ALPHA-BOND CLEAVAGE UPON ELECTRONIC EXCITATION OF ACETYL-CHLORIDE - STUDY OF CI SINGLE SURFACES BY FULL GEOMETRY OPTIMIZATION

The photoexcited C-X (X = C, Cl) cleavage of a bond alpha to the carbo nyl group in acetyl chloride has been studied using ab initio methods. A configuration interaction method among all single-substituted deter minants using a spin-restricted Hartree-Fock reference state (RCIS) is used to locate directly, by full geometry optimizations, the minimum- energy conformation and the transition state for the C-X bond cleavage in both the first excited singlet state (S-1) and the first triplet s tate (T-1). Our results clearly indicate that the C-Cl bond cleavage i n the S-1 state can take place directly upon photoexcitation as the ve rtical transition energy is clearly above the corresponding C-Cl trans ition state. The same is true for both the C-Cl and C-C bond cleavages in the T-1 state. However, these last processes are not competitive b ecause a previous intersystem crossing must occur. Our results also sh ow that bond breaking can be understood as a crossing between the init ial np(Cl) --> (C=0) excitation and an np(Cl) --> (C-Cl)* configurati on which leads directly to the bond fission.