Mode selective dynamics and kinetics of the H2 + F2 → H + HF + F reaction

Abstract

The reaction between vibrationally excited H₂ and F₂ had previously been suggested to be a critical chain-branching step in the combustion of mixtures containing H₂ and F₂. In the present study, the vibrational state specific dynamics and kinetics for the reaction H₂ + F₂ → H + HF + F (R1) were investigated by quasiclassical trajectory (QCT) and quantum mechanical (QM) reactive scattering calculations on an accurate potential energy surface that was constructed based on a large number of quantum chemical calculations at the MRCI-F12(CV)+Q/cc-pCVTZ-F12 level. The reaction probabilities for (R1) in collinear configurations were obtained from the QCT and QM calculations, and the state specific rate constants were evaluated by the full-dimensional QCT calculations. Both the collinear and full-dimensional results demonstrated that (R1) can be significantly promoted by vibrational excitation of F₂, whereas excitation of H₂ vibration has a smaller effect on the reactivity. This indicates that the rate constants for the presumed chain-branching reaction, H₂(ν = 1) + F₂ → H + HF + F, used in the previous kinetic modeling study of H₂–F₂ combustion were overestimated. The mode-selective reactivity observed for (R1) was interpreted in terms of the coupling between the vibrational modes of the reactants and the reaction coordinate motion.