Theoretical study of the radiationless decay channels of triplet state norbornene

Four possible radiationless decay channels for the lowest triplet state (T1) of norbornene to the singlet ground state (S0) have been investigated by ab initio CASSCF, MRD–CI and unrestricted density functional calculations. Neither breaking a single bond, yielding a triplet biradical with an allylic substructure, nor pyramidalization of the double bond, was found to be an energetically reasonable decay pathway. All theoretical models agree that twisting around the double bond is the most favourable decay coordinate. Spin–orbit coupling matrix elements employing the CASSCF method are small (0.5–1.5 cm−1) however, so the results for the gas-phase reaction do not support the short lifetime observed for the T1 state in benzene solution, but show an interesting mechanism for decay in rigid systems.