Quantum dynamics of the N(4S) + O2 reaction on the X2A' and a4A' surfaces: reaction probabilities, cross sections, rate constants, and product distributions

Abstract

We report real wave packet (WP) calculations of reaction probabilities, cross sections, rate constants, and product distributions of the reaction N(4S) + O2(X3 ) → NO(X2Π) + O(3P). We propagate initial WPs corresponding to several O2 levels, and employ reactant coordinates and a flux method for calculating initial-state-resolved observables, or product coordinates and an asymptotic analysis for calculating state-to-state quantities. Exact or J-shifting calculations are carried out at total angular momentum J=0 or J>0, respectively. We employ the recent X 2A' S3 potential energy surface (PES) by Sayós et al. and the earlier a 4A' PES by Duff et al. In comparing S3 results with the WP ones of a previous X 2A' S2 PES, we find lower S3 energy thresholds and larger S3 probabilities, despite the higher S3 barrier. This finding is due to the different features of the doublet PESs in the reactant and product channels, at the transition state, and in the NO2 equilibrium region. We analyze the effects of the O2 initial level and discuss Cs abstraction or insertion mechanisms. Tunnel effects through the S3 barrier enhance the room-temperature rate constant by ~ 3.7 times with respect to the previous S2 WP rate, thus improving the agreement with the experimental result. The NO vibrational distribution is inverted and the rotational ones are strongly oscillating. We explain these non-statistical results showing that the reaction partners approach each other with a large impact parameter. The WP vibrational distribution is however different from that observed, which is oscillating. WP calculations show that the new S3 PES describes accurately several features of the X 2A' state, although a lowering of its barrier height by ~ 0.56 kcal/mol should bring calculated and observed rate constants in full agreement.