THEORETICAL-STUDY OF THE RADIATIONLESS DECAY CHANNELS OF TRIPLET-STATE NORBORNENE

Four possible radiationless decay channels for the lowest tripler stat e (T-1) of norbornene to the singlet ground state (S-0) have been inve stigated by ab initio CASSCF, MRD-Cl and unrestricted density function al calculations. Neither breaking a single bond, yielding a triplet bi radical with an allylic substructure, nor pyramidalization of the doub le bond, was found to be an energetically reasonable decay pathway. Al l 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 th e results for the gas-phase reaction do not support the short lifetime observed for the T-1 state in benzene solution, but show an interesti ng mechanism for decay in rigid systems. (C) 1998 Elsevier Science B.V . All rights reserved.