Vibrational Interactions In Pariser-Parr-Pople Theory .2. 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.