Abstract
The end-to-end dynamics of the smallest energized Criegee intermediate, HCOO, 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 s of molecular dynamics simulations using a neural network-representation of CASPT2/aug-cc-pVTZ reference data, the branching ratios into molecular products HCO+OH, CO+H, or HO+CO was quantitatively determined. Consistent with earlier calculations and recent experiments, decay into HCO+OH was found to be rare whereas the other two molecular product channels are accessed with fractions of and , respectively. On the 1 ns time scale, which was the length of an individual MD simulation, more than 40 \% of the systems remain in the reactant state due to partial intramolecular vibrational redistribution (IVR). Formation of CO+H occurs through a bifurcating pathway, one of which passes through formic acid whereas the more probable route connects the di-radical OCHO 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 \%. This work demonstrates that atomistic simulations with global reactive machine-learned energy functions provide a quantitative understanding of the chemistry and reaction dynamics for atmospheric reactions in the gas phase.