An ab initio molecular orbital study of the potential energy surface of the HO2+NO reaction

The potential energy surface of the HO2+NO reaction has been investigated at second order Moller–Plesset perturbation (MP2) and density functional (DFT) methods with the 6-311++G** basis set and at complete active space CAS(8,8) self-consistent field level using the 6-31G** basis set. The reaction is shown to give three different groups of products, viz., HO–NO2, NO2+OH, and HNO+O2. The thermodynamically stable HO–NO2 can be formed from the energized ONOOH adduct by the 1,2 migration of the OH group via a loose transition state (referred to as TS2) with a relatively higher barrier height compared to O–O bond fission. The other exothermic product, NO2+OH, arises from a direct O–O dissociation of ONOOH and is expected to be the most favorable process on account of its low barrier height. HNO+O2 can be formed by two different channels: (i) the direct hydrogen abstraction and/or (ii) the barrierless association of the reactants to form the peroxynitrous acid, ONOOH, which then undergoes 1,3 hydrogen migration, giving rise to the HN(O)OO biradical followed by N–O dissociation. Of the two channels, channel (i) has been found to be dominant. Owing to their higher barrier heights, HNO formation is expected only at high temperatures. NOH+O2 and HONO+O are not expected to compete in the kinetics of the HO2+NO system. The energetic of the key reactions, namely HO2+NO→HO–NO2 and HO2+NO→NO2+OH, has also been obtained at the QCISD/6-311++G(2df,2pd)//MP2/6-311++G** level.