Vibrational interactions in pariser-parr theory. I. Bandwidths and transition intensities

It is shown that bandwidths of some transitions can be calculated with good accuracy in the Pariser-Parr theory. For these calculations the critical point is a knowledge of the variation of the bond resonance integral β with respect to distance, which is given by κ= -d In(-β)/dR = 2.0 Å-1. Calculations are carried out for the bandwidths of the N-V transition of ethylene and the 1A1g→ 1E1u, 1E2g transitions of benzene and the intensities of the 1A1g→1B 2u, 1E2g transitions of benzene. Satisfactory agreement with experiment is obtained; the neglect of electronic repulsion terms is demonstrated to have a relatively small effect. Selection rules are established for the strength of vibrational coupling in alternant hydrocarbons.