Title : Study of the kinetics of gas-phase reactions within the framework of a non-equilibrium approach
Abstract:
Within the framework of the non-equilibrium approach, an analysis of the kinetics of H-atom transfer reactions in the gas phase was carried out. The main objects of the study are the reactions of methane with methyl and hydroxyl radicals ((energy calculation level CCSD(T)6-311+GB3LYP6-31+G). The study leads to the conclusion about significant differences in the mechanisms of symmetric (first) and asymmetric reactions. Common in both cases is the first stage of the reaction: the movement of the system along the minimum energy path (MEP) due to the translational energy of the reactants to a certain point b. At this point, the energy of the reactants is zero (the stopping point of the reactants). In the case of a symmetric reaction, the second stage of the reaction, tunneling of the H-atom (movement along the tunneling coordinate r, orthogonal to the MEP), in accordance with the generalized Franck – Condon principle (GFCP), occurs in two stages. In the first, a reorganization of the system occurs, leading to symmetrization of the system potential along the coordinate r, and in the second, the actual tunneling of the H-atom in the double-well potential. Unlike in this case, in the asymmetric reaction, the tunneling of the H-atom occurs via a single-stage mechanism, in which the movement of the H-atom is accompanied by a reorganization of the system. As in the first case, the tunneling of the H-atom occurs in a double-well potential at a fixed distance between the donor and acceptor atoms. The second difference between the two reactions concerns the behavior of the system at the moment of reactants collision. In the symmetric reaction, the tunneling of the H-atom occurs directly from point b. In the asymmetric case, due to the oscillation of the bond between the donor and acceptor atoms in the collision complex, further movement of the system along the MEP is possible and, as a consequence, an increase in the tunneling rate. Finally, as shown by the kinetic analysis, the lifetime of the collision complex has a certain influence on the shape of the kinetic dependence in this case. The calculated values of the thermal rate constants make it possible to reproduce the experimental data with good accuracy over the entire studied temperature range. In general, the study confirms the validity of the non-equilibrium approach as an alternative to modern dynamic models for describing the kinetics of H-atom transfer reactions in a gas phase.
