A theoretical determination of the low-lying electronic states of the p-benzosemiquinone radical anion

The low-lying electronic states of the p-benzosemiquinone radical anion are studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbital (ANO) basis sets. Vertical excitation energies at the optimized geometries of the anion and the neutral molecule are computed. In light of the present theoretical results, the information provided by different spectroscopic techniques, such as UV/vis absorption, excitation, fluorescence, electron photodetachment, and electron scattering, is rationalized. CASSCF force fields are employed to compute vibronic intensities of the two lowest-energy pi-pi* transitions in order to solve controversial assignments and to give an interpretation of the available resonance Raman data.