Selectivity of the α and β bond fissions for bromoacetyl chloride upon n→π[sup *] excitation: A combined complete-active-space self-consistent field and multireference configuration interaction study.

The potential energy surfaces for the BrCH[SUB2]COC1 dissociations into Br+CH[SUB2]COC1, BrCH[SUB2]CO +C1, and BrCH[SUB2]+COC1 in the S[SUB0], S[SUB1], and T[SUB1] states have been investigated at the complete-active-space self-consistent field, density functional theory, and multireference configuration interaction levels with the 6-31G* and cc-pVDZ basis sets, which provide some new insights into the mechanism of the BrCH[SUB2]COC1 photodissociation at 248 nm. It is found that the most probable pathway is the S[SUB1] C-C1 α and C-Br β bond fissions, which are a pair of competitive dissociation channels with some preference of the α C-C1 bond cleavage. The C-C α bond fission can take place along the S[SUB1] pathway upon photoexcitation ar 248 nm, but it is not in competition with the C-C1 α bond cleavage. These results are consistent with the experimental findings. The relative strength of the C-C and C-C1 α bonds is one of the factors that influences the selectivity of the α bond fissions. However, the selectivity is mainly determined by the mechanism of dissociation upon n→&pie;* excitation. The preference of the C-C1 α bond fission over the C-Br β bond cleavage was attributed to the nonadiabatic recrossing in previous studies. The present calculations predict that the distance dependence of intramolecular energy relaxation is another important factor that influences the selectivity of the α and β bond cleavages. [ABSTRACT FROM AUTHOR]