Title : Study of hydrogen transfer reactions within the framework of the non-equilibrium approach
Abstract:
The kinetics of H-atom transfer reactions of methane with methyl and hydroxyl radicals in the gas phase was studied within the framework of the non-equilibrium approach (energy calculation level CCSD(T)/6-311++G**//B3LYP/6-31G**). Analysis of experimental data leads to the conclusion that there is a significant difference 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 subsequent tunneling of the H-atom from point b, in accordance with the generalized Franck–Condon principle (GFCP), occurs in two stages: reorganization and the H-atom transfer. In the reorganization stage, the tunneling coordinate r is used as the reaction coordinate (in the equilibrium approximation); the latter is assigned the symbol ρ. The actual tunneling of the H-atom occurs in the symmetric double-well potential V(r; Q,ρ*), where Q is the distance between the C-atoms of methane and the methyl radical and ρ* is the position of the non-equilibrium activated complex on the structural coordinate ρ. In contrast, in the case of an asymmetric reaction of methane with a hydroxyl radical, a one-stage tunneling mechanism in the potential V(r;Q) is assumed, in which the process of transition of the H-atom is accompanied by a reorganization of the system. 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.
