Vibrational interactions in pariser-parr-pople theory. II. Ground- and excited-state force fields

Expressions are derived for the interaction between ground and excited states to give resonance contributions to the vibrational force field. Two contributions are found: The dominating term reflects largely the change in the bond resonance integral due to bond length variation with some contribution of the electrostatic terms; the other, a relatively small term, is generated by the polarization due to the field generated by a vibrational distortion. Calculations are made for the ground states of a number of aromatics; adequate agreement with experiment is found for benzene and naphthalene. The possible distortion of molecules and the Born-Oppenheimer approximation are investigated when the resonance interactions become strong. Application is made to the 1B1u and 3B1u states of benzene. Distortion and violation of the Born-Oppenheimer approximation is predicted to occur for the two B1u states of benzene.