Reaction Pathway Dynamics for Atmospheric Decomposition Reactions: Unimolecular Dissociation of H₂COO

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.