‘Classical wavepacket’ dynamics through a conical intersection. Application to the S1/S0 photochemistry of benzene

Excited state dynamics calculations (using semiclassical surface-hopping trajectories first described by Tully and Preston) have been carried out in order to explain the low quantum yield for the photochemical production of benzvalene from benzene. Trajectories are propagated using a series of local quadratic approximations to the potential energy surface. Initial conditions are determined by random sampling of each excited state normal mode within an energy threshold which generates an ensemble of trajectories - a ‘classical wavepacket’. Points on the potential energy surface are computed as required using a parametrised quantum-mechanical/force-field method (molecular mechanics-valence bond, MMVB) which has been designed to simulate the CASSCF potential for ground and covalent excited states. The low benzvalene quantum yield is shown to result from the fact that, after decay via an S1/S0 conical intersection, most trajectories lead back to S0 benzene and do not reach the region of the prefulvene intermediate.