Excited electronic states of p-benzoquinone Original Research Article

Vertical excitation energies are calculated for the valence and Rydberg states of p-benzoquinone using the CASSCF and LS–CASPT2 methods. Singlet as well as triplet states are considered. Optimized geometries and adiabatic excitation energies are determined for the states 1Au, 1B1g, 1B3g and 1B1u in both manifolds. 1 1Au and 1 3Au are predicted to be the lowest excited states in the respective multiplicity. The geometry of 1 1B3g and 1 1B1u may deviate from the D2h symmetry of the ground state. Normal modes and frequencies for the low lying valence excited states are refined by scaling the calculated CASSCF force fields in internal coordinates. Comparison with previous experimental and theoretical work leads to a revision of several earlier assignments.