A CI-all-single-excitations study of Norrish type I process in acetyl chloride

The photocleavage of a bond alpha to the carbonyl group, the Norrish type I process, in acetyl chloride has been studied by means of ab initio methods. A recently developed methodology that makes use of configuration interaction among all single-substituted determinants using a spin-restricted Hartree-Fock reference state (RCIS) is used in order to study the carbon-carbon and carbon-chlorine bond cleavages in both the first excited singlet state and the first triplet state. In good agreement with experimental results it is found that the C-Cl bond cleaves preferentially over the C-C bond. Due to the evolution of the excited states toward a np(Cl) → σ* (C-Cl) configuration, the C-Cl bond cleavage involves a low energy barrier in both the triplet and singlet excited states, though as intersystem crossing in not likely to occur, the experimentally observed behavior can be attributed to the singlet excited state. These results confirm the ability of the RCIS methodology to deal with chemical reactions in excited states. © 1993 American Chemical Society.