Combined nonadiabatic transition-state theory and ab initio molecular dynamics study on selectivity of the α and β bond fissions in photodissociation of bromoacetyl chloride

The selectivity of the alpha C-Cl and beta C-Br bond fissions upon n ->pi(*) excitation of bromoacetyl chloride has been investigated with combined nonadiabatic Rice-Ramsperger-Kassel-Marcus theory and ab initio molecular dynamics calculations, which are based on the potential energy profiles calculated with the complete active space self-consistent field and multireference configuration interaction methods. The Zhu-Nakamura [J. Chem. Phys. 101, 10630 (1994); 102, 7448 (1995)] theory is chosen to calculate the nonadiabatic hopping probability. It is found that nonadiabatic effect plays an important role in determining selective dissociations of the C-Cl and C-Br bonds. The calculated rate constants are close to those from experimentally inferred values, but the branching ratio of the alpha C-Cl and beta C-Br bond fissions is different from the experimental findings. The direct molecular dynamics calculations predict that fission of the C-Cl bond occurs on a time scale of picoseconds and cleavage of the beta C-Br bond proceeds with less probability within the same period. This reveals that the initial relaxation dynamics is probably another important factor that influences the selectivity of the C-Cl and C-Br bond fissions in photodissociation of BrCH2COCl at 248 nm.