New analytical (2A′,4A′) surfaces and theoretical rate constants for the N(4S)+O2 reaction

Abstract

We report two new analytical fits of the ground potential energy surface (PES) (2A′) and the first excited PES (4A′) involved into the title reaction and its reverse, using ab initio electronic structure calculations from Papers I and II along with new grids of ab initio points by means of the second-order perturbation theory on CASSCF wave function [CASPT2 (17,12) G2/aug-cc-pVTZ] reported here (1250 points for the A′2 PES and 910 points for the A′4 PES). Some experimental data were also introduced to better account for the exoergicity and the experimental rate constant at 300 K. The final root-mean-square deviations of the fits were 1.06 and 1.67 kcal/mol for A′2 and the A′4 PESs, respectively, for the NOO Cs abstraction and insertion regions of the PESs. Thermal rate constants were calculated (300–5000 K) for both the direct and reverse reactions by means of the variational transition state theory with the inclusion of a microcanonical optimized multidimensional tunneling correction, obtaining a very good agreement with the experimental data within all the temperature range. The new analytical A′2 PES presents several stationary points not introduced in previous analytical surfaces, and describes accurately the NO2 (X 2A1) minimum, which seems to be very accessible according to the trajectories run in a preliminary quasiclassical trajectory study. The new analytical A′4 PES has a lower energy barrier than the previous one, which increases significantly the contribution of this PES to the total rate constant at high temperatures. Moreover, the new analytical PESs not only describe accurately the Cs regions of the NOO system but also the ONO C2v or near C2v regions.