Conical intersection pathways in the photocycloaddition of ethene and benzene: A CASSCF study with MMVB dynamics

Pathways involving possible conical intersections and radicaloid intermediates on the potential energy surface for the photocycloaddition of ethene to benzene have been studied using CASSCF geometry optimization and MMVB simulation of conical intersection decay dynamics. This study is intended as a model for arene-alkene photochemistry. The formation of one C-C bond occurs without barrier to yield a low energy conical intersection. Whilst many of the previously postulated intermediates do exist, the reactivity is shown to be controlled by this single conical intersection that has the geometry of a distorted tetrahedron with 4 carbon centers at its vertices. Such a structure is similar to the one encountered in the sigmatropic migration of the methyl group in but-l-ene and of the allyl group in hexa-1,5-diene. The dynamics of the decay at this conical intersection, studied with MMVB, show that one may form either the ortho, meta, or para products. However, ground state relaxation valleys have been located for the meta and ortho products only. This is consistent with experiment in solution where ortho and meta products are formed with approximately equal quantum yields.