Photochemistry of the Simplest Criegee Intermediate, CH2OO: Photoisomerization Channel toward Dioxirane Revealed by CASPT2 Calculations and Trajectory Surface-Hopping Dynamics

The photochemistry of Criegee intermediates plays a significant role in atmospheric chemistry, but it is relatively less explored compared with their thermal reactions. Using multireference CASPT2 electronic structure calculations and CASSCF trajectory surface-hopping molecular dynamics, we have revealed a dark-state-involved A(1)A -> X(1)A photoisomerization channel of the simple Criegee intermediate (CH200) that leads to a cyclic dioxirane. The excited molecules on the A(1)A state, which can have either originated from the B(1)A state via B(1)A -> A(1)A internal conversion or formed by state selective electronic excitation, is driven by the out-of-plane motion toward a perpendicular A/X(1)A minimal-energy crossing point (MECI) then radiationless decay to the ground state with an average time constant of similar to 138 fs, finally forming dioxirane at similar to 254 fs. The dynamics starting from the A(1)A state show that the quantum yield of photoisomerization from the simple Criegee intermediate to dioxirane is 38%. The finding of the A(1)A -> X(1)A photoisomerization channel is expected to broaden the reactivity profile and deepen the understanding of the photochemistry of Criegee intermediates.