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Anharmonicity contributions to the vibrational second hyperpolarizability of conjugated oligomers

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 σ-conjugated polymer, polysilane, as well as the nonplanar π 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 π-conjugated polymers. The role of zero-point vibrational averaging remains to be evaluated

© Journal of Chemical Physics, 2000, vol. 112, núm. 2, p. 1011-1019

American Institute of Physics

Author: Champagne, Benoît
Luis Luis, Josep Maria
Duran i Portas, Miquel
Andrés, José Luís
Kirtman, Bernard
Date: 2000
Abstract: 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 σ-conjugated polymer, polysilane, as well as the nonplanar π 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 π-conjugated polymers. The role of zero-point vibrational averaging remains to be evaluated
Format: application/pdf
Citation: Champagne, B., Luis, J.M., Duran, M., Andrés, J.L., i Kirtman, B. (2000). Anharmonicity contributions to the vibrational second hyperpolarizability of conjugated oligomers. Journal of Chemical Physics, 112 (2), 1011-1019. Recuperat 10 febrer 2011, a http://link.aip.org/link/doi/10.1063/1.480651
ISSN: 0021-9606 (versió paper)
1089-7690 (versió electrònica)
Document access: http://hdl.handle.net/10256/3236
Language: eng
Publisher: American Institute of Physics
Collection: Reproducció digital del document publicat a: http://dx.doi.org/10.1063/1.480651
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Is part of: © Journal of Chemical Physics, 2000, vol. 112, núm. 2, p. 1011-1019
Rights: Tots els drets reservats
Subject: Dinàmica molecular
Polarització (Electricitat)
Polímers
Molecular dynamics
Polarization (Electricity)
Polymers
Title: Anharmonicity contributions to the vibrational second hyperpolarizability of conjugated oligomers
Type: info:eu-repo/semantics/article
Repository: DUGiDocs

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