Ab initio molecular orbital study of potential energy surface for the NH+NO₂ reaction

The reaction of NH with NO₂, which can produce N₂O+OH and HNO+NO by two distinct reaction paths, has been studied by ab initio molecular orbital calculations. The first reaction path taking place by initial N-N association forms an intermediate HNNO₂, 1, which undergoes H-migration yielding NN(O)OH, 3, before reaching the N₂OH-OH product The transition state 2 for the rate-determining 1→3 rearrangement, with the activation barrier of 30 kcal/mol at the G2-level of calculation, lies below the energy of the reactants. The O migration for the HNNO₂ 1 intermediate to produce HNO+NO is inaccessible at low temperatures due to the presence of a high migration barrier. The second path via initial N-O association forms an intermediate HNONO, 9, which is expected to dissociate readily to HNO+NO via a loose transition state lying 24 kcal/mol below the reactants. Since the initial N-N and N-O association reactions effectively occur with no barriers, the overall activation energy for NH+NO₂ is expected to be negligible or slightly negative as was found experimentally.