A Theoretical Study on the Photodissociation of Acetone: Insight into the Slow Intersystem Crossing and Exploration of Nonadiabatic Pathways to the Ground State

Structures of transition states (TSs) and minima on seam of crossing (MSXs) for potential energy surfaces (PESs) of acetone of the S0, S1, and T1 states were explored. On the basis of the results, we propose a new mechanism, slow intersystem crossing from S1 to T1 without seam of crossing, followed by CH3 dissociation via a TS on T1; this slow pathway will be overtaken by a more efficient S1 pathway for higher energy. This is consistent with the observed long lifetime of the S1 species. Moreover, four channels, including three new ones, were found to regenerate the ground state acetone from the S1 PES, and they all may be involved in the roaming channel that has been proposed recently as a new route of CO generation in a 230 nm photolysis. There are significant differences in MSX structures and energies between the present CASPT2 results and previous CASSCF results.