Reaction Pathway Dynamics for Atmospheric Decomposition Reactions: Unimolecular Dissociation of H ₂ COO

Branching ratios for fragmentation channels of important meta- and unstable species are essential for a molecular-level characterization of atmospheric chemistry. Here, the molecular product channels for the decomposition dynamics of the smallest Criegee intermediate, H2COO, are investigated. Using a high-quality, full-dimensional machine learned potential energy surface (CASPT2/aug-cc-pVTZ), the translational, rotational, and vibrational energy distributions of the CO2 + H2, H2O + CO, and HCO + OH fragmentation channels were analyzed to elucidate energy partitioning. The CO2+H2 product forms through two pathways that bifurcate after formation of the OCH2O intermediate. The first is a “direct pathway”, for which CO2 is preferentially vibrationally excited and H2 remains in its vibrational ground state. Alternatively, along an “indirect pathway” passes through formic acid, whereby H2 can populate levels with v > 0. For all channels passing through energized formic acid, lifetime distributions are described by stretched exponentials with β ranging from 0.9 to 1.3, indicating non-RRKM effects and the need for explicit molecular dynamics.