A theoretical study of the photochemical isomerization reactions of (+)-2-carene-4α-methanol from the triplet state

Abstract

The mechanisms for the photochemical isomerization reactions are investigated theoretically using a model system of (+)-2-carene-4α-methanol (2-carene) 1, using the CASSCF (eight-electron/six-orbital active space) and MP2-CAS methods and the 6-311(d) and 6-311++G(3df, 3pd) basis sets, respectively. The structures of the intersystem crossings, which play a crucial role in these photo-isomerization reactions, are identified. The intermediates and transition structures of the ground state are also calculated to provide a qualitative explanation of the reaction pathways. The model investigations suggest that the preferred reaction route for 1 is as follows: singlet reactant → Franck–Condon region → triplet reactant → triplet transition state → intersystem crossing → intermediate → singlet transition state → photoproduct. In particular, the intersystem-crossing mechanism in this work gives a better explanation and supports the available experimental observations. Two types of reaction pathways that lead to final photoproducts are identified: (path I) ring expansion to form a cycloheptene ring and (path II) ring closure to form a methylcyclohexene structure. Both exhibit a biradical character. These theoretical results agree well with the available experimental observations.