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 (E_coll) range the b = 0 reaction probability increases first and then decreases with increasing E_coll, reaching around 15% at the medium E_coll. No significant E_coll 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 E_coll increases. Unlike in the case of HBr + CH₃, the integral cross-section decays sharply as E_coll 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 E_coll increases, and very few reactive trajectories violate zero-point energy.