Potential energy surfaces and quasiclassical trajectory study of the O + H2+ → OH+ + H, OH + H+ proton and hydrogen atom transfer reactions and isotopic variants (D2+, HD+)

Abstract

The rate constants (k; T: 200–900 K) and cross-sections (σ; E_col: 0.010–0.50 eV) of the O + H₂⁺ → OH⁺ + H (1), OH + H⁺ (2) reactions, which occur on the ground (1²A′′) and first excited (1²A′) potential energy surfaces (PESs), respectively, were investigated for the first time, considering also the rate constants for D₂⁺ and HD⁺. Ab initio multireference configuration interaction calculations were performed on both barrierless PESs (where the minimum energy path involves the insertion of the O atom into the middle of the H₂⁺ bond), and suitable analytical expressions were developed for the first time and used in quasiclassical trajectory (QCT) calculations. k(1) ≈ 3k(2) independent of the isotopic variant, k(H₂⁺) > k(HD⁺) > k(D₂⁺) for reactions (1) and (2), and the intermolecular and intramolecular isotopic effects are essentially independent of T. Comparison with the Langevin–Gioumousis–Stevenson (LGS) simple capture model shows that these results are similar to the QCT ones, especially for reaction (2); and the isotopic effects are coincident with the QCT ones for both reactions. For O + H₂⁺, σ(1) ≈ 3σ(2) at E_col ≤ 0.10 eV, and σ(1) = 1.5σ(2) at 0.40 and 0.50 eV. The larger value of σ(1²A′′) with respect to σ(1²A′) arises from the larger value of b_max(1²A′′) with respect to b_max(1²A′), and this results from the more attractive character of the former PES. Besides, the reaction probabilities are quite large [0.78–0.98 (1²A′′) and 0.78–0.93 (1²A′)], and the decreasing trend of both cross-sections as E_col increases arises from the barrierless character of both PESs. We expect that these results (in particular, the competition between proton transfer and hydrogen atom transfer) will encourage experimentalists to carry out investigations on this interesting reaction.