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Experiments on benzene have established that its lowest triplet state (3B1u) is conformationally unstable owing to vibronic coupling with the next higher state (3E1u). This instability was found to be critically dependent on the influence of a crystal field. An analogous vibronic coupling is to be expected in the singlet manifold, but here no direct evidence is available for a conformational instability. The distortion behavior of benzene is of importance for the interpretation of its photophysical and photochemical properties. We have therefore determined the potential-energy surfaces of the 1,3B1u and 1,3E1u states along the two-dimensional distortion coordinate S8(,) using ab initio multireference single and double excitation-configuration-interaction calculations. The results show that for both B1u states the hexagonal conformation is unstable and lies 800 cm-1 above a wide, virtually cylindrical trough. A calculation of the vibrational spacing in the 3B1u state yields good agreement with the experimentally observed frequency. The calculation of intensities in the absorption and emission spectrum for this state qualitatively agrees with the experiment. An estimate is made of the interaction of the excited molecule with neighboring molecules in a crystal, which indicates that the crystal-field induced energy variations in the trough should be of the order of 10 cm-1. Combination of our calculations with experimental data shows that the vibronic coupling in the B1u states of benzene should not be looked upon as a static coupling in which the molecule is permanently distorted to one conformation but as a dynamic one in which the molecule makes excursions over the entire potential-energy surface. The Journal of Chemical Physics is copyrighted by The American Institute of Physics

Conformational instability of the lowest triplet state of the benzene nucleus. I. The unsubstituted molecule