The influence of initial energy on product vibrational distributions and isotopic mass effects in endoergic reactions: the Mg+FH case

Abstract

Extended Quasiclassical Trajectory and quantum Reactive-Infinite Order Sudden calculations were performed on a previously developed potential energy surface to investigate the dynamics of the endoergic (1.33 eV) Mg+FH→MgF+H reaction. The study focused on both the product vibrational distributions and the spectator-atom isotopic mass effect. In particular, their dependence upon varying, over a wide range, both translational and vibrational energy of reactants was investigated in detail. It was found that an increase of the translational energy shifts the maximum of the product vibrational distribution to a higher product vibrational state (v′) when the reactant vibrational state (v) is low. However, the maximum of the product vibrational distribution is shifted to lower v′ values when v is high. At the same time, it was found that the vibrational energy has less influence on the shape of the product distributions than does the translational energy, except when several (four in our case) vibrational quanta are added. In this case, a product vibrational distribution having a vibrational adiabatic-like shape was obtained. At high translational and vibrational energy, collisions were found to be direct enough to allow for the kinematic heavy heavy–light constrictions to largely determine the product vibrational distribution, as confirmed by the analysis of quantum state-to-state opacity functions. Isotopically substituted reactions showed a generally good agreement between quasiclassical and quantum results for all initial v values. Despite that, an unexpected shift of quasiclassical reactive thresholds towards higher translational energies was found for the D and T isotopic variants at low vibrational energies. A rationale for these and other dynamical effects is discussed.