α-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 carbonyl group in acetyl chloride has been studied using ab initio methods. A configuration interaction method among all single-substituted determinants 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 (S1) and the first triplet state (T1). Our results clearly indicate that the C-Cl bond cleavage in the S1 state can take place directly upon photoexcitation as the vertical transition energy is clearly above the corresponding C-Cl transition state. The same is true for both the C-Cl and C-C bond cleavages in the T1 state. However, these last processes are not competitive because a previous intersystem crossing must occur. Our results also show that bond breaking can be understood as a crossing between the initial np(Cl) → (C=O)* excitation and an np(Cl) → (C-Cl)* configuration which leads directly to the bond fission.