Insights into photodissociation dynamics of acetaldehyde from ab initio calculations and molecular dynamics simulations

In the present paper we report a theoretical study on mechanistic photodissociation of acetaldehyde (CH3CHO). Stationary structures for H-2 and CO eliminations in the ground state (S-0) have been optimized with density functional theory method, which is followed by the intrinsic reaction coordinate and ab initio molecular dynamics calculations to confirm the elimination mechanism. Equilibrium geometries, transition states, and intersection structures for the C-C and C-H dissociations in excited states were determined by the complete-active-space self-consistent field (CASSCF) method. Based on the CASSCF optimized structures, the potential energy profiles for the dissociations were refined by performing the single-point calculations using the multireference configuration interaction method. Upon the low-energy irradiation of CH3CHO (265 nm < 318 nm), the T-1 C-C bond fission following intersystem crossing from the S-1 state is the predominant channel and the minor channel, the ground-state elimination to CH4+CO after internal conversion (IC) from S-1 to S-0, could not be excluded. With the photon energy increasing, another pathway of IC, achieved via an S-1/S-0 intersection point resulting from the S-1 C-C bond fission, becomes accessible and increases the yield of CH4+CO.