Cross sections of the O++H2→OH++H ion-molecule reaction and isotopic variants (D2, HD): Quasiclassical trajectory study and comparison with experiments

Accurate global potential energy surface for SiH2 +(X2A1) and quantum dynamics of related reaction H(2S) + SiH+(X1Σ+)

With the many-body expansion method, an accurate global potential energy surface (PES) is constructed for SiH₂ ⁺(X²A₁) by mapping 4762 ab initio energy points calculated on the multireference configuration interaction level including Davidson corrections with aug-cc-pV6Z Dunning's basis set. The dissociation energies and equilibrium geometries of SiH⁺(X¹Σ⁺) and H₂(X¹Σ_g ⁺) agree well with the experimental results. The topographical characteristics of all stationary points for the SiH₂ ⁺(X²A₁) PES are discussed in detail and compared with other theoretical and experimental results. In order to verify the validity and usability of the present PES, the dynamics calculations based on the Chebyshev quantum wave packet method are performed for the H(S2)+SiH⁺(X¹Σ⁺)→Si⁺(P2)+H₂(X¹Σ_g ⁺) reaction. The probabilities, the total integral cross sections, and the rate constants are computed, and the analogies with the corresponding ones of reaction H(S2) + CH⁺(X¹Σ⁺)→C⁺(P2) + H₂(X¹Σ_g ⁺) are also made. The reasonable dynamical behavior throughout the entire configuration space indicates that the PES is suitable for relevant dynamics investigations and serves as a building block for constructing the PES of larger molecular systems containing Si⁺/H.

A new potential energy surface of the OH2+ system and state-to-state quantum dynamics studies of the O+ + H2 reaction

A new global potential energy surface of the O⁺ + H₂ system was constructed with neural network method, using about 63000 ab initio points, which were calculated by employing the multi-reference configuration interaction method with aug-cc-pVTZ and aug-cc-pVQZ basis sets.

Competition between the H- and D-atom transfer channels in the H2O+ + HD reaction: reduced-dimensional quantum and quasi-classical studies

The ion-molecule reaction between a water cation and a hydrogen molecule has recently attracted considerable interest due to its importance in astrochemistry. In this work, the intramolecular isotope effect of the H₂O⁺ + HD reaction is investigated using a seven-dimensional initial state-selected time-dependent wave packet approach as well as a full-dimensional quasi-classical trajectory method on a full-dimensional ab initio global potential energy surface. The calculated branching ratios for the formation of H₃O⁺ and H₂DO⁺via H- and D-transfer agree reasonably well with the experimental values. The preference to the formation of the H₃O⁺ product observed using the experiment at low collision energies is reproduced by theoretical calculations and explained by a one-dimensional effective potential model.

Stereodynamics study of the O+ + D2 (υ = 0, j = 0–2) → OD+ + D reaction

The vector correlations between products and reagents for the ion–molecule reaction O+ + D2 → OD+ + D with different rotational quantum numbers (j = 0, 1, or 2) were explored theoretically using the quasi-classical trajectory method (QCT) on a Martìnez–Millán–González (MMG) surface. The three angular distributions P(θr), P([Formula: see text]), and P(θr,[Formula: see text]), as well as four polarization-dependent differential cross sections (PDDCSs) were calculated. The results indicate that a reagent’s rotational excitation greatly influences both the vector correlations of k–k′, k–j′, and k–k′–j′ and the PDDCSs of the title reaction, which means the reactivity is very sensitive to the rotational quantum number.

Time dependent quantum dynamics study of the O++H2(v=0,j=0)→OH++H ion-molecule reaction and isotopic variants (D2,HD)

The time dependent real wave packet method using the helicity decoupling approximation was used to calculate the cross section evolution with collision energy (excitation function) of the O++H2(v=0,j=0)-->OH++H reaction and its isotopic variants with D2 and HD, using the best available ab initio analytical potential energy surface. The comparison of the calculated excitation functions with exact quantum results and experimental data showed that the present quantum dynamics approach is a very useful tool for the study of the selected and related systems, in a quite wide collision energy interval (approximately 0.0-1.1 eV), involving a much lower computational cost than the quantum exact methods and without a significant loss of accuracy in the cross sections.

Exact quantum dynamics study of the O(+)+H2(v=0,j=0)-->OH(+)+H ion-molecule reaction and comparison with quasiclassical trajectory calculations

The close-coupling hyperspherical (CCH) exact quantum method was used to study the title barrierless reaction up to a collision energy (E(T)) of 0.75 eV, and the results compared with quasiclassical trajectory (QCT) calculations to determine the importance of quantum effects. The CCH integral cross section decreased with E(T) and, although the QCT results were in general quite similar to the CCH ones, they presented a significant deviation from the CCH data within the 0.2-0.6 eV collision energy range, where the QCT method did not correctly describe the reaction probability. A very good accord between both methods was obtained for the OH(+) vibrational distribution, where no inversion of population was found. For the OH(+) rotational distributions, the agreement between the CCH and QCT results was not as good as in the vibrational case, but it was satisfactory in many conditions. The kk(') angular distribution showed a preferential forward character, and the CCH method produced higher forward peaks than the QCT one. All the results were interpreted considering the potential energy surface and plots of a representative sampling of reactive trajectories.