Full-dimensional potential energy surface development and dynamics for the HBr + C2H5 → Br(2P3/2) + C2H6 reaction.

Abstract

We report a full-dimensional spin-orbit-corrected analytical potential energy surface (PES) for the HBr + C2H5 → Br + C2H6 reaction and a quasi-classical dynamics study on the new PES. For the PES development, the ROBOSURFER program package is applied and the ManyHF-based UCCSD(T)-F12a/cc-pVDZ-F12(-PP) energy points are fitted using the permutationally-invariant monomial symmetrization approach. The spin-orbit coupling at the level of MRCI-F12+Q(5,3)/cc-pVDZ-F12(-PP) is taken into account, since it has a significant effect in the exit channel of this reaction. Our simulations show that in the 1-40 kcal mol-1 collision energy (Ecoll) range the b = 0 reaction probability increases first and then decreases with increasing Ecoll, reaching around 15% at the medium Ecoll. No significant Ecoll dependence is observed in the range of 5-20 kcal mol-1. The reaction probabilities decrease monotonically with increasing b and the maximum b where reactivity vanishes is smaller and smaller as Ecoll increases. Unlike in the case of HBr + CH3, the integral cross-section decays sharply as Ecoll changes from 5 to 1 kcal mol-1. Scattering angle distributions usually show forward scattering preference, indicating the dominance of the direct stripping mechanism. The reaction clearly favors H-side attack over side-on HBr and the least-preferred Br-side approach, and favors side-on CH3CH2 attack over the CH2-side and the least-preferred CH3-side approach. The initial translational energy turns out to convert mostly into product recoil, whereas the reaction energy excites the C2H6 vibration. The vibrational and rotational distributions of the C2H6 product slightly blue-shift as Ecoll increases, and very few reactive trajectories violate zero-point energy.