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 A1A → X1A photoisomerization channel of the simple Criegee intermediate (CH2OO) that leads to a cyclic dioxirane. The excited molecules on the A1A state, which can have either originated from the B1A state via B1A → A1A internal conversion or formed by state-selective electronic excitation, is driven by the out-of-plane motion toward a perpendicular A/X1A minimal-energy crossing point (MECI) then radiationless decay to the ground state with an average time constant of ∼138 fs, finally forming dioxirane at ∼254 fs. The dynamics starting from the A1A state show that the quantum yield of photoisomerization from the simple Criegee intermediate to dioxirane is 38%. The finding of the A1A → X1A photoisomerization channel is expected to broaden the reactivity profile and deepen the understanding of the photochemistry of Criegee intermediates.