Quasiclassical trajectory studies of N+OH, O+NH, and H+NO collisions using global ab initio potential energy surfaces

Abstract

We present a quasiclassical trajectory study of the collisions which occur on the 1A′, 3A″, and 1A″ potential surfaces of HNO using recently developed global potential surfaces that were derived from ab initio calculations. Each of these surfaces was assumed to be uncoupled from the other surfaces for the purposes of the calculations, and the appropriate statistical average of electronic states for each process was calculated. For N+OH and O+NH, we specifically studied reactive collisions which give H+NO as products, and we also studied the production of N+OH from O+NH. Overall rate constants calculated for either N+OH or O+NH are in good agreement with most experiments, and in addition, the product NO vibrational distribution from the N+OH reaction is in good correspondence with recent measurements, revealing modest excitation that is close to what would be expected from a statistical distribution. For O+NH, the calculated NO vibrational distributions are much hotter than statistical, in apparent disagreement with recent measurements. However, a careful analysis of limitations on the measurements due to spectral interference and to collisional relaxation indicates that it is not possible to say if theory and experiment are at odds for this reaction. We find a significant cross section for O+NH→N+OH on the 3A″ surface (roughly 5% of the total reactive cross section, independent of energy), and this leads to rate constants at low temperatures that are orders of magnitude higher at low temperature than estimates made earlier based on H atom abstraction on the 5Π surface. The mechanism of this reaction is found to involve three steps: addition to form HNO, isomerization to HON, and dissociation to produce N+OH. We have also studied nonreactive vibrational and rotational excitation in H+NO collisions, and we obtain distributions that are somewhat closer to experiment than obtained in previous theoretical studies, although there are still points of disagreement. We find that reactive H+NO→N+OH collisions have absolute cross sections and average NO rotational excitation that are in excellent agreement with recent fast H atom measurements.

Chemical Reaction Dynamics Beyond the Born-Oppenheimer Approximation

“…If one lowers the collision energy of the H + NO reactants so they do not have enough energy to encounter the problematic region of the potential energy surface (near the conical intersection) in the entrance channel as they try to surmount the barrier to forming the HNO intermediate, one can still not assume that the reaction will proceed adiabatically. This is because in the O + NH and N + OH exit channels there are regions of the multidimensional energy surface that come energetically close to another adiabat, and the energies of these narrowly avoided crossings are lower than the energy required to surmount the entrance channel barrier (111).…”

Cold collisions of an open-shell S-state atom with a 2Π molecule: N(4S) colliding with OH in a magnetic field

“…V summarizes and concludes the paper. [46][47][48][49][50][51] . This reaction is the major source of the NO radical in the interstellar medium and is one of the key elementary processes in nitrogen chemistry.…”

Accurate time dependent wave packet calculations for the N + OH reaction

“…21) and wave packet (WP) methods. 23 It was found that in the QCT calculations including the electronic degeneracy factor related to the fine structure of reactants, the total rate constant in the studied range of temperature was nearly independent of temperature.…”