The potential energy surfaces for the C-C, C-Br and C-Cl dissociations of BrCH 2 COCl(S 1 ) are investigated at the CAS(8,7)/6 31G * and MR CI/cc pVDZ levels of theory, which provides some new insights into the mechanism of the BrCH 2 COCl photodis sociation at 248nm. It is found that the relative strength of the C-C and C-Cl α bonds is one of the factors that influence the selectivity of the α bond fissions. However, the selectivity is mainly determined by the mechanism of dissociation upon n→π * excitation. The preference of the C-Cl α bond fission over the C-Br β bond cleavage is attributed to the nonadiabatic recrossing in previous studies. The present calculations predict that distance dependence of intramolecular energy relaxation is another important factor that influences the selectivity of the α and β bond cleavages.
