Excited-state vibrations of benzene and polycyclic aromatic hydrocarbons: simple force field models based on molecular orbital characteristics of hexagonal carbon networks

Simple force field models for excited-state vibrations of benzene and polycyclic aromatic hydrocarbons (PAH) have been established. Root-mean-square frequency errors by the present MO/8ΔE and MO/8ΔΠ methods were found to be comparable with or even better than the conventional CCSD(T) and CIS calculations. The very strong vibronic coupling of the ν8 mode for the S2(1B1u) and T1(3B1u) states of benzene proposed in jet-spectroscopic studies by Hiraya and Shobatake and in EELS studies by Swiderek et al. has given confirmation by the present calculations. The present methods hold promise for predicting excited-state vibrations for PAH in the accuracy of 40 cm−1.