Competition between the H-abstraction and the X-abstraction pathways in the HX (X = Br, I) + C2H5 reactions

Abstract

The recently-developed high-level full-dimensional spin–orbit-corrected potential energy surfaces based on ManyHF-UCCSD(T)-F12a/cc-pVDZ-F12 + SO_corr(MRCI-F12+Q(5,3)/cc-pVDZ-F12) (cc-pVDZ-PP-F12 for the Br and I atoms) energy points for the reactions of HX (X = Br, I) with C₂H₅ are improved by adding three to four thousand new geometries with higher energies at the same ab initio level to cover a higher-energy range. Quasi-classical trajectory simulations in the 30–80 kcal mol⁻¹ collision energy range on the new surfaces are performed and show that as collision energy increases, the reaction probability of the submerged-barrier H-abstraction reaction pathway decreases a bit but the reactivity of the X-abstraction reaction, which has an apparent barrier, increases significantly, which leads to the co-domination of the two reaction pathways at high collision energies. The excitation in HX vibrational mode helps both reaction pathways, but more for X-abstraction. The mode-specific excitations in C₂H₅ inhibit the H-abstraction, especially for CH₂ wagging mode, but almost no effect is found for X-abstraction. The deuterium effect is similar for both pathways. The sudden vector projection model can only predict the HX-stretching vibrational enhancements in X-abstraction. Forward/backward scattering is favored for H/X-abstraction, indicating the dominance of the direct stripping/rebound mechanism. The decrease of reactivity for the H-abstraction reaction pathway partly comes from the fact that the H-abstraction is much pickier about the initial attack angle. The reactivity of both reaction pathways increases when side-on CH₃CH₂ attack happens. The major part of the initial translational energy is preserved as translational energy in the products in H-abstraction, while for X-abstraction a large amount of it is transferred into the internal energy of C₂H₅X.