Ab initio variational transition-state-theory reaction-rate calculations for the gas-phase reaction H+HNO→H2+NO

We report ab initio calculations of the potential-energy surface and variational transition-state-theory calculations of the temperature dependence of the reaction rate for the chemical reaction H+HNO→H2+NO. This reaction is very exothermic and proceeds nearly without activation. Steepest-descent reaction pathways on the potential-energy surface for the abstraction reaction are presented both at the five-electron, five-orbital complete-active-space self-consistent-field level, and at the single- and double-substitution configuration-interaction level. Multireference configuration-interaction calculations are reported for several points along each of the reaction pathways. All electronic structure calculations employ a correlation consistent polarized valence double-zeta basis set. Canonical variational transition-state-theory calculations using the calculated potential-energy surface information predict a rate constant for this reaction that is an order of magnitude greater than that predicted previously by flame experiments. Our calculated rate constant is well represented by the three-parameter expression, k(T)=7.406×10−13T0.720 exp(−0.655/0.001 99T) cm3 molecules−1 s−1 over the range 200–3000 K.