Photodissociation dynamics of energized H2COO: Formation of molecular products

The photodissociation dynamics of the smallest energized Criegee intermediate, H2COO, was characterized for vibrational excitation close to and a few kcal/mol above the barrier for hydrogen transfer. From an aggregate of at least 5 μs of molecular dynamics simulations using a neural network-representation of CASPT2/aug-cc-pVTZ reference data, the branching ratios into molecular products HCO + OH, CO2 + H2, or H2O + CO on the nanosecond time scale were quantitatively determined. Consistent with earlier calculations and recent experiments, decay into HCO + OH was found to be rare (∼2%), whereas the other two molecular product channels are accessed with fractions of ∼30% and ∼20%, respectively. On the 1 ns time scale, which was the length of an individual molecular dynamics simulation, more than 40% of the systems remain in the reactant state due to efficient, partial intramolecular vibrational redistribution. Formation of CO2 + H2 occurs through a bifurcating pathway, one of which passes through formic acid, whereas the more probable route connects the di-radical OCH2O with the product through a low-lying transition state. Notably, none of the intermediates along the pathway accumulate, and their maximum concentration always remains well below 5%.