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.
The geometries of the reactants, transition states, intermediates and products for the titled reactions are optimized with CAS(17,13)/cc pVDZ, CAS(9,7)+1+2/cc pVDZ and CAS(9,7)+1+2/cc pVTZ. All stationary points are characterized with vibrational analysis, and the rate constants for the titled reaction are computed with transition state theory. The obtained values are in good agreement with the experimental ones.
The geometries of the reactants and product for the titled reaction are optimized at CAS(3,3)+1+2/cc-pvDZ level. The energetics of some crucial points along the minimum energy path are refined by CAS(3,3)+1+2/cc-pvTZ method. Based on RRKM theory, the rate constants for the titled reaction are calculated with the geometries and frequencies of CAS(3,3)+1+2/cc-pvDZ. The obtained values are in good agreement with the experimental ones.
