Theoretical study of the O(3P) + C2H6 reaction based on a new ab initio-based global potential energy surface

Abstract

A new analytical potential energy surface was developed for the first time for the nine-body O(³P) + C₂H₆ hydrogen abstraction reaction, named PES-2020, which was fitted to explicitly-correlated high-level electronic structure calculations. This surface simulates the topography of the reactive system, from reactants to products, OH(v,j) + C₂H₅. The adiabatic energy of reaction, ΔH_r(0 K) = −2.33 kcal mol⁻¹, reproduces the experimental evidence, and the barrier height, 10.70 kcal mol⁻¹, agrees with the ab initio calculations used as input. In addition, an intermediate complex in the exit channel is observed, which is stabilized with respect to the products of the reaction. Based on PES-2020 a dynamics study was carried out, where quasi-classical trajectory calculations were performed for collision energies in the range of 7.0–60.0 kcal mol⁻¹, which covers high collision energy regions. The reaction cross section increases with collision energy; the largest fraction of available energy is deposited as translational energy (44–66%), and the scattering distribution evolves from backward to forward with collision energy. These findings reproduce previous theoretical calculations using electronic structure calculations of lower levels. However, where these previous studies failed, viz. in rotational and vibrational OH(v,j) distributions, PES-2020 reproduces practically quantitatively the experimental evidence, i.e., cold vibration and rotation, the rotational distribution peaking at j = 1–3 depending on the collision energy. In sum, this behaviour is typical of gas-phase hydrogen abstraction reactions with direct mechanism and high reaction barrier.