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

摘要

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 BrCH_2COCl at 248 nm.