Vibronic Contribution to the Infrared Intensities of Benzene

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Abstract
Vibronic contributions from the π‐electron system to the intensities of the in‐plane normal modes of benzene have been evaluated theoretically in terms of Pariser—Parr—Pople semiempirical π‐electron theory. The vibronic terms arise in the benzene intensities because of changes in the two‐center electron‐repulsion integrals, and in the one‐electron one‐center core integrals, during the normal coordinate displacements. They are very important in determining the intensities of all three in‐plane modes. The results indicate that a model which treats the intensities in terms of localized bond dipoles and bond dipole‐moment derivatives is untenable in any molecule where vibronic terms may enter. The vibronic terms do not appear to give rise to unusual intensity changes in the 1033‐ and 1489‐cm−1 bands in condensed media as compared with the gas phase, but there may be a major vibronic term involved in the unusual decrease in the 3080‐cm−1 band intensity in solid benzene.