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+O-2. The thermodynamically stable HO-NO2 can be formed from the energized ONO'OH 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, NP2+OH, arises from a direct O-O dissociation of ONO'OH and is expected to be the most favorable process on account of its low barrier height. HNO+O-2 cain 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, ONO'OH, 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+O-2 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. (C) 1997 American Institute of Physics.