Photochemistry of the Simplest Criegee Intermediate, CH<sub>2</sub>OO: 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 <i>A</i><sup>1</sup>A → <i>X</i><sup>1</sup>A photoisomerization channel of the simple Criegee intermediate (CH<sub>2</sub>OO) that leads to a cyclic dioxirane. The excited molecules on the <i>A</i><sup>1</sup>A state, which can have either originated from the <i>B</i><sup>1</sup>A state via <i>B</i><sup>1</sup>A → <i>A</i><sup>1</sup>A internal conversion or formed by state-selective electronic excitation, is driven by the out-of-plane motion toward a perpendicular <i>A</i>/<i>X</i><sup>1</sup>A 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 <i>A</i><sup>1</sup>A state show that the quantum yield of photoisomerization from the simple Criegee intermediate to dioxirane is 38%. The finding of the <i>A</i><sup>1</sup>A → <i>X</i><sup>1</sup>A photoisomerization channel is expected to broaden the reactivity profile and deepen the understanding of the photochemistry of Criegee intermediates.