Low-temperature chemical reactions. Effect of symmetrically coupled vibrations in collinear exchange reactions

Abstract

The problem of fluctuational barrier preparation for a collinear exchange reaction is considered in the Leggett formalism for arbitrary temperature, frequencies and coupling parameters γ j of the reaction coordinate with intermolecular vibrations. The transition probability w is shown to increase and decrease due to symmetrically and antisymmetrically coupled vibrations, respectively. When both effects are combined the symmetrically coupled vibrations suppress the antisymmetrically coupled ones. As an example of the separation of vibrations by their symmetry type the vibrations of a one-dimensional harmonic chain containing the reactants are studied. Consideration of the many-dimensional motion allows us to explain the main characteristic features of solid state cryochemical reactions, such as the low activation energies (despite the large barriers at equilibrium Van der Waals distances between reactants), the strong isotopic H/D effect that occurs at the same time as a weaker dependence of w on mass for heavier particle transfer, and the connection of cross-over temperature with symmetrically coupled vibration frequencies.