Franck–Condon structure of the S→S1 and S→S2 transitions in norbornadiene

Abstract

We report an ab initio study of the geometry and force field of the S0, S1, and S2 electronic states of norbornadiene at the Hartree–Fock/6–31G* and configuration interaction singles/6–31+G levels of theory. The calculated Franck–Condon structures of the S0→S1 and S0→S2 transitions agree well with the structures observed in the optical absorption and energy loss spectra. It is argued that the 47 000 cm−1 absorption band which undergoes changes in the vibronic structure upon going from the gas to the solid phase owes its different activity to the freezing of the angle between the two ‘‘wings’’ of the cyclohexadiene part of the molecule. The complete neglect of differential overlap/spectroscopic parametrization method is used to locate valence states of the title molecule and to calculate vibronic coupling parameters to predict vibronic activity of the nontotally symmetric modes in the spectra. It is demonstrated that the highest frequency C=C stretching mode of the b2 symmetry is the most effective in bringing intensity into the S0→S1 transition. The positions of the five lowest valence states obtained by the complete neglect of differential overlap/spectroscopic parametrization method agree very well with the peaks of bands observed in the electron energy loss spectra.