ELECTRON RELAXATION EFFECTS IN VIBRATIONAL SPECTRA.
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Publisher:Ohio State University
The direct effects of electron relaxation in a molecular vibration are seldom considered. However, the observable manifestations of such a phenomenon are sometimes quite striking. Qualitative measures of the magnitude of such an effect are readily obtained from the theory described. Mathematically, the treatment is very similar to the pseudo (or second-order) Jahn-Teller effect, where a second order perturbation term is responsible for a softening of force constant in a given symmetry coordinate. This idea has been used in the present study to rationalise the low wagging and rocking force constants of ketene and diazomethane with respect to ethylene. The theory has been extended to interaction constants, and via third and fourth order perturbation theory, to the third and fourth order parts of the vibrational potential function. Theory indicates that where a significant second order effect exists, the signs and relative magnitudes of the third and fourth order coefficients in the vibrational potential may be largely determined by the electron relaxation terms. Such an analysis has been shown to be successful for the potential coefficients up to fourth order for carbon dioxide, and is able to rationalise the negative anharmonicities seen in diazomethane and ketene, the methyl radical and certain planar excited electronic states of ammonia. The prediction of significant contributions to the coefficients of the crossterms of the symmetry displacements, in potential terms higher than second order, indicates a breakdown in the concept of normal modes in such systems.
Author Institution: Department of Chemistry, University of Michigan
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