ELECTRON-VIBRATION INTERACTION AND THE RAMAN INTENSITIES OF A QUINOIDMOLECULE

The effect of electron correlation on the Raman intensities is examine d for p-benzoquinodimethane. The calculated Raman intensity of nu(3), characterized as the in-phase C=C stretching mixed with the C-C shrink ing, is reduced significantly by taking the effect of electron correla tion into account at the second-and third-order Moller-Plesset perturb ation levels. As a result, the observed Raman intensity pattern charac teristic of the quinoid structure is well reproduced by the calculatio ns at these correlated levels. Such a strong effect of electron correl ation is rationalized by considering the change in the electronic stru cture induced by applying an electric field and the concomitant change in the potential energy gradients (partial derivative V/partial deriv ative x(k)) arising from the electron-vibration interaction, since pol arizability derivatives (partial derivative alpha(rho sigma)/partial d erivative x(k)) are obtained as the second derivatives of the potentia l energy gradients with respect to the applied electric field. (C) 199 7 Elsevier Science B.V.