中文摘要：

采用量子化学从头算CASSCF和CASPT2方法对邻氯甲苯在低激发态上的光解机理进行了理论研究．研究结果表明，在266姗的光激发下邻氯甲苯可以激发到第一单重态上，然后存在两种可能的离解途径：一种是经过S1／S0交叉点内部转换驰豫到基态，然后甲基上的一个H转移形成邻-5-亚甲基-6-氯-1，3-环己二烯，进而C-C1键断裂生成苄基：另一种是先后经过S1/T2和T2/T1交叉点驰豫到三重态，然后进行C-C1键断裂，形成邻甲苯基．这两种途径具有相近的反应几率，与实验结果很吻合．

英文摘要：

The photodissociation mechanism of o-chlorotoluene on the low-lying electronic states has been investigated with the use of the CASSCF and CASPT2 techniques. Our calculations indicate that o-chlorotolubene can be excited to the first singlet state （s1） at 266 nm, and then two possible channels would be appeared. In one channel, the system could relax from the S1 state to the So state via the S1/S0 intersection point, then undergoes the intramolecular hydrogen transfer to form o-5-methylene-6-chloro-1,3- cyclohexadiene, which further dissociates into benzyl and C1 radicals. In another channel, the system could relax to the T1 state via the S1/T2 and T2/T1 intersection points, then directly dissociates into o-tolyl and C1 radicals. The two channels were found to proceed with probabilities of similar magnitudes, which is in good agreement with the recent experiment.