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  • 1
    ISSN: 1572-9001
    Keywords: Urocanic acid ; propenoic acid ; imidazoie ; semiempirical calculations ; ab initio calculations ; protonation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The neutral, cationic, and anionic structures of both prototropic tautomers oftrans- andcis-urocanic acid [(E)- and (Z)-3-(1′H-imidazol-4′(5′)-yl)propenoic acid, respectively] were studied by using semiempirical andab initio gas-phase calculations. Potential energy surfaces of the structures were calculated by using the semiempirical AM1 method, and the geometries corresponding to global minima on these surfaces were optimized up to the MP2/6-31G* level of theory. The calculated protonation forms of each urocanic acid isomer have a planar molecular structure due to a delocalized π-electron system, and all of them prefer thes-trans conformation with respect to the bond between the imidazole and the propenoic acid moieties. Thecis-urocanic acid structures are stabilized by an intramolecular hydrogen bond. The chargedcis-urocanic acid isomers have a lower molecular energy than the correspondingtrans-isomers, whereas the neutral molecules have, after inclusion of thermodynamic corrections, approximately the same energy. The cationic urocanic acid structures have about 2500 kJ mol−1 lower energy than the anionic ones and about 1000 kJ mol−1 lower energy than the neutral ones. The nonzwitterionic forms of the neutral urocanic acid isomers have about 200 kJ mol−1 lower energy than the zwitterionic ones. These energy differences are explained by the proton affinities of the imidazole and the propenoic acid moieties of the urocanic acid structures.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 51-68 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Methods will be developed for variational calculations of steady-state wave functions and the corresponding quasienergies. These functions appear as solutions to the time-dependent Schrödinger equation for a molecule interacting with an oscillating electromagnetic field. A variation principle is used to obtain coupled equations for each Fourier component (FC) of the steady-state wave function. The FCs are expanded in configuration state functions using the structure of the complete active space (CAS) SCF approach. Two methods are outlined, one which is based on a common set of molecular orbitals for all FCs and one based on an individual optimization of both CI coefficients and molecular orbitals for each FC. The latter method leads to molecular orbital sets for the different FCs which are mutually nonorthogonal. Formulas are derived for the calculation of higher-order dynamic susceptibilities for steady states. An application is made to the H2 molecule, and results are presented for the frequency-dependent polarizability and hyperpolarizability along the molecular axis. The static values obtained are α = 6.413 (6.387) and γ = 676 (674) atomic units, with exact numbers within parentheses.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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