Ya Peng, Hui Zhang
The photodissociation dynamics of peroxynitric acid (HO2NO2) in the atmospherically relevant ultraviolet region are investigated using the complete active space SCF electronic structure calculations combined with nonadiabatic molecular dynamics based on the fewest-switches surface hopping method. Minimum-energy crossing points between the lowest two singlets and between the lowest two triplet states are characterized and classified according to their local topological features in the branching plane. The S0/S1 crossing is identified as a sloped single-path intersection, whereas the T1/T2 crossing exhibits a peaked bifurcating topology. Nonadiabatic dynamics simulations for the photodissociation of HO2NO2 reveal ultrafast S1 → S0 relaxation mediated by the sloped intersection and a dominant NO2 + HO2 dissociation channel, with limited triplet involvement due to weak spin-orbit coupling. The predicted strong preference for the NO2 + HO2 pathway is consistent with updated experimental photolysis measurements in the actinic UV region, and the calculated rotational constants of HO2NO2 show very good agreement with available spectroscopic data. These results elucidate how changes in excited-state molecular geometry govern the photodissociation behavior of HO2NO2, highlighting the structural origin of its dominant reaction pathway.