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  • 1990-1994  (582)
  • 1985-1989  (226)
  • 1992  (582)
  • 1985  (226)
  • Computational Chemistry and Molecular Modeling  (808)
Materialart
Erscheinungszeitraum
  • 1990-1994  (582)
  • 1985-1989  (226)
Jahr
  • 101
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 413-413 
    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: No Absract.
    Materialart: Digitale Medien
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  • 102
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 405-411 
    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 one-range addition theorem [J. Fernández Rico, R. López, and G. Ramírez, Int. J. Quantum Chem. 34, 121 (1988)] for the Yukawa potential is rederived in a very simple manner using weakly convergent expansions [E. J. Weniger, J. Math. Phys. 26, 276 (1985)] of plane waves in the Fourier representation. It is discussed in which sense the addition theorem converges. The relation to Sturmian expansions of Coulomb Green functions is pointed out. © 1992 John Wiley & Sons, Inc.
    Materialart: Digitale Medien
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  • 103
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 421-422 
    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
    Materialart: Digitale Medien
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  • 104
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 423-425 
    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
    Materialart: Digitale Medien
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  • 105
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 469-476 
    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: Semiempirical AM1 SCF—MO models for the cycloaddition between monosubstituted alkenes (RCH = CH2, R = Me, Ph, CF3) and tbutyl cyanoketene predict the lowest-energy transition state to be relatively polar and asynchronous in character and to have a cis relationship between the R and the tbutyl groups, in agreement with the experimentally determined product for R = Ph. We interpret this stereochemistry as resulting from the presence of a significant antarafacial component in the transition state, which enables these two bulky groups to adopt pseudoequatorial positions in which steric interactions are minimized. The alternative higher energy trans geometry forces the R and the cyano groups to adopt more sterically hindered pseudoaxial positions. Alternative biradical-like transition states inferred by others from ab initio MCSCF calculations on the reaction between ketene and ethene do not retain any significant antarafacial component on the ketene and, therefore, form less satisfactory models for this specific reaction. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
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  • 106
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 477-495 
    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 densities of states for small (TiO2)x-clusters, x = 1, 3, 6, 9, and 14, have been calculated by means of the INDO method. The shape of the valence bands' density of states (DOS) are discussed in terms of the distribution of coordination numbers. A one-slab cluster with uniform distribution of the coordination numbers was used to compare our calculations with experimental spectra. The photoelectric DOS and DOS for a cluster with an oxygen vacancy are in very good agreement with experimental findings for the TiO2 (001) surface. O1s core level shifts between a surfacelike and a bulklike oxygen atom have been estimated. It is concluded that the observed surface-bulk shift for the TiO2 (001) surface contains a substantial relaxation contribution. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 10 Ill.
    Materialart: Digitale Medien
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  • 107
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 427-447 
    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: Two kinds of quantum mechanical treatment are needed in chemistry: First, a treatment accurate enough to provide definitive solutions of chemical problems, in particular, the prediction of reactions and reaction mechanisms, without reference to experiment. As yet, treatments of this degree of accuracy are restricted to a few small atoms and molecules. Second, a treatment that chemists can use as a practical aid to their own research, on the same basis as mass spectrometry or NMR. Here, the need has been largely met by the series of progressively better semiempirical methods, in particular, those developed by our group. Since only the best and most expensive ab initio procedures are superior to ours and since the computing time they need is far greater, their use as a practical aid in chemistry is restricted. Our procedures, therefore, serve as a useful supplement to ab initio ones in areas where the latter cannot be applied effectively, as the leading ab initioists now fully recognize. Since the semiempirical approach is capable of almost unlimited further development, this situation is likely to continue in the foreseeable future. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
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  • 108
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 517-531 
    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 parametrization of semiempirical molecular orbital methods is reviewed. The parametrization procedures are classified in three categories. The newly proposed category of consistency of parameters in a row of elements is applied in SINDO1 to the Si atom. The calculations on test molecules and silicon clusters demonstrate that significant improvements in accuracy can be obtained in this way. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 12 Ill.
    Materialart: Digitale Medien
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  • 109
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 497-515 
    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 dipole moments (μ), polarizabilities (α), hyperpolarizabilities (β), and second-order hyperpolarizabilities (γ) of polyenes, polyynes, and polyphenyls have been calculated by a finitefield method with the PM-3 parameterization of the semiempirical MNDO Hamiltonian at the optimum geometries. These results were compared to experimental values obtained from EFISH and THG measurements. The calculations reproduce the magnitudes of β and γ, as well as the effect of the substituents and the effect of bond alternation on β and γ. The coefficients of the power law, which describes the dependence of β and γ on the number of π centers, were calculated. For β, exponents of 1.5-2.2 and 0.03-0.04 were obtained for polyenes and polyynes, respectively, and for γ, exponents of 3.9-4.9, 2.9-3.3, and 2.5-2.7 were obtained for polyenes, polyynes, and polyphenyls, respectively. These results confirm the efficiency of enhancing γ by insertion of C = C double bonds into a chain. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
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  • 110
    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: Contact ion pairs, essential intermediates of both chemical and biochemical reactions in solution, can be satisfactorily approximated by MNDO calculations as exemplified by the lithium and potassium salts of 2,5-trimethylsilyl-p-benzosemiquinone radical anion and by sodium tetraphenylallyl anion diethyl ether. Topics discussed are the structural changes of p-benzoquinone derivatives on single-electron reduction, the dynamics of the countercations in semiquinone radical contact ion pairs measured by temperature-dependent ESR spectra, as well as structural details of the sodium tetraphenyl-allylanion complex isolated such as its intramolecular Na dibenzene sandwich or its diethylether solvation. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 12 Ill.
    Materialart: Digitale Medien
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  • 111
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 543-548 
    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 explain observed electron transfer rates between the terminal aromatic moieties in the arylsubstituted alanine dimer 1, a conformational analysis of the dipeptide was performed using a standard molecular mechanics force field. The resulting low-energy conformers could conveniently be grouped into two families with an average energy difference of ca. 2 kcal mol-1 and populations of 64% and 30%, respectively. These results correlate well with the 78:22 ratio of the two experimentally distinguishable decay processes for the radical anion of 1. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
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  • 112
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 549-563 
    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 AM1 semiempirical SCF MOmethod developed by Dewar and co-workers has been employed to compute possible interaction processes of the odorant molecules cocaine, methyl benzoate, and benzaldehyde with the biological substrate lysine. The only process predicted as possible in each case was nucleophilic attack on the carbonyl carbons of each odorant by the nitrogen lone pairs on lysine. The remote amino group of lysine in each case was predicted to react with lower activation enthalpy than did the α-amino group, a prediction consistent with preliminary experimental data from these laboratories. Although the computed activation enthalpies were high (ca. 40 kcal mol-1) for the gas-phase processes, it is conceivable that such processes could occur by alternate pathways with considerably lower ctivation energies in solvated media such as exists in the presence of mucus-containing olfactory binding protein in vertebrate olfactory reception. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 10 Ill.
    Materialart: Digitale Medien
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  • 113
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 869-877 
    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: Structural properties of cyclopropabenzene and its heteroanalogs are studied at the SCF 6-31G level of theory. It is found that the Mills-Nixon (MN) type of localization within the benzene ring is present in these systems. The origin of the bond fixation lies in rehybridization at the carbon junction atoms. π-electron bond orders usually follow changes within the sigma framework but can be misleading sometimes. It is shown that a judicious choice of heteroatoms can considerably enhance the MN effect. A refined and better definition of the MN effect is offered. Finally, present calculations and analysis of the results in terms of hybridization model and π-bond orders strongly indicate that experimental 4J(H—C—C—Me) proton-proton spin-spin couplings over four bonds can be used in identification of the MN effect only with extreme caution.
    Zusätzliches Material: 4 Tab.
    Materialart: Digitale Medien
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  • 114
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 879-887 
    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: Photochromic spironaphthoxazines and their open merocyanine forms were studied via semiempirical Hartree-Fock SCF quantum chemical calculations. The most stable of the merocyanine isomers were found to have a cis-trans-cis conformation and the zwitterionic form is, generally, more stable than is the quinoidal tautomer. The calculated activation energy for the thermal bleaching reaction is in good agreement with the experimental one.
    Zusätzliches Material: 4 Tab.
    Materialart: Digitale Medien
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  • 115
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 907-916 
    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: A recent theory of nonadiabatic effects in triatomic molecules is specialized to the four-state Renner-Teller and Jahn-Teller 1Πu/1Φg interactions and is then generalized by including the electronic spin and by considering the 2Λ Renner-Teller effect.
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
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  • 116
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 889-905 
    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: A heuristic fitting procedure to obtain an analytical potential function for describing a reactive potential energy surface in the neighborhood of the intrinsic reaction coordinate (IRC) has been developed. For discussion, the pairwise potential function form, ∑anr-n, is assumed in order to fit ab initio quantum mechanical calculations of intramolecular (or intrasupermolecular) interaction energies and its use is found advantageous because all the calculation can be carried out by the linear least squares method. Normal modes perpendicular to IRC are utilized to prepare an initial data base for the potential fitting in the neighborhood of IRC. Some trial molecular dynamics (MD) simulations are performed in order to check the fitted potential function and, unless they lead to reasonable energies within the tolerance assumed, their results are utilized to construct an improved data base (the dynamic sampling). The present systematic optimization procedure has been applied to the proton transfer reaction of the formamidine-water (FW) system. The normal mode analysis in both the transition state (TS) and the stable state (SS) regions suggests that the present fitted potential function can reproduce satisfactorily the Born-Oppenheimer (BO) adiabatic surface obtained by ab initio molecular orbital (MO) calculations. We conclude that our procedure works well for the chemical reaction molecular dynamics (CRMD) simulation.
    Zusätzliches Material: 4 Ill.
    Materialart: Digitale Medien
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  • 117
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 929-939 
    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 structure of silacarbonyl ylide in the singlet gound state is obtained by ab initio quantum chemical calculations. This proves the recent experimental observation of silacarbonyl ylide by the photolysis of oxasilitane [W. Ando, K. Hagiwara, and A. Sekiguchi, Organometallics 6, 2270 (1987)]. The dynamic stability of the silacarbonyl ylide is studied by the TCSCF/3-21G(*) method. The second-order Jahn-Teller effect is examined in connection with the active role of frontier orbitals. Configuration analysis is used to clarify characteristic electronic structure of the silylene-carbonyl interaction.
    Zusätzliches Material: 6 Ill.
    Materialart: Digitale Medien
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  • 118
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 917-928 
    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 recently proposed ab initio method for calculation on many-electron molecular systems with the approximation of the inactive part of a molecule by a frozen molecular fragment was tested further in a case of the dissociation reaction of the C—F bond in n-fluoropropane. Results from the Hartree-Fock, multiple reference double-excitation configuration-interaction and second-order Møller-Plesset methods are presented. The reproduction of potential energy surfaces as well as the reproduction of electron density distribution are in excellent agreement with extended basis-set calculations. Different choices of fragments to be frozen have been examined.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
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  • 119
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 941-952 
    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: A previously proposed [Sutcliffe and Tennyson, Int. J. Quantum Chem. 29, 183 (1991)] body-fixed Hamiltonian is applied to AB2 systems in Radau coordinates with the x-axis embedded along the bisector of the angle. It is shown that by using a discrete variable representation for the angular coordinate it is possible to avoid singular regions of the Hamiltonian. A two-step variational procedure is used to obtain rotationally excited states of the system. The results of test calculations H2S and D2S with J = 0, 1, 5, and 10 are discussed along with computer-usage characteristics.
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
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  • 120
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 691-698 
    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: Both ab initio and semiempirical electronic structure calculations are used to investigate the molecular and electronic structures and eneregetic stabilities of an unusual bridged compound with the general formula [Y—SiH3—X—SiH3—Y]-, with Y = H or F and X = H, CH3, NH2, OH, F, or Cl. Most of these bridged anions are quite stable relative to YSiH3 + XSiH3Y-, and the stability is predicted to increase considerably when Y = H is replaced with Y = F.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
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  • 121
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 679-690 
    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: Stretching force constants for formamide and its seven associated species involving two to four molecules hydrogen-bonded through linear and cyclic configurations and 10 structures containing formamide hydrogen-bonded with one to five water molecules are reported. Since ab initio calculations are rather inconvenient to perform on such big clusters and are time-consuming, CNINDO MO calculations were carried out using the gradient method. The results demonstrate, on the one hand, the feasibility of semiempirical calculations for the evaluation of trends in force constants for big clusters where generally ab initio calculations become much involved and, on the other hand, explain the effect of hydrogen bonding and cooperativity on force constants and vibrational spectra of biologically important systems composed of formamide in the condensed phase and its aqueous solutions. The C=O and N—H stretching force constants are found to reduce significantly on hydrogen bonding. The reduction in force constant is further enhanced when two cyclic dimers become associated through a linear hydrogen bond. The results indicate justification for the stabilization of the formamide structure with two cyclic dimers hydrogen-bonded together. The reduction in the force constants on hydrogen bonding also reflect the cooperativity contribution. The C=O and C—N stretching force constants for the structures corresponding to formamide in liquid and aqueous solution phases are in agreement with the experimental vibrational frequencies reported.
    Zusätzliches Material: 2 Ill.
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  • 122
    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: Ab initio molecular quantum mechanics has been applied to a number of coupled ring systems including biterahedryl and cubylcubane. Basis sets at least as large as double zeta augmented by carbon d functions (DZ+d) were used throughout. For biterahedryl, electron correlation effects qualitatively alter the molecular structure, decreasing the central C—C bond distance while increasing the adjacent C—C distance. Carbonyl substituents, contrary to some previous thinking, do not qualitatively alter the bitetrahedryl structure. This finding points to a structural problem with the monomer bicyclo[3.1.0.02,6]hexane that carries over to the coupled bicyclo[3.1.0.02,6]hexyl molecule. Finally, for the cubylcubane molecule synthesized in 1988, the only significant difference between Hartree-Fock theory and experiment occurs for the central C—C distance, which, as in the other coupled molecules, is too long. These results significantly lengthen the (still very short) list of closed-shell hydrocarbon molecules for which Hartree-Fock theory encounters structural difficulties.
    Zusätzliches Material: 7 Ill.
    Materialart: Digitale Medien
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  • 123
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 997-1008 
    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: Ab initio band structure results of polypyrrole (PPY) and polythiophene (PTP) obtained with a double-zeta basis set are reported. The electronic density of states (DOS) of the various quasione-dimensional copolymers (superlattices) of the type (AmBn)x (A = pyrrole; B = thiophene) have been calculated numerically within the ab initio SCF tight-binding approximation. These copolymers on the basis of the band positions of PPY and PTP are found to belong to the class of type II (staggered) superlattices. The trends in their electronic properties as a function of (i) composition (m/n), (ii) block sizes m and n, and (iii) arrangement of blocks in the copolymer chain are discussed.
    Zusätzliches Material: 8 Ill.
    Materialart: Digitale Medien
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  • 124
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 1009-1024 
    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: An ab initio method for calculating the longitudinal linear polarizability of polymeric chains is described. This method is equivalent to an uncoupled Hartree-Fock scheme. It is applied to polyethylene and polysilane in minimal STO-3G and extended 4-31G basis sets. The study describes important techniques for solving the difficulties met in actual calculations: band reordering of the band structures, calculation of analytical derivatives of the energy bands ∊n(k) and LCAO coefficients cnp(k), and errors caused by the improper lattice sum truncations of the Hartree-Fock matrix.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
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  • 125
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 1025-1035 
    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: A great deal of effort in recent years has been put into the development of a new band theory technique based on multiple-scattering theory. Some of the highlights of this development are described.
    Materialart: Digitale Medien
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  • 126
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 1037-1045 
    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: Some time ago Min et al. [Phys. Rev. B 30, 5076 (1984)] found the near-equilibrium sequence of phases of paramagnetic crystalline H to be (in order of increasing energy) simple cubic (SC), hexagonal close packed (HCP), body-centered cubic (BCC), and face-centered cubic (FCC), with FCC and HCP substantially separated (about 60 mRy/atom). This result is counterintuitive on three counts when compared with crystalline Li. That comparison suggests that HCP should lie only slightly below FCC and both should be substantially below BCC. (Simple cubic should lie lowest in H but not in Li because of the difference in orbital occupancy.) We have redone the calculations with our published full-potential linearized augmented plane wave code WIEN. We find the sequence of phases with increasing energy to be SC, HCP, FCC, and BCC, the ordering expected from solid Li. By direct exploration of dense k-space meshes, we show that the difference in predictions results from a k-space mesh extrapolation technique used by Min et al.
    Zusätzliches Material: 1 Ill.
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  • 127
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 1047-1071 
    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: A survey of the development of the application of quantum chemical techniques to Ziegler-Natta polymerization starting from the pioneering work of Clementi and co-workers is presented. In the latter, the role of the catalyst was elucidated, a feasible reaction pathway was proposed, and several properties of the catalyzed reaction (heats of formation, activation barriers, physisorption and chemisorption energies, etc.) were obtained without semiempirical approximations. Furthermore, a few proposals for enhancing catalytic activity for the oligomerization process proved to be of great practical value and led to several patents. This turns out to be a good example of the usefulness of the computational techniques to provide intense interactions between theoretical and applied chemistry.
    Zusätzliches Material: 10 Ill.
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  • 128
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992) 
    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
    Materialart: Digitale Medien
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  • 129
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 1073-1089 
    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: An efficient ab initio method for electronic structure calculations on extended molecular systems is presented, along with some illustrative applications. A division of the system into subunits allows the interactions to be separated into short- and long-range contributions, leading to a reduction of the computational effort from the original fourth-power size-dependence to one that is approximately quadratic. The short-range contributions to the Fock matrix are obtained in an essentially conventional fashion, while the long-range interactions are evaluated using a two-center multipole expansion formalism. The number of short-range contributions grows only linearly with the number of subunits, while the long-range contributions grow as N2. Systematic studies of the computational efforts for systems of up to 99 water molecules organized as one-stranded chains, three-stranded chains, and three-dimensional clusters, as well as alkane chains with up to 69 carbon atoms, have been performed. In these model systems, the overall computational effort grows as NK where 1 〈 K 〈 2.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
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  • 130
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 1091-1099 
    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: Ab initio Hartree-Fock calculations are reported for the chemisorption of K on Ag using three different types of clusters to model the system. Geometry optimization is done in 4 degrees of freedom. It is found that since there is an absence of complete charge transfer between the adsorbate and substrate, the interaction can be interpreted as being predominately covalent in nature.
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
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  • 131
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 1115-1139 
    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 interaction of CO with the MgO(100) surface has been investigated by means of all electron cluster model calculations. The CO molecule is bound on the Mg2+ site of MgO with a chemisorption energy of about 0.2 eV. The binding mechanism is electrostatic in nature and arises almost entirely from the interaction of the weak electric field generated by the ionic surface and the CO charge distribution, with negligible contributions from chemical effects as the CO σ donation. When CO is bound through carbon, its vibrational frequency increases with respect to the gas-phase value. This shift, Δ, has been analyzed and decomposed into the sum of different contributions. It is found that the positive Δω does not arise entirely from the field-dipole interaction but is due, in part, to the increase in Pauli repulsion occurring when the C—O molecule vibrates in the presence of the surface “wall.” A stronger electrostatic interaction, bringing the CO adsorbate closer to the surface, increases this wall effect and results in a more pronounced positive ω shift. It is also found that the two CO orientations exhibit opposite shifts in ωe, thus, the two orientations can be distinguished, in principle, by IR spectroscopy. The analysis of our ab initio cluster wave functions gives a very different picture than the standard view of the metal-CO bond as arising from σ donation and π back donation.
    Zusätzliches Material: 5 Ill.
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  • 132
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 1101-1114 
    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: Reactions of a hydrogen molecule with a ZnO surface are studied by an ab initio method. For simulating the ZnO (10 1 0) surface, one ZnO molecule both with and without a Madelung potential is used. Since the electrostatic potential due to the ionic layer decreases exponentially, the effect of the layers deeper than the second one can be neglected. The Madelung potential is, therefore, expressed by the 32 point charges of ±0.5 situated on the first and second layers. Several low-lying states of ZnO and the ZnO + H2 system have been calculated by the symmetry-adapted cluster (SAC) and SAC-CI methods. It is found that the 1Σ+ state of ZnO is the ground state and catalytic active and the other states are inactive. ZnO (1Σ+) reacts with H2 and dissociatively adsorbs it with making Zn—H and O—H bonds. This occurs both with and without the Madelung potential. Without the Madelung potential, the heat of reaction is 81.3 kcal/mol and the reaction barrier is 14.0 kcal/mol. With the Madelung potential, the heat of reaction decreases to 73.5 kcal/mol and the barrier decreases to 11.5 kcal/mol. The mechanism of this reaction is the electron donation from the 2pπ orbital of O to the antibonding σu MO of H2 and the back-donation from the bonding σg MO of H2 to the LUMO of ZnO. In the intermediate stage of the reaction, the dipole of ZnO works to increase the overlap of the active MOS to make the reaction easier. Throughout the reaction, the in-plane 2pπ orbital of O and the HOMO of ZnO are inactive and work to keep the ZnO bond stable during the catalytic process.
    Zusätzliches Material: 8 Ill.
    Materialart: Digitale Medien
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  • 133
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 831-837 
    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 all-valence INDO method has been modified for the inclusion of spin-orbit coupling effects. In the method presented, the Hamiltonian includes spin-orbit coupling and the basis set constitutes the singlet and triplet determinental wave functions constructed from molecular orbitals resulting from nonrelativistic calculations. Eigenvectors obtained are later used for the evaluation of transition probabilities among different states. The results presented include lifetimes of different states of organic molecules and transition energies for halogen molecules and they are in a good agreement with experimental results. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 6 Tab.
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  • 134
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 853-867 
    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: INDO ground-state calculations have been performed to elucidate the electronic structure of the two para-dimetallated σ-phenylene complexes [(Cp2*M)2C6H4], M = Sc, Lu. The lutetium complex exhibits an extraordinarily small M—Cαaryl—Cβaryl angle of about 88°. This unusual geometry is attributed to an agostic interaction between the metal center and the (C—H)β group of the σ-bonded aryl ring. For the analogous scandium compound, the corresponding angle is 118°, rather close to the “normal” value of 120°. Both experimental findings are very well reproduced in the computational models. An energy-partitioning analysis that exploits the features of the INDO formalism is applied in order to rationalize this geometry difference. The bending of the aryl ligand out of the plane defined by the metal coordination is favored by covalent ligand-metal interactions, but is counteracted by electrostatic repulsion effects. For the lutetium complex, the covalent interactions dominate due to the extended nature of the Lu 5d orbitals and an agostic distortion of the geometry results.
    Zusätzliches Material: 5 Ill.
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  • 135
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 887-895 
    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: Valence-bond calculations have been carried out on ZnO and HgO using a basis set of Slatertype atomic orbitals and the one- and two-electron integrals as computed in the semiempirical AM1 molecular orbital method. The zero differential overlap approximation has been used to calculate integrals between atomic orbital Slater determinants using the rules for matrix elements between determinants formed by orthogonal orbitals. Diabatic and adiabatic curves have been analyzed for the two systems, and results compared with molecular orbital AM1 results. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 136
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 869-885 
    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: AM1 calculations for various intermolecular and internally hydrogen-bonding species are compared with experiment. The calculations generally provide a useful, but not perfect, means for modeling H-bonds. The average stabilization energy for 33 gas-phase hydrations of ions was 1.4, whereas that for amination of 10 ions was 4.2 kcal/mol, too weak when compared to experimental results. H-bonding distances have a tendency to be long. The tendency of AM1 to predict multicentered H-bonds is discussed in the context of recent surveys of crystallographic data banks. Caution is recommended in comparing AM1 results with inappropriate experiments and inadequate ab initio calculations. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Tab.
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  • 137
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 897-930 
    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: A computational approach, which involves the combination of the OPLS force field and molecular orbital MNDO, AM1, and PM3 methods, has been developed to describe the effects of a large, molecular mechanically simulated environment on the Hamiltonian of a quantum chemical system. To test the validity of the combined quantum mechanical/molecular mechanical (QM/MM) potential, a systematic study of the structures and energies of neutral and charged hydrogen-bonded complexes has been carried out, including comparisons with pure semiempirical calculations and available experimental and ab initio data. It is shown that, in many cases, the hybrid QM/MM potential behaves better than do related MNDO/M, AM1, and PM3 methods. As a case in point, the draw-back of AM1 favoring bifurcated H-bonded structures over single ones is not presented in the combined AM1/OPLS scheme. Possible ways of improvement of the combined QM/MM potential are discussed. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 14 Tab.
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  • 138
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992) 
    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
    Materialart: Digitale Medien
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  • 139
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 935-947 
    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 calculations of the magnetic susceptibility (χ) of the selected derivatives of cellulose have been performed using the localized molecular orbitals with the basis set of Gaussian functions. The advantages of the variational procedure in the framework of the approximation of the additivity of the localized molecular fragments for the evaluation of the magnetic susceptibility and nuclear magnetic shielding (σ) and the comparison with the NMR of high-resolution data have been discussed. Theoretical values for χ of the 50 sizable organic molecules have been presented. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 4 Ill.
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  • 140
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 949-983 
    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: Instabilities of the restricted Hartree-Fock (HF) state of the two-dimensional extended Hubbard model under doping are studied with the linear response theory employing the linearized Bethe-Salpeter equation. Boundaries of the instabilities for the ordering vectors q = Q ≡ (π, π) and q = Q0 ≡ (0, 0) are given in terms of the interaction strength parameters at various doping levels. With doping, dominant instabilities generally change from ones for q = Q to ones for q = Q0. Broken symmetry phases obtained from the instabilities are also studied and classified using the group-theoretical technique. For q = Q, two more new phases are obtained in addition to the four nonmagnetic and five magnetic phases that have been already derived by one of the authors. For q = Q0, three nonmagnetic and five magnetic phases are obtained. Deriving the explicit forms of the charge current and spin current operators in the HF approximation, charge current and spin current densities in the broken symmetry states are discussed in detail. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 9 Ill.
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  • 141
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 1041-1044 
    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 recently proposed Harbola-Sahni local potential in its exchange-only version [Phys. Rev. Lett. 62, 689 (1989)] is found to give the static dipole and quadrupole polarizabilities for the Neisoelectronic series in excellent agreement with the corresponding Hartree-Fock estimates. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 1 Tab.
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  • 142
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 1069-1069 
    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
    Materialart: Digitale Medien
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  • 143
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 1045-1056 
    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: A method is described for obtaining a modified virtual orbital space that improves the rate of convergence in configuration interaction studies of electron correlation for both ground-state and excited-state properties. The validity of the method has been compared with other procedures by performing a series of test calculations of the electric dipole properties of the CO molecule in the X1Σ+ ground state and the first excited singlet states A1∏, I1Σ-, and D1Δ using different optimization schemes. The low-lying Rydberg and valence excited states of the acetone molecule have been investigated using different N - 1-electron potentials in the construction of virtual orbitals. The examples indicate that the presented average virtual orbitals (AVOs) constitute a compromise set of orbitals suited for simultaneously improving the description of several electronic states. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 7 Tab.
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  • 144
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 1071-1073 
    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
    Materialart: Digitale Medien
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  • 145
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 1057-1067 
    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: State-of-the-art experimental electron diffraction and computational information on the structure of alkaline earth dihalide molecules are in agreement for the shape of these symmetric triatomic molecules (linear/bent/quasi-linear). However, the computed and measured bond lengths show differences that are not only considerably larger than the experimental error but also have the wrong sign. The physical meaning of experimental bond lengths depends on the physical techniques used in their determination and the ways of averaging over molecular vibrations. The choice of model potentials in the elucidation of experimental information is also important, especially for floppy molecules. When improved computational bond lengths become available, their comparison with experimental information will have to take account of the physical meaning of the experimentally determined bond lengths. The computed equilibrium distance (re) should be smaller than the experimental distance-average bond length (rg). The differences may range from a few thousandths of an Å to a few hundredths with increasing temperature and, especially, with increasing floppiness of the molecule. For truly accurate comparison, experimental bond lengths should be compared with computed ones only following necessary corrections, bringing all information involved in the comparison to a common denominator. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 1 Ill.
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  • 146
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 1449-1468 
    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 original polarizable continuum model (PCM) of solvation based on the boundary element method for the numerical solution of the Poisson equation for a realistic molecular shape is modified through a classical description of solute charge distribution and solute polarization. In this approach, the solute charge distribution is represented by point charges and polarizabilities and the solute-solvent polarization is evaluated. The solvent-solute back polarization is also considered. The effect of electrolyte ionic strength is included via a dielectric function based on the Debye-Hückel model. A more realistic description of molecular cavity boundaries is introduced with the help of variable van der Waals radii. Approaches of evaluation of dispersion-repulsion and cavitation contributions to solvation Gibbs free energy suitable for large biopolymers are also presented. Test calculations illustrating the performance of the model are given together with examples of its application for larger biopolymer structures.
    Zusätzliches Material: 6 Ill.
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  • 147
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 1469-1477 
    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: Theoretical models of divalent cobalt bound to the active site of the enzyme, carbonic anhydrase, are constructed to study the effect of the first-shell geometry and coordination number on the experimentally observed visible spectra. Favorable comparisons of the ab initio calculations of the hexa-aquo complex of Co(II) with experimental results provides a test of the method. The d-d spectra variation with pH in the enzyme is found to be determined by the first-shell complex geometry as obtained from energy optimization. The low pH complex is predicted to be predominantly five-coordinate with two waters in the first-shell, whereas the high pH complex is predominantly four-coordinate. Equilibria between the four- and five-coordinate structures at low pH is not indicated by the calculation.
    Zusätzliches Material: 5 Tab.
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  • 148
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 1491-1498 
    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 use of 57Fe Mössbauer radiation allows the study of protein crystal dynamics by a time-resolved analysis of X-ray scattering. In myoglobin cystals, the main source of the root mean squared amplitude of motions comes from intramolecular protein dynamics. Segments of the size of an α-helix move collectively. Long-range correlated motions give only a minor contribution. Comparison with Mössbauer absorption spectroscopy shows that protein-specific dynamics is frozen out below 200 K and the lattice dynamics is mainly responsible for the low-temperature behavior.
    Zusätzliches Material: 4 Ill.
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  • 149
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 1479-1489 
    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: Time-resolved fluorescence of single tryptophan proteins have demonstrated the complexity of protein dynamic and protein structure. In particular, for some single tryptophan proteins, their fluorescence decay is best described by a distribution of fluorescence lifetimes rather than one or two lifetimes. Such results have provided further confirmation that the protein system is one which fluctuates between a hierarchy of many conformational substates. With this scenario as a theoretical framework, the correlations between protein dynamic and structure are investigated by studying the time-resolved fluorescence and anisotropy decay of holo and apo human superoxide dismutase (HSOD) at different denaturant concentrations. As a function of guanidine hydrochloride (GdHCl), the width of the fluorescence lifetime distribution of HSOD displays a maximum which is not coincident with the fully denatured form of HSOD at 6.5M GdHCl. Furthermore, the width of the fluorescence lifetime distribution for the fully denatured forms of holo and apo HSOD is greater than that of the native forms.
    Zusätzliches Material: 3 Ill.
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  • 150
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 1499-1514 
    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 results of the Monte Carlo Metropolis simulation of water structure and of the hydration of nucleic acid fragments, complementary base pairs and mispairs, base pair stacks, and duplex fragments have been summarized. Systematic investigations suggest some general conclusions: (1) the hydration shell structure of the major and minor grooves of the duplex depends significantly on DNA conformation (or stack configuration) and nucleotide sequence, while global hydration characteristics (average energy, the number of water-water and water-base H-bonds) are only slightly dependent on these factors, (2) hydration economy takes place in the B-A transition due to an increase of the number of water molecules forming hydrogen bonds with two or more atoms of bases (water bridging), and (3) the hydration of the duplex could contribute to nucleic acid functioning via water-bridged mispair formation and stabilization of specific conformations.
    Zusätzliches Material: 3 Ill.
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  • 151
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 1529-1536 
    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: Various methods are used to surmount the multiple-minima problem that is encountered in the multidimensional conformational energy surface of a polypeptide. A summary is given here of two of these methods: (i) The build-up procedure that is modified to include statistical data on the positional frequencies of occurrence of amino acid residues along the chain, and (ii) the diffusion equation method that smoothes out the potential surface, leaving only the potential well containing the global minimum.
    Zusätzliches Material: 4 Ill.
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  • 152
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 1515-1528 
    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: In this study is presented a general algorithm for computing Voronoi volumes of atoms of group of atoms in condensed phases. The method is essentially an extension of the Medvedev procedure to allow vertice determination for any Voronoi polyhedron, primitive or with degenerate vertices. The algorithm has been employed for computing time-averaged volumes in the hydrated crystal of met-myoglobin, using the data of a molecular dynamics simulation. The results, compared to previous volume determination in myoglobin, emphasize the fundamental role of solvent structure close to the protein surface in relation to the packing density properties of the residues. Relative volumes fluctuations of myoglobin residues have been found to be correlated to the corresponding mean square displacements from X-ray diffraction studies and to the theoretical hydration energies.
    Zusätzliches Material: 7 Ill.
    Materialart: Digitale Medien
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  • 153
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 87-107 
    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: We have investigated four computational models of the resting state and HOOH-bound complex of a heme protein, cytochrome c peroxidase (CCP), to determine the most efficient methodology that yields reliable results. Previous studies have focused on the overall dynamic structure of proteins and not explicitly on the motion of small molecules and their effect on nearby residues. In this study, the effects of dielectric, boundary constraints, explicit inclusion of solvent, and charges on the heme unit have specifically been examined. For the case studied here, in which interactions with water are very important, it was found that, contrary to commonly accepted procedures, a distance-dependent dielectric (ε = 4R) with a constrained protein and no solvent resulted in unacceptably large deviations from more accurate models. In this case, even a buried Trp residue hydrogen bonded to the water network showed large deviations due to the exaggerated motions of the waters. However, use of a constant dielectric (ε = 1) with the same constraints resulted in a computationally efficient model that yielded properties very similar to the more accurate models. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 7 Ill.
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  • 154
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 109-124 
    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: Potential energy surfaces are generated by ab initio calculations for the transfer of two protons in the pentameric chains (H2O ·· H2O ·· H2OH+ ·· OH2 ·· OH2) and (H3N ·· H3N ·· H3NH+ NH3 ·· NH3). These surfaces are then fit by the sum of (a) two functions, each of which reproduces the transfer potential of a single proton plus (b) a function that models the coupling between the two protons. Several candidates are tested for the latter coupling function. A simple linear dependence upon the motion of the two protons reproduces the full two-dimensional surface to an accuracy of some 1 kcal/mol. The popular quadratic function provides a poorer approximation as also do exponential and Coulombic-type functions. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 8 Ill.
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  • 155
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 125-143 
    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 principle of macromolecular self-assembly is used to construct a model of computing that exploits quantum effects to achieve enhanced real-time capabilities. Signals impinging on a device (or biological cell) trigger the appearance of macromolecules that self-assemble into a mosaic. Adaptor enzymes recognize features of the mosaic and link these to the output of the device. In this way, a symbolic pattern recognition problem is converted to a free-energy minimization process. A Hartree-type self-consistent-field formalism is developed for treating the self-assembly process. The formalism demonstrates that the parallelism inherent in the quantum mechanical wave function (the superposition of electronic states) can speed up the exploration of the potential surface, thereby increasing computational search power over what can be achieved with conventional models of computation. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 156
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. ix 
    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
    Materialart: Digitale Medien
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  • 157
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 255-272 
    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: Energy distributions of six alanine-based peptides were generated by combining backbone and sidechain rotamers with a matrix algorithm and by classifying the obtained chain conformations in order of increasing energy. The interaction range was varied from 3 to 6 to allow a maximum of two contiguous i,i+4 hydrogen bonds. An examination of these results has shown an agreement between the energy distributions of the interaction intervals of the lysine residues located in positions 7, 12, and 17 and the basic hypothesis of Zimm-Bragg Theory. It was found, however, that the alanine residues have energy distributions in which the α-helix is the preferred conformation even when a single i,i+4 hydrogen bond is formed so that α-helix nucleation beginning with these residues is noncooperative. As a consequence, instead of being an all or nothing process, the helix-coil transition of short peptide chains should have the participation of small stretches of α-helix and the nucleation (σ) and stabilization (s) parameters of Zimm-Bragg theory should have values larger than those determined by host-guest experiments. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 6 Ill.
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  • 158
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. xi 
    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
    Materialart: Digitale Medien
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  • 159
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. xxvii 
    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
    Materialart: Digitale Medien
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  • 160
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 1-11 
    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 the second-order self-energy of electron propagator theory, one can obtain total energies for the initial, N-electron state and the final, N ± l-electron states. Recent derivations and computational studies have demonstrated the feasibility of calculating effective first-order density matrices corresponding to the electron-binding energies. One-electron properties and energy gradients of the final states are thereby made accessible. Applications to. the ground and excited states of CaCN and to C3+ illustrate the capabilities of this method. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Tab.
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  • 161
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 55-83 
    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: Diagrammatic and Coupled Cluster techniques are used to develop an approach to the single-particle Green's function G which concentrates on G directly rather than first approximating the irreducible self-energy and then solving Dyson's equation. As a consequence the ionization and attachment parts of the Green's function satisfy completely decoupled sets of equations. The proposed Coupled Cluster Green's Function method (CCGF) is intimately connected to both Coupled Cluster Linear Response Theory (CCLRT) and the Normal Coupled Cluster Method (NCCM). These relations are discussed in detail. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 14 Ill.
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  • 162
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 31-53 
    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 second-order self-energy is expressed in terms of the HF zeroth-order polarization propagator. The polarizability dependence of the many-body features in the ionization spectra is assessed by means of second-order Green's function simulation of the XPS valence spectra of the 1,3-hexadiene, 1,4-hexadiene, 1,5-hexadiene, 1,3-cyclohexadiene, 1,4-cyclohexadiene molecules, the open forms being considered in several conformations. Variations in the one-particle and many-body features determining the shape of the spectra are also interpreted in terms of the molecular primary and secondary structures. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 7 Ill.
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  • 163
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 13-30 
    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: A new method is presented for the calculation and analysis of static linear and nonlinear response properties. The method involves the use of a perturbed electron propagator formalism, and is a correlated generalization of standard SCF-level coupled Hartree-Fock (or RPA) schemes. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 4 Tab.
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  • 164
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 27-36 
    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: A simplified three-level model for γ, the molecular third-order nonlinear optical susceptibility, is presented and discussed. The perturbation theory-based approach suggests that there are three primary avenues to optimizing molecular four-wave mixing susceptibilities and that with each of these is associated a particular class of molecular electronic structures. The three electronic structure classes consist of (1) conjugated donor-acceptor dipolar molecules with a large secondorder susceptibility, β; (2) even-member conjugated chains such as -enes, -ynes, and ene-ynes with large two-photon dominated susceptibilities; and (3) “charged” odd-member conjugated chaines with large linear absorption dominated third-order susceptibilities such as squaryliums (perhaps, more generally, the polymethine dyes). Classes (1) and (2) have been known and investigated in the past, while recent results of ours suggest the existence of the third and perhaps final class.
    Zusätzliches Material: 2 Tab.
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  • 165
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 91-118 
    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 theory for the calculation of the frequency-dependent hyperpolarizabilities β(-2ω; 0, ω), β(-ω; 0, ω), and β(0; ω, -ω) is discussed. New relations between these tensors are derived for those wave functions that obey the time-dependent Hellmann-Feynman theorem (e.g., the self-consistent field [SCF] or the exact wave function). Using second-order Møller-Plesset perturbation theory (MP2), expressions are obtained for the hyperpolarizabilities in terms of derivatives of appropriately defined linear polarizability tensors with respect to a static electric field. Results are presented for ammonia and formaldehyde for the optical Kerr effect and for secondharmonic generation. These results indicate that it is desirable to determine the frequency-dependent contribution to the hyperopolarizability at the MP2 rather than the SCF level of theory, in cases where the static hyperpolarizability has a large contribution from electron correlation and/or where the frequency-dependent contribution may be more significant, such as for secondharmonic generation.
    Zusätzliches Material: 2 Ill.
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  • 166
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 147-158 
    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: A review of our own work on ab initio calculation of nonlinear optical properties by the finite oligomer method is presented. Various computational considerations such as geometry determination and choice of basis set are discussed as well as extrapolation to the long-chain limit. For cases where the extrapolation is problematic, two potential solutions are suggested. We also analyze several important areas ripe for investigation including the frequency-dependence, the effect of vibrational distortion, and the role of electron correlation.
    Zusätzliches Material: 3 Ill.
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  • 167
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 119-134 
    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: A new formalism and algorithm is developed for solving the general-order time-dependent Hartree-Fock (TDHF) problem. It is shown that for any order a generalization of the TDHF equations can be derived where all lower-order solutions constitute a constant term. This makes it very easy to obtain high-order solutions. As the space required for the mapping of density matrices to Fock matrices in a problem of a giver order is largely reduced, we can perform the most time-consuming steps within the core memory of the machine and easily manipulate vector products via optimum routines. The second hyperpolarizability γ is obtained from the secondorder TDHF solution via a 2n rule. The formalism also allows for expressing all terms in the equation diagrammatically, which provides additional physical insight and a more systematic evaluation of terms. To illustrate the method, TDHF results are presented for trans-butadiene and carbon monoxide for several optical processes, including correlation corrections to their static hyperpolarizabilities obtained via coupled cluster (CCSD) and many-body perturbation theory. The hybrid TDHF/CCSD method provides excellent agreement with the DC-SHG experiments (χ||(2) = 11.4 x 10-32 esu/mol compared to 12.9 ± 11.4 × 10-32 esu/mol and χ||(3) = 149 compared to 144 ± 4 × 10-39 esu/mol).
    Zusätzliches Material: 6 Tab.
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  • 168
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 135-146 
    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: Ab initio and model calculations on linear clusters of acetylene molecules have been carried out to identify the effect of weak intermolecular interaction on the intrinsic dipole polarizability (-) and the second hyperpolarizability (γ) and, in particular, to determine the three-body and higher-order effects. Ab initio and model calculations on a linear dimer show clearly that the evolution of properties from the separated limit is largely a consequence of the mutual polarization as the monomers begin to interact. This reveals that intermolecular quantum features are not significant. Furthermore, higher-order response properties play a negligible role in the evolution of lower-order cluster properties. For instance, the hyperpolarizability γ of the monomer is relatively unimportant in the evolution of α of the dimer; the change in α due to dimer formation is dictated largely by the mutual polarization via the intrinsic α′s of the monomer. With this determination, an electrical interaction model can be used to explore α and γ of long acetylene chains, and this serves as a guide to the vibronic coupling effects on hyperpolarizabilities of longchain species. We find, in particular, that the hyperpolarizability of the subunits of long acetylene chains are dramatically enhanced by intermolecular electrical interaction.
    Zusätzliches Material: 4 Ill.
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  • 169
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 171-173 
    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
    Materialart: Digitale Medien
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  • 170
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992) 
    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
    Materialart: Digitale Medien
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  • 171
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 159-169 
    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: Components of the crystal quadratic susceptibility tensor χ(2) for second-harmonic generation are calculated for the title compound, 4-(N, N-dimethylamino)-2-acetamido-nitrobenzene. Input data are the crystal structure and refractive indices and CNDO hyperpolarizabilities; two different sets of refractive indices give similar results. The calculations yield effective polarizabilities as well as both rigorous and anisotropic Lorentz local electric fields. Susceptibility components are close to 10 pm V-1, rather smaller than those deduced experimentally and showing a weaker dependence on direction; Kleinman symmetry is mostly observed within 10%. Screened dipoledipole interactions using CNDO dipole moments amount to -80 kJ mol-1, implying permanent fields of 2 GV m-1.
    Zusätzliches Material: 1 Ill.
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  • 172
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 175-195 
    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 correlation between topological features of molecular structure and physicochemical as well as biological properties of molecules were examined for some sets of compounds, using graph-theoretical desciptions. It was found that the topological similarity plays a role in the determination of molecular properties for almost all compound sets studied. As a conclusion, one can presume that the less elaborated and complicated similarity measure based on the graphtheoretical description is sufficient in the case of distinct structure-property dependence.
    Zusätzliches Material: 2 Ill.
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  • 173
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 209-214 
    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: We generalize the usual complex virial theorem using a procedure of complex scaling for only some of the coordinates of the system. This result can also be considered as an extension of the virial theorem obtained in a previous work using real scaling for a subset of coordinates.
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  • 174
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 281-300 
    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: A simple valence-bond approach is considered for treating extended systems of equivalent atoms. In this approach, a limited nonorthogonal basis set of atomic orbitals is utilized and only nearest-neighbor interactions are supposed. By exploiting then the translational invariance of the system, expressions for the matrix elements between antisymmetrized products of such nonorthogonal orbitals are derived in an algebraic form suitable for the numerical implementation on a computer. The one-band linear case is explicitly considered. Some possible applications of the proposed approach and its connections with model Hamiltonians such as the Hubbard and the Heisemberg ones are also discussed.
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  • 175
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992) 
    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
    Materialart: Digitale Medien
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  • 176
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 301-309 
    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 ground states of atoms and molecules Li-, Be, LiH, LiH2-, and Li2 have been calculated using the n-electron wave functions built up with two kinds of geminals. As a comparison, the above systems have been calculated with the Hartree-Fock self-consistent field and the multi-configuration self-consistent field method as well. The results show that the wave functions in this work are capable of describing the electron correlations, and both kinds of geminals can be taken as a starting point in building up n-electron ground states.
    Zusätzliches Material: 8 Tab.
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  • 177
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 313-320 
    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: We propose a novel method for calculation of the electronic correlation energy of molecular systems in the path integral formalism. The procedure consists of saddle-point expansion of the path integral around the Hartree-Fock energy. By one loop-expansion of the molecular path integral into the generalized space phase of the molecular orbitals, the shift energy is obtained.
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  • 178
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 321-326 
    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: Ab initio Hartree-Fock crystal orbital results were used as input for the calculation of exciton energies in the Tamm-Dancoff and random-phase approximation for polymers with helical symmetry. The calculations were applied to a hydrogen fluoride chain. We show that the Tamm-Dancoff method is a good approximation to the random-phase theory. Furthermore, the influence of the band-band interaction in the exciton calculation has been investigated.
    Zusätzliches Material: 1 Ill.
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  • 179
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 343-352 
    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: 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.
    Zusätzliches Material: 6 Ill.
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  • 180
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 327-342 
    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 local origin/local orbital (LORG) method of Hansen and Bouman has been implemented with the intermediate neglect of differential overlap Hamiltonian for spectroscopy (INDO/S). The method is shown capable of demonstrating the inductive effects associated with electron-withdrawing substituents through the diamagnetic shielding term. In addition, the method is capable of differentiating chemical shift in differing bond environments. The calculated paramagnetic contribution, however, is deficient for substituents that saturate the minimal basis such as oxygen and fluorine, which severely limits the general utility of the procedure. Through the utilization of reduced linear equations for the paramagnetic term, the method is amenable to any molecule for which a self-consistent field can be performed and therefore can potentially be used to study very large systems. At present, however, the LORG method when used with the rapid INDO/S model Hamiltonian does not reliably reproduce the paramagnetic contribution to the shielding.
    Zusätzliches Material: 3 Ill.
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  • 181
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 353-374 
    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: Following the recent developments of the charge sensitivity analysis (CSA) in the atoms-in-molecules (AIM) resolution, the corresponding CSA quantities in the orbital (or shell) resolution (OR) are defined. The OR electron population variables, in the ordinary closed-shell SCF problem, are the elements of the bond-order matrix P, and their conjugates, “chemical potentials,” FT = ∂E/∂P, are the respective Fock matrix elements, appropriate for the representation in question; here E is the SCF energy. The second derivatives ∂2E/∂P∂P define the OR hardness tensor from which all related OR CSs, e.g., the hardness, softnesses, Fukui function (FF) indices, etc., can be determined. The rigid potentials and hardness tensor, corresponding to the “frozen” orbital approximation, are examined in more detail, and the decoupled representation of the normal orbitals (NoO) is introduced, in which the rigid hardness tensor becomes diagonal. Illustrative valence-shell NoO contours for the water molecule are given and discussed. The new approximation for the OR FF indices, as the orbital occupation probabilities, is proposed on the basis of the density matrix functional development of Donnely and Parr for natural orbitals, and the relevant expressions for the molecular fragment (collection of orbitals) quantities are summarized.
    Zusätzliches Material: 3 Ill.
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  • 182
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 375-392 
    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: Two numerical codes, a complex face vector F and a real face vector D are developed for the characterization of square-cell configurations (lattice animals), used for representing the shapes of molecular monolayers and cross sections of molecular surfaces. The real face vector D represents all the intrinsic properties, size, and shape of the lattice animal. The complex face vector F contains complete information about the size, the shape, and also the placement of the particular lattice animal with respect to the lattice. Based on the properties of the face vectors, a method is developed for the classification of similar animals into equivalence classes. The face vector method is proposed for an algorithmic, nonvisual computer analysis of similarity of shapes of molecular monolayers and planar domains of cross sections of molecular surfaces, approximated by lattice animals.
    Zusätzliches Material: 5 Ill.
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  • 183
    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 electron-vibrational problem of the general nonadiabatic molecular systems has been solved by means of the quasi-particle transformations. The SCF ab initio solution of the nonadiabatic fermion Hamiltonian yields stabilization of the electronic ground-state energy due to electron-phonon interaction and it also gives the corrections to the one- and two-particle terms. Two two-particle correction yields effective attractive electron-electron interaction, but in the form different from Frölich's effective electron-electron interaction term. In contrast to the standard electron-phonon Hamiltonian of solid-state physics that does not take into account the possible effects of nonadiabaticity of a system, the presented nonadiabatic theory yields also one-particle corrections. The presence of this term in the Hamiltonian might play a crucial role in the theory of superconductivity since the superconductors are nonadiabatic systems.
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  • 184
    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: An exact solution for the electron-vibrational problem of the nonadiabatic molecular system has been obtained. By the quasi-particle transformation technique, the fermionic Hamiltonian has been derived and solved at the ab initio level. Results clearly and unambiguously show that the gap formation due to nonadiabatic electron-phonon coupling is mediated by the one-particle electron-phonon interaction term, whereas the two-particle one represents just a correction to the correlation energy. The temperature dependence of the gap and electronic specific heat connected with the electron-phonon coupling have also been derived.
    Zusätzliches Material: 2 Ill.
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  • 185
    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: Conditions for low and high Tc superconductors following from the nonadiabatic electron-vibrational theory at ab initio level have been obtained. According to the presented results, the supercurrent is realized by the motion of the ground-state electronic-charge distribution of the fully occupied band. The motion of the electronic-charge distribution is conditioned by the newly arisen, nondissipative degrees of freedom of nuclear, Jahn-Teller-like, microflows. The Meissner effect is also interpreted.
    Zusätzliches Material: 2 Ill.
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  • 186
    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
    Materialart: Digitale Medien
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  • 187
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 443-443 
    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
    Materialart: Digitale Medien
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  • 188
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 449-461 
    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: A vibrational coupling model to treat the solvation effects in chemical reaction rate calculations is proposed and applied to the intramolecular hydrogen transfer reaction CH3O· → ·CH2OH in the condensed phase. The effect of solvation is taken into account in two ways: (1) the solvent effect on the activation energy of the reaction is simulated by including 39 surrounding water molecules, represented by fractional charges at the assumed atomic positions, in the potential energy surface calculation; and (2) the vibrational couplings between the 10 nearest solvent molecules and the molecules constituting the reaction system are explicitly included in a vibrational frequency calculation. RRKM theory with Miller's tunneling correction included is employed to calculate the rate constants. The effect of solvation causes a significant change in the chemical reaction rate, mainly through a lowering of the activation energy. The vibrational coupling causes a slight increase of the rate constant in the tunneling region by perturbing the vibrational frequencies of the reactant and transition states, which appear in the rate-constant expression, but has little effect at higher temperatures.
    Zusätzliches Material: 4 Ill.
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  • 189
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 511-529 
    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 sulfur mustard, HD, a chemical warfare agent, has been studied by ab initio quantum molecular computations (HF/6-31G* and 6-31G) on its various forms (neutral, ethylene sulfonium, and free carbonium ions). The geometries of these molecules have been completely optimized and the minimal energy conformations determined with their associated charge distributions. We discuss these results on electrostatic properties with respect to the mechanism of DNA alkylation by HD and compare them with our previous study of the nitrogen mustard mechlorethamine.
    Zusätzliches Material: 6 Ill.
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  • 190
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 531-550 
    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: A new method is presented for calculating the normalization factor for continuous spectrum functions in the L2 basis. One variant of this method is applicable to operators given in the form of a tridiagonal matrix and does not require explicit calculation of the roots of the characteristic equation. The other variant of the method can be used in the general case, where after the basis is introduced, its length is gradually increased.
    Zusätzliches Material: 4 Ill.
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  • 191
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 573-578 
    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: For the CF4+, PF5+, SF6+, and MoF6+ ions appearing after the F1s photoionization, the possibility of dissociation has been shown by the ab initio MO LCAO method within the Z + 1 core equivalent model. According to the calculations, the decay channel AFn+ → AFn-1+ + F(1s12p6) is energetically open for the ions. So the interpretation of the gas-phase emission FKα spectra, in which the bands are assigned to the discrete transition energies, can be unacceptable for these ions. The conditions and signs of such failure are discussed.
    Zusätzliches Material: 1 Ill.
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  • 192
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992) 
    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
    Materialart: Digitale Medien
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  • 193
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 579-589 
    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: Four esters of 5-cyano-1,6-dihydro-6-oxo-3-pyridinecarboxylic acid involving substitutions of methyl, isopropyl, tert-butyl, and phenyl groups at the 2-position, which exhibit varying degrees of cardiotonic activities, have been studied using the electric-field and electrostatic potential mapping techniques. The net charge distributions in the molecules were obtained using the MNDO method. Electric-field mapping has been performed in three-dimension in order to obtain information about the possible structure-activity relationships of these compounds. The results show a prominent electric-field region in the methyl-substituted compound extending from near the oxygen atom O6 to near the methyl group. This fact and orientation of the electric field in this region, in view of the Erhardt model, may be related to the greater cardiotonic activity of the methyl-substituted molecule in comparison to those of the other molecules of this class. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
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  • 194
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 647-658 
    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: A general scheme is proposed for the study of the interrelations and transformations of flexible functional groups within the configuration space M of all possible chemical species of a fixed overall stoichiometry S. The methods presented provide a concise but sufficiently detailed description of the occurrence of various functional groups in different families of nuclear arrangements, as well as their transformations into one another during chemical reactions and conformational changes. The approach uses experimental information on the importance of various functional groups, topological criteria for the flexibility of these functional groups, and the fundamental pattern of distribution of configurational families within the nuclear configuration space. The resulting schemes have simple, easily programmable representations. © 1992 John Wiley & Sons, Inc.
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  • 195
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 669-676 
    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 cycloaddition reaction of ketene and methylenimine, leading to 2-azetidinone, has been studied theoretically by RHF/3-21G and IRC. This reaction is believed to be nonsynchronous and concerted, taking place through a twisted transition state. Four π orbitals are involved in this reaction, which is a “2 × [1 + 1]”-type cycloaddition. In the course of the reaction, rotation of the methylene group instead of oxygen in ketene was ascertained. The activated barrier is calculated to be 33.9 kcal/mol. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
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  • 196
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 677-699 
    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: Two things were done in this paper: (i) A generalization of Schwinger's variational principle to a subsystem was developed within the framework of quantum field theory and applied to the theory of atoms in molecules. This work generalizes the previous derivation given in the Schrödinger formulation. (ii) It is demonstrated that Feynman's path integral, when expressed in terms of the coherent-state representation, can be constructed for a subsystem of a many-electron system if a divergence term, which serves as a variational constraint in the definition of an atom in a molecule, is added to the Lagrangian. This formulation is equivalent to the atomic statement of the variational principle if the divergence term is suitably constructed. It is shown that the path integral can be expressed as a product of the individual atomic contributions to the steps along the paths with the action being determined by a corresponding sum of the atomic contributions to the action integral. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 1 Ill.
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  • 197
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 701-712 
    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 total (elastic plus inelastic) intensities of 51 keV electrons scattered by H2CO and H2CCO have been measured over a range of K = (4π/λ) sin(θ/2) = 1-9.5 Å-1 and compared with the theoretical intensities calculated with SCF and CI wave functions. Significant discrepancies are found between the experimental intensities and the theoretical ones based on the SCF wave functions. Most of the chemical binding and electron correlation effects observed in the total scattered intensities are reproduced by the theoretical intensities based on the CI wave functions calculated with the basis set including polarization functions on all atoms. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 6 Ill.
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  • 198
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992) 
    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
    Materialart: Digitale Medien
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  • 199
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 769-782 
    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 concept of a self-consistent field is developed within the version of density functional theory based on local-scaling transformations. It is shown that in this context there arise two types of consistency: one relating to the charge-consistency within an orbit and another to “orbit jumping.” The latter is analyzed in terms of one-particle equations. The connection with other methods is discussed. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 1 Ill.
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  • 200
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 747-753 
    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: Using the BCH theorem, we express the Hamiltonian of a Morse oscillator as a complete series of powers of the creation and annihilation operators for the harmonic oscillator. In this way, we improve the results of a previous work that uses a Bogoliubov-Tyablikov tranformation to calculate the Franck-Condon factors by means of equivalent harmonic oscillators potentials. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 4 Tab.
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