Orientational dependence of the N(4S)+ NO(X 2Π) and N(4S)+ O2(X 3Σ –g) reactions: comparison of the angle-dependent line-of-centres model with quasiclassical trajectories

Abstract

The study of orientational or steric effects in the field of reaction dynamics is a rapidly expanding field both experimentally and theoretically. In this theoretical work, we report the results of a comparison between simple angle-dependent line-of-centres model predictions for the dynamics of the N(⁴S)+ O₂→ NO + O(³P) and N(⁴S)+ NO → N₂+ O(³P) reactions and the results of extensive quasiclassical trajectory calculations on Sorbie–Murrell analytical potential-energy surfaces for both systems (lowest ²A′ and ³A″ surfaces, respectively). The use of a simple bending potential around the saddle-point geometry in the above-mentioned model furnishes in both cases results that compare very well with the quasiclassical trajectory results, although the comparison is not so good in the case of the N(⁴S)+ O₂ reaction. By using a better choice for the angle-dependent barrier height functions only small improvements are achieved. Both surfaces exhibit marked anisotropic behaviours and seem to be capable of reorienting the systems even if they start with rather unfavourable geometrical arrangements for reaction, especially in the case of the N(⁴S)+ NO reaction. It is concluded that this model could be a good and computationally cheap way of providing an a priori estimate of orientational effects on the reaction cross-sections and opacity functions of reactions similar to the ones studied here, by using a simple angle-dependent barrier height function based on a few ab initio points, even for bent transition states.