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  • Computational Chemistry and Molecular Modeling  (19)
  • Electronic structure and strongly correlated systems
  • 1990-1994  (19)
  • 1
    Electronic Resource
    Electronic Resource
    A population analysis based on hybrid orbitals (1990)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 38 (1990), S. 597-605 
    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: By a way of partitioning of Mulliken overlap populations, the hybrid orbitals of central atoms and ligands are obtained by the CNDO method. The dipole moments of some hybrids are calculated using the hybrid orbitals obtained, and they are mostly in accordance with experimental values. In addition, overlap integrals may be decomposed into components, and then σ, π, and δ hybrid orbitals can be obtained.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560380409
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  • 2
    Electronic Resource
    Electronic Resource
    Electrophilic substitution in the benzofuran series: A theoretical (AM1) study (1991)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 40 (1991), S. 325-333 
    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 electrophilic substitution in a series of 5-aminobenzofurans was investigated by all-valence electron AM1 molecular orbital calculations. Various molecular properties of the reactants, intermediates, and products were computed and compared with available experimental data. The results indicate that while calculated ground-state charge densities cannot be used to predict reactivity and orientations, the relative thermodynamic stabilities of the reaction intermediates (arenium ions, σ complexes) reflected by calculated heats of formation (ΔHƒ) accurately indicated the isomers resulting from the kinetically controlled substitutions. The relative stabilities of the reaction products could be used to predict reaction preferences for the thermodynamically controlled electrophilic substitutions. Positions 4 for both aniline and 5-aminobenzofurans were the most susceptible toward electrophilic attack. The calculated ΔHƒ of the reaction intermediates, in good agreement with the experimental data, were capable of distinguishing between slightly nonequivalent positions such as the ortho and ortho prime to a substituent. Calculated AM1 ΔHƒ may represent practical guides in the prediction of sites of electrophilic aromatic substitutions for molecules with complex structures.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560400832
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  • 3
    Electronic Resource
    Electronic Resource
    Variational calculations on ammonia using two symmetrical normal modes (1991)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 40 (1991), S. 441-450 
    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: Variational calculations have been carried out on the ammonia molecule using two-dimensional potential energy functions. The two dimensions used are the inversion and symmetrical stretching coordinates. If the potential function includes a term to describe the interaction between these two coordinates, the results are as good as those obtained in other studies using a six-dimensional potential function. The barrier height for the new potential function is 1857.5 cm-1.
    Additional Material: 10 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560400841
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  • 4
    Electronic Resource
    Electronic Resource
    Study on some metal-metal quadruple bonds (1992)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 343-352 
    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: In the present paper, the role of (n - 1)ƒ orbitals in metal-metal quadruple bonds was studied. It was shown by the calculations that the probabilities for finding the σ-, π-, and δ-electrons between two metal atoms, under the influence of the ƒ orbitals on the metal-metal quadruple bonds, increased while their mean kinetic energy components along the metal bond axis decreased. In addition, the effects of the ƒ orbitals upon the σ, π, and δ metal-metal bonds were different. In general, σ 〈 π 〈 δ.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560430305
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  • 5
    Electronic Resource
    Electronic Resource
    Intermolecular interactions of methyl acetate, β-propiolactone, ethyl acetate, and γ-butyrolactone: An AM1 semiempirical study (1992)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 81-89 
    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: Closely related structures, like esters and lactones, have vastly different physical properties. This is apparently due to differences in the intermolecular interactions. The intermolecular interactions of methyl acetate, β-propiolactone, ethyl acetate, and γ-butyrolactone have been studied using the AM1 semiempirical method. Some of the “arranged clusters” were also compared to possible covalently bound trimers and tetramers of β-propiolactone and γ-butyrolactone. © 1992 John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560440107
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  • 6
    Electronic Resource
    Electronic Resource
    Perturbation theory to determine the stable isomer of triatomic linear molecules (1993)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 249-256 
    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 electronegativity perturbation theory up to the fourth order has been used to study the preferred isomer of linear triatomic molecules. Systems with 10-16 valence-electron systems are discussed and our results are in good agreement with the experiment. The only exception is the isomers of PNO. The usefulness of the perturbation theory is briefly discussed. © 1993 John Wiley & Sons, Inc.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560480406
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  • 7
    Electronic Resource
    Electronic Resource
    Theoretical study on the reaction mechanism of the alcoholysis of isocyanates (1992)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 403-409 
    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: In this paper, the reactions of HNCO or CH3NCO with methanol have been studied using the ab initio MO method. The geometries of the reactants and products have been optimized by the energy gradient method. The calculated results are in accordance with experiment. Using Powell's method, by the minimization of the Euclidean norm σ of the gradient, the structures of the transition state (TS) for the two reactions were obtained. The structures have been further confirmed as TS by finding that there is one and only one negative eigenvalue for their force constant matrix. The imaginary vibration mode corresponding to the TS was also discussed. The calculated activation barriers of these two reactions are 96.02 and 95.13 kJ/mol, respectively. It can be concluded that the alcoholysis reaction of isocyanate is a nucleophilic addition reaction with methanol as an electronic donor and isocyanate as an acceptor and that the hydroxyl hydrogen of CH3OH plays an important role in the reaction.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560420303
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  • 8
    Electronic Resource
    Electronic Resource
    Band structures of al-doped superconductors YBa2Cu3-xALxO7 + δ (1994)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 233-242 
    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: Based on the EHMO approach, an approximate treatment of electronic energy-band structures is suggested. By employing this treatment, computations of the band structures for the Al-doped superconductors YBa2Cu3-xAlxO7 + δ were carried out. It is shown by analysis of the band structures and the density of states that the 2D Cu-O planes in the Y—Ba-Cu—O superconducting system play a dominant role in superconductivity, whereas the 1D Cu—O ribbons have indirectly an influence on superconductivity through the connection of the O(4) atoms to two Cu—O planes. © 1994 John Wiley & Sons, Inc.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560500402
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  • 9
    Electronic Resource
    Electronic Resource
    A theoretical study of the hydrolysis of some sterically hindered phenolic esters (1992)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 77-85 
    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 hydrolysis of sterically hindered phenolic esters is of practical importance. The influence of the steric bulk of 2,6-substituents (R) in various esters and the volume of the acyl group (R1) on these reactions have been studied theoretically. The differences (ΔΔHf) between semiempirical heats of formation (ΔHf) of the products and reactants were calculated for the formation of the tetrahedral intermediates - the rate-determining step of a base-catalyzed process. An analysis of the ΔΔHf values and of the calculated optimized low-energy geometries indicated that the reactivity of esters generally decreased with the increase of the bulk of R, while the size of R1 had less influence. However, electronic (resonance) effects of R1 were noticed in the case of unsubstituted phenolates. On the other hand, calculated global enthalpy changes for the reactions do not correctly reflect the influence of the steric factors over the hydrolysis of the hindered esters. © 1992 John Wiley & Sons, Inc.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560440711
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  • 10
    Electronic Resource
    Electronic Resource
    Hydroxyl stretching in substituted phenols: An AM1 study (1993)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 7-15 
    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: Hydroxyl-stretching frequencies (vOH) were estimated for a number of 4-substituted and 2,6-disubstituted phenols using the AM1 all-valence electron molecular orbital approximation. There was significant correlation between calculated and experimentally observed frequencies with better correlation observed with the gas phase as compared with dilute solution-phase values. Significant Hammett sigma-rho relationships were recorded for the 4-substituted phenols, consistent with experimental observations. The vOH stretching frequency associated with the equilibrium structure of 2,6-di-t-butylphenol was calculated to be 15 cm-1 higher than that of phenol and this increase was attributed to steric interactions that act to effectively shorten the OH bond length. © 1993 John Wiley & Sons, Inc.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560480705
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