Theoretical studies on the photochemical reaction mechanisms of o-xylylene. Effects of phenyl group for electrocyclic reaction mechanism

Abstract

The potential energy surfaces (PESs) of the electrocyclic reactions of o-xylylene at the ground and the lowest excited states are calculated by CASSCF molecular orbital and MRMP2 methods. The lowest excited state geometry of o-xylylene has C_2v symmetry and is about 65 kcal mol⁻¹ in energy above the ground state. The PESs in the vicinity of the conical intersection are different from those of the electrocyclic reaction of cis-butadiene. In the vicinity of the conical intersection, the transition state at the ground state relating to methylene-cycloheptadienyl carbene is located. The transition state is only 4.3 kcal mol⁻¹ lower in energy than the conical intersection at the CASSCF(10,10)/6-31G(d) level and 0.5 kcal mol⁻¹ lower at the MRMP2/6-311+G(d,p) level. The transition state corresponding to benzocyclobutene does not locate in the vicinity of the conical intersection because of the resonance energy between benzene ring and methylene group.