Radical addition to HNO. Ab initio reaction path and variational transition state theory calculations for H+HNO→H2NO and H+HNO→HNOH

Abstract

Features of the potential energy surface for hydrogen atom addition to both the nitrogen atom and the oxygen atom of HNO have been investigated at high levels of ab initio theory. For both reactions, vibrational frequencies, moments of inertia, and energies were determined along steepest descent paths at the multiconfigurational self-consistent field level of theory, correlating all valence electrons except the 2s electrons on oxygen (complete active space multiconfigurational self-consistent field distributing 11 electrons among 9 active orbitals). Energies were refined along these reaction pathways using large-scale multireference configuration interaction calculations (all single and double excitations from all configurations generated by a five-electron-in-four-orbital reference space) and large basis sets (correlation-consistent polarized valence triple zeta). The calculated barriers for H-atom addition to the nitrogen atom and the oxygen atom of HNO are 2.68 and 8.99 kcal/mol, respectively. Because of the relatively large barrier for addition to the oxygen atom of HNO, this channel cannot compete with addition to the nitrogen to form the H2NO radical. However, the HNOH radical can likely be formed indirectly by a 1,2 hydrogen migration. Variational transition state theory rate calculations performed over a wide range of temperatures using the ab initio potential energy surface information as input show a substantial variational effect for the calculated association rate constant. The rate of H+HNO association to form a radical complex is slower than the rate of hydrogen atom abstraction at all temperatures; by a factor of more than 20 at room temperature to about a factor of 3 at 2000 K.