Excited electronic states of p-benzoquinone

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 1A(u), 1B(1g), 1B(3g), and 1B(1u) in both manifolds. 1 (1)A(u) and 1 (3)A(u) are predicted to be the lowest excited states in the respective multiplicity. The geometry of 1 B-1(3g) and 1 B-1(1u), may deviate from the D-2h 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. (C) 2001 Elsevier Science B.V. All rights reserved.