Theoretical study of the photochemical reaction mechanism of bicyclo[4.1.0]hept-2-ene upon direct photolysis

Ab initio multiconfigurational CASSCF/MP2 method with the 6-31G* basis set has been employed in studying the photochemistry of bicyclo[4.1.0]hept-2-ene upon direct photolysis. Our calculations involve the ground state (S,) and excited states (S-1, T-1, and T-2). The ground-state reaction pathways corresponding to the formation of the six products derived from bicyclo[4.1.0]hept-2-ene via two important diradical intermediates (D1 and D2) were mapped. It was found that there are various crossing points (conical intersections and singlet-triplet crossings) in the regions near D1 and D2. These crossing points imply that direct photolysis can lead to two possible radiationless relaxation routes: (1) S-1 -> S-0, (2) S-1 -> T-2 -> T-1 -> S-0. Computation indicates that the second route is not a competitive path with the first route during direct photolysis. The first route is initiated by barrierless cyclopropane bond cleavage to form two singlet excited diradical intermediates, followed by efficient decay to the ground-state surface via three S-1/S-0 conical intersections in the regions near the diradical intermediates. All six ground-state products can be formed via the three conical intersections almost without barrier after the decays. The barriers separating the diradical minima on S-1 from the S-1/S-0 conical intersections were found to be very small with respect to the vertical excitation energy, which can explain why the product distribution is independent of excitation wavelength. Triplet surfaces are not involved in the first route, which agrees with the fact that the product contribution was unchanged by the addition of naphthalene. (c) 2005 Wiley Periodicals, Inc.