THEORETICAL-STUDY OF THE REACTION OF OH WITH HNO

A complete-active-space (CAS) multiconfiguration self-consistent field (MCSCF) wavefunction with a polarized correlation-consistent basis set was used to determine the stationary points on the OH + HNO potential energy surface. The single-point energies were determined with a multireference configuration interaction (MRCI) wavefunction consisting of single and double excitations from selected configurations chosen from the reference wavefunction. Saddle points were confirmed at the five-electron, five-active-orbital CASSCF level by calculating analytical second derivatives. Segments of the minimum-energy paths (MEPs) for the hydrogen abstraction reaction and for the radical addition to nitrogen were calculated. It was found that the zero-point energy contribution for the abstraction pathway increases as one moves away from the MCSCF saddle point toward the reactants. An evaluation of the MRCI energy and the MCSCF zero-point energy contribution on the MEP at a few points towards the reactants suggests that the true transition state is earlier than the computed MCSCF saddle point, and that abstraction occurs with little or no activation barrier. At the MRCI level, the maximum in the vibrationally adiabatic potential along the MEP for the addition of OH to the nitrogen of HNO occurs at 5.6 kcal mol-1 above the reactants and, like the abstraction, this structure is earlier than the MCSCF saddle point. The resulting product of the OH addition, HN(O)OH, is not expected to decompose to H + HONO because the barrier to H elimination (31.4 kcal mol-1) is significantly higher than the barrier for the reaction back to OH + HNO (20.3 kcal mol-1). Statistical rate calculations have been performed for the abstraction and addition reactions, and the results are fitted to standard three-parameter temperature-dependent rate expressions.