Anharmonicity contributions to the vibrational second hyperpolarizability of conjugated oligomers

被引:55
作者
Champagne, B
Luis, JM
Duran, M
Andrés, JL
Kirtman, B
机构
[1] Fac Univ Notre Dame Paix, Lab Chim Theor Appl, B-5000 Namur, Belgium
[2] Univ Girona, Dept Chem, Girona 17071, Catalonia, Spain
[3] Univ Girona, Inst Computat Chem, Girona 17071, Catalonia, Spain
关键词
D O I
10.1063/1.480651
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
Restricted Hartree-Fock 6-31G calculations of electrical and mechanical anharmonicity contributions to the longitudinal vibrational second hyperpolarizability have been carried out for eight homologous series of conjugated oligomers-polyacetylene, polyyne, polydiacetylene, polybutatriene, polycumulene, polysilane, polymethineimine, and polypyrrole. To draw conclusions about the limiting infinite polymer behavior, chains containing up to 12 heavy atoms along the conjugated backbone were considered. In general, the vibrational hyperpolarizabilities are substantial in comparison with their static electronic counterparts for the dc-Kerr and degenerate four-wave mixing processes (as well as for static fields) but not for electric field-induced second harmonic generation or third harmonic generation. Anharmonicity terms due to nuclear relaxation are important for the dc-Kerr effect (and for the static hyperpolarizability) in the sigma-conjugated polymer, polysilane, as well as the nonplanar pi systems polymethineimine and polypyrrole. Restricting polypyrrole to be planar, as it is in the crystal phase, causes these anharmonic terms to become negligible. When the same restriction is applied to polymethineimine the effect is reduced but remains quantitatively significant due to the first-order contribution. We conclude that anharmonicity associated with nuclear relaxation can be ignored, for semiquantitative purposes, in planar pi-conjugated polymers. The role of zero-point vibrational averaging remains to be evaluated. (C) 2000 American Institute of Physics. [S0021-9606(00)30602-X].
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页码:1011 / 1019
页数:9
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