CONFORMATIONAL INSTABILITY OF THE LOWEST TRIPLET-STATE OF THE BENZENE NUCLEUS .2. P-XYLENE, THE INFLUENCE OF SUBSTITUENTS

A calculation of the potential-energy surface of the lowest triplet state of p-xylene as a function of the S8(p,φ) distortion coordinate of the benzene skeleton has been made to learn more about the influence of substituents on the vibronically induced distortion of benzene in its metastable triplet state.The results show the hexagonal conformation of the benzene nucleus in p-xylene to be unstable with respect to distortions along S 8(p,φ). In contrast with benzene, for which a flat, virtually cylindrical trough was calculated, the lowest triplet state of p-xylene shows a preference for quinoidal conformations. The variation in energy with the position of the methyl groups in the quinoidal structure is insignificant within the accuracy of the calculation. © 1990 American Institute of Physics.