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-1-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.