    Details
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The chemical bonding in the interesting class of refractory transition metal compounds is illustrated for TiC, TiN, and TiO. Self-consistent augmented plane wave (APW) calculations are already available for these compounds. Using the respective potentials we have repeated the band calculations on a finer k grid with the linearized APW method to obtain accurate densities of states (DOS). These DOS can be divided into local partial contributions to characterize the bonding. Further information can be obtained from a decomposition of the metal d DOS into t2g and eg symmetry components. These partial local DOS are compared with the LCAO counterpart and give a first picture of the chemical bonding in these compounds. The electron densities corresponding to the occupied valence states are obtained from the LAPW calculations. They provide further insight into characteristic trends in the series from TiC to TiO: around the nonmetal site the density shows increasing localization; around the metal site the deviation from spherical symmetry changes from eg to t2g. These effects can be traced back to the three types of valence bands. Electron density plots of characteristic band states (all energies of a selected k point in the Brillouin zone) will be shown. These plots can describe the different types of bonding occurring in these systems.
    Zusätzliches Material: 6 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/qua.560230435
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  • 5
    Digitale Medien
    Digitale Medien
    Charge distribution and electric field gradients in YBa2Cu3O7 by band structure calculations (1990)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 38 (1990), S. 339-347 
    Details
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: From calculations using the full potential linearized augmented plane wave (LAPW) method, in which exchange and correlation effects are treated by the local density approximation (LDA), the electric field gradients (EFGs) of YBa2Cu3O7 are obtained on a first principles basis. The EFGs are computed at all atomic positions and a comparison with experimental values (magnitude and corresponding anisotropies) shows good agreement for Ba, Cu(1), and all oxygen positions while at Cu(2) the computed EFG agrees in symmetry but has only half the experimental value. However, the theoretical EFG would agree with the experimental value if only 0.07 electrons are transferred from dx2-y2 to dz2 symmetry.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/qua.560382434
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  • 6
    Digitale Medien
    Digitale Medien
    Understanding relativistic effects of chemical bonding (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 641-653 
    Details
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: To elucidate the physical origin of relativistic changes of molecular properties, exact theorems, perturbation theory, and Hartree-Fock-Slater-Pauli calculations are exploited. The relativistic molecular virial theorem offers insight into the relativistic and nonrelativistic, kinetic, and potential energy contributions to the bond energy. In general, there exist two contributions to the relativistic correction of a molecular property: the relativistic change at the nonrelativistic equilibrium geometry and the change of the nonrelativistic property due to the relativistic change of the equilibrium geometry. Sometimes the first and sometimes the second contribution is the dominant one. Accurate numerical results for H+2-like systems are obtained using direct relativistic double perturbation theory. In some cases, near-degenerate perturbation theory is mandatory. Relativistic changes of chemical bond energies are often proportional to the density change in the K-shell when the bond is formed. Relativistic corrections to many properties (and also to the 1s2-correlation energy) are often proportional to Z2α2. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 4 Ill.
    Materialart: Digitale Medien
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  • 7
    Digitale Medien
    Digitale Medien
    Selectivity effects and hydrocarbon-chain growth during Fischer-Tropsch synthesis (1982)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 3 (1982), S. 135-139 
    Details
    ISSN: 0192-8651
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Biochemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Informatik
    Notizen: It is well known that if the formation of hydrocarbons during Fischer-Tropsch synthesis occurs via the condensation polymerization mechanism, there is little hope for selectivity enhancement in the desired range of products. Recent data on low surface area model catalysts, where readsorption is unlikely, have shown that at low conversions, the product distribution obeys the condensation polymerization mechanism and the distribution of products is shifted to lower-molecular-weight hydrocarbons. We have used a computer simulation of the growth of hydrocarbon chains to obtain a picture of the catalyst surface under synthesis conditions. Such an approach could prove useful in distinguishing between various theoretical models. We have applied the simulation to compare the changes in selectivity when readsorption occurs and when it does not. The dynamic behavior of the reacting system which is obtained from the computer results has shown that selectivity to lower-molecular-weight hydrocarbons is a stronger function of the extent of reaction than the incorporation of readsorption into the chain growth mechanism.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/jcc.540030204
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  • 8
    Digitale Medien
    Digitale Medien
    The calculated structures of the C4H8NO+ cations resulting from the unimolecular gas-phase dissociation (CH3)2NCOCH2X·+ → C4H8NO+ + X· (X = CL, NO2) (1982)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 3 (1982), S. 251-259 
    Details
    ISSN: 0192-8651
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Biochemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Informatik
    Notizen: The structures and rearrangements of various cyclic and acyclic C4H8NO+ cations resulting from the title reaction have been investigated by means of semiempirical (MNDO) and ab initio (4-31G) calculations.
    Zusätzliches Material: 3 Tab.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/jcc.540030217
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  • 9
    Digitale Medien
    Digitale Medien
    Theoretical Investigations on Fluorine-Substituted Ethylene Dications C2HnF4-n 2+(n = 0-4)Part of this research has been published as a short communication: W. Koch, G. Frenking, F. Maquin, D. Stahl, and H. Schwarz, J. Chem. Soc. Chem. Commun. 1187, 1984. (1986)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 7 (1986), S. 406-416 
    Details
    ISSN: 0192-8651
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Biochemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Informatik
    Notizen: The optimized geometries and energies of fluorine-substituted ethylene dications C2HnF4-n 2+ (n = 0-4) have been investigated by means of ab initio methods. At the MP3/6-31G**//6-31G* + zero-point energy level of theory, the results predict that C2F42+ and C2HF32+ are planar, while C2H42+, C2H3F2+ and 1,1 - C2H2F22+ prefer a perpendicular geometry. For 1,2 - C2H2F22+ an energy difference of only 0.3 kcal/mol is found between the (trans) planar and perpendicular structure. The stabilizations attributed to hyperconjugation, fluorine lone-pair donation, and (C—F) double-bond conjugation are discussed. A comparison is made for the C—C and C—F stretching frequencies determined at 6-31G*//6-31G* between the neutral and dicationic species. The theoretically determined ionization energies for the vertical process N+ → N2+ at the MP3/6-31G*//3-21G level are compared with experimental Qmin values.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/jcc.540070404
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