The photodissociation mechanism of o-chlorotoluene on the low-lying electronic states has been investigated with the use of the CASSCF energy gradient techniques. Our calculations indicate that o-chlorotolubene can be excited to the first singlet state (S1) at 266 nm, and then two possible channels can appear. In one channel, the system could relax from the S1 state to the S0 state via the S1/S0 intersection point, then carries out the intramolecular hydrogen transfer to form o-5-methylene- 6-chloro-1,3-cyclorhexadiene, which further dissociates into benzyl and Cl radicals. In another channel, the system could relax to the T1 state via the S1/T1 intersection point, then directly dissociates into o-tolyl and Cl radicals. The two channels were found to proceed with probabilities of similar magnitudes, which is in good agreement with the recent experiment.

Key words: ab initio CASSCF, o-chlortoluene, photodissociation, potential intersection, intramolecular hydrogen transfer