Quantum Mechanical Study of the Photoisomerizations of Bicyclo[4,1,0]hept-2-ene (2-Norcarene)

Abstract

The mechanisms of the photochemical isomerization reactions were investigated theoretically using a model system of bicyclo[4,1,0]hept-2-ene (2-norcarene) 1 with the CASSCF (eight-electron/eight-orbital active space) and MP2-CAS methods and the 6-311(d) basis set. The structures of the conical intersections and intersystem crossings, which play a crucial role in such photoisomerization reactions, were obtained. The intermediates and transition structures of the ground-state were also calculated to assist in providing a qualitative explanation of the reaction pathways. Our model investigations suggest that the preferred singlet photoreaction route for 1 is as follows: singlet reactant --> Franck-Condon region --> conical intersection --> intermediate --> transition state --> photoproduct. On the other hand, our theoretical findings indicate that the preferred triplet photoreaction route for 1 is as follows: singlet reactant --> Franck-Condon region --> triplet minimum --> triplet transition state --> intersystem crossing --> intermediate --> singlet transition state --> photoproduct. In particular, the intersystem crossing mechanism found in this work gives a better explanation and supports the available experimental observations. Two kinds of reaction pathways, which can lead to final photoproducts, have been identified: (paths I or III) ring-expansion to form a cycloheptene ring and (paths II or IV) ring-closure to form a methylcyclohexene structure. Both exhibit biradical character. Also, our theoretical investigations strongly indicate that substantial interaction occurs between the cyclopropane moiety and the isolated carbon-carbon double bond in the excited state of (1).