Automated full-dimensional potential energy surface development and quasi-classical dynamics for the HI(X1Σ+) + C2H5 → I(2P3/2) + C2H6 reaction

Abstract

A full-dimensional spin–orbit-corrected analytical coupled-cluster-quality potential energy surface (PES) is developed for the HI(X¹Σ⁺) + C₂H₅ → I(²P_3/2) + C₂H₆ reaction using the ROBOSURFER program package, and a quasi-classical trajectory (QCT) study on the new PES is reported. The stationary-point relative energies obtained on the PES reproduce well the benchmark values. Our simulations show that in the 0.5–40 kcal mol⁻¹ collision energy (E_coll) range, the b = 0 reaction probability, where b denotes the impact parameter, increases first and then stays steady with increasing E_coll, reaching around 10% when E_coll = 5 kcal mol⁻¹. No significant E_coll dependence is observed in the range of 5–40 kcal mol⁻¹. The reaction probabilities decrease monotonically with increasing b, and the maximum b where the reactivity vanishes becomes smaller and smaller as E_coll increases. Scattering angle distributions show a forward scattering preference, indicating the dominance of the direct stripping mechanism, which is more obvious than in the case of HBr + C₂H₅ → Br + C₂H₆. The reaction clearly favors H-side attack over side-on HI and the least-preferred I-side approach, and favors side-on CH₃CH₂ attack marginally over CH₂-side and the least-preferred CH₃-side approach at high E_coll. At low E_coll, however, the dominant effect of H-side attack becomes weaker, while the side-on CH₃CH₂ attack becomes comparable with CH₂-side and the former is a little less favored when E_coll = 0.5 kcal mol⁻¹. It turns out that the initial translational energy is converted 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 none of the reactive trajectories violates the zero-point energy (ZPE) constraint. The energy transfer in the HI + C₂H₅ → I + C₂H₆ reaction is very similar to the case in the HBr + C₂H₅ → Br + C₂H₆ system that we investigated recently.