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  • 1
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 54 (1995), S. 305-311 
    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 half-projected Hartree-Fock wave function (HPHF) is one of simplest models for introducing some electronic correlation effects. In this model, the wave function is built up with only two Slater determinants. This simple form suggests its application for the direct determination of singlet excited states. On the other hand, because the HPHF model does not mix singlet and triplet states with Ms = 0, it can be used for determining independently singlet and triplet states without any mutual contamination. In the present work, we applied this model to determine nine electronic states of the lithium molecule; one of them exhibits even the same symmetry of the fundamental one. For this purpose, the 6-311G(d) basis was used. Potential energy curves were determined and some spectroscopic constants derived. The numerical results were compared with the available experimental data, as well as with other theoretical values. © 1995 John Wiley & Sons, Inc.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 22 (1982), S. 681-690 
    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 influence of a small deformation of CĈO angle in phenol (tilt), into the rotational far-infrared (FIR) spectrum is analyzed using several approaches. In all of them, the CNDO/2 method is used to determine the potential energy functions. In a first step, the C—O bond and the rotation axis are both supposed to coincide with the C2 symmetry axis of the phenyl group. With this assumption the torsional frequencies are calculated in both the symmetric and asymmetric rotor approximations. In a second step, the tilt of the C—OH bond is determined theoretically and found to be -3°, measured from the C2 symmetry axis, the C—OH bond crossing this axis, Using this second geometry, and taking as the rotation axis the C2 axis, the torsional frequencies are again determined in both approximations. An improvement of the calculated transition energies is encountered at each stage of the calculation, when compared with experimental data. Finally the importance of the introduction of a tilt into the FIR torsional frequency calculations is discussed.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 33 (1988), S. 61-68 
    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 possible reaction path for the polymorphic transformation of trans-trans solid diacetamide into its trans-cis from is envisaged. A description of the two crystalline forms in the same crystallographic system is adopted, and the energy profile of the crystallographic reaction path is calculated by using an empirical 6-exp. atom-atom potential method and a 6-13G ab initio procedure for the inter- and intramolecular contributions, respectively. Two barriers appear along the reaction path. The first corresponds to the destruction of the trans-trans chains and the second to the formation of the trans-cis dimers. These barriers, being lower than the acetyl rotation barier, are in agreement with the ability to isomerize diacetamide in solid state. The presence of a small energy minimum between these barriers would suggest the existence of an intermediate crystallographic form.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 38 (1990), S. 727-740 
    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 potential energy hypersurface of the histamine monocation is determined by ab initio methods at the STO-4G level using analytical gradient techniques. Three transition states and two minima have been found for the Nτ—H tautomer. One of the transition states connects the trans conformational region with a minimum gauche structure, where the proton of the ammonium group is approximately halfway between the Nπ of the imidazole group and the N of the ammonium group, but nearer to the Nπ. This minimum connects the potential energy surface of the Nτ—H tautomer with the imidazolium one. In the latter region, three transition states and two minima have been found. Critical points are discussed in relation with experimental data and histamine H2 receptor models.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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