The molecular structure of the lowest triplet state of benzene: Quinoid or anti-quinoid?

Abstract

The molecular structure and vibrational frequencies for the 1 3B 1u state of benzene are examined using the ab initio MC SCF method. Vibrational analysis shows that the hexagonal D 6h structure has a degenerate pair of imaginary frequencies, 924i cm −1 for the ν 8 (E 2g) mode. The equilibrium structure is calculated to have the quinoid form, with the anti-quinoid geometry being a transition state for the pseudo-rotation of the benzene ring. Although this supports the existence of the pseudo-Jahn-Teller effect, the energy lowering from D 6h to D 2h symmetry is only 1.4 kcal/mol, and the energy order reverses when harmonic zero-point vibrational energy corrections are included. Consequently, the present result suggests that the molecular structure of the 1 3B 1u state of benzene would be difficult to determine experimentally because the ring distortion is very flexible along the ν 8 vibrational mode in the lowest triplet state.