α-Bond cleavage upon electronic excitation of acetyl chloride study of CI single surfaces by full geometry optimization

Abstract

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)→(CO)* excitation and an np(Cl)→(C—Cl)* configuration which leads directly to the bond fission.