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

Extended Quasiclassical Trajectory and quantum Reactive-Infinite Order Sudd en calculations were performed on a previously developed potential energy s urface to investigate the dynamics nf the endoergic (1.33 eV) Mg + FH --> M gF + H reaction. The study focused on both the product vibrational distribu tions and the spectator-atom isotopic mass effect. In particular, their dep endence upon varying, over a wide range, both translational and vibrational energy of reactants was investigated in detail. It was found that an incre ase of the translational energy shifts the maximum of the product vibration al distribution to a higher product vibrational state (upsilon') when the r eactant vibrational state (upsilon) is low. However, the maximum of the pro duct vibrational distribution is shifted to lower upsilon' values when upsi lon 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 tra nslational energy, except when several (four in our case) vibrational quant a are added. In this case, a product vibrational distribution having a vibr ational adiabatic-like shape was obtained. At high translational and vibrat ional energy, collisions were found to be direct enough to allow for the ki nematic heavy heavy-light constrictions to largely determine the product vi brational distribution, as confirmed by the analysis of quantum state-to-st ate opacity functions. Isotopically substituted reactions showed a generall y good agreement between quasiclassical and quantum results for all initial upsilon values. Despite that, an unexpected shift of quasiclassical reacti ve 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.