Insights into mechanistic photodissociation of chloroacetone from a combination of electronic structure calculation and molecular dynamics simulation.

The stationary and intersection structures on the S(0) and S(1) potential energy surfaces of CH(3)COCH(2)Cl have been determined by the CAS(10,8)/cc-pVDZ optimizations and their relative energies are refined by the CASPT2//CAS(10,8)/cc-pVDZ single-point calculations. Non-adiabatic molecular dynamics simulations were performed on the basis of the state-averaged CAS(10,8)/cc-pVDZ calculated energies, energy gradients, and Hessian matrix for the S(0) and S(1) states. It is found that the features of the S(1) potential energy surface and non-adiabatic effect control the selectivity of the two α-C-C bond fissions, which provides a reasonable explanation why one α-C-C bond was observed as a primary channel and the other is ruled out even if CH(3)COCH(2)Cl is excited at 193 nm. The β-C-Cl fission is determined to be a dominant channel once the CH(3)COCH(2)Cl molecule is excited to the S(1) state and the β-C-Cl:α-C-C branching ratio is estimated by the RRKM rate theory to be 15:1 at 193 nm, which is overestimated in comparison with the value of ~11:1 inferred experimentally. The present calculation reveals that the α-C-C fission might take place in the ground electronic state as a result of the S(1) → S(0) internal conversion upon photolysis at 308 nm. However, the measured kinetic energy distributions of the α-C-C fission products suggest that the fission does not involve internal conversion to the ground state. To solve this issue, we need to perform non-adiabatic quantum dynamics simulation on accurate S(0), S(1), and S(2) potential energy surfaces, which is still a challenging task currently.