An ab initio study of the singlet potential-energy surface for the reaction of NO3 with HO2

The geometries of transition states and intermediates on the singlet potential-energy surfaces for the reactions NO, + HO2 --> OH + NO2 + O-2 (1) and NO3 + HO2 --> HNO3 + O-2 (2) have been optimised in MP2 calculations with a 6-31G* basis set. Energetics have been derived from single point MP4/6-311G** calculations at the MP2/6-31G* geometries. Reaction (1) proceeds without an overall activation barrier other than the reaction endothermicity via three O2NO ... OOH rotational isomers. Reaction (2), which is exothermic, has no overall activation energy and involves hydrogen atom transfer in a NO3 ... HO2 complex which then yields products via a HNO3 ... O-2 adduct. No evidence was found for a cyclic transition state.