Non-equilibrium rate coefficients and isotope effect with bimolecular ion-(polar) molecule reactions in xenon

Using the Fokker-Planck version of an approximate Boltzmann equation for the ion (translational) energy distribution function f_Ithe departure, k, of the non-equilibrium ion-(polar) molecule reaction rate coefficient k_nonfrom its equilibrium value k^(eq) is calculated. k enhances considerably with an increase of the dipole moment of the reacting molecular species (locked dipole reaction model). But the k-values, e.g. for reactions of H^–(D^–) and nitromethane in xenon enhance also with increasing ratio of the concentrations of CH₃NO₂ and Xe and decrease with enhancing gas temperature T. The reaction-induced (translational) non-equilibrium effect leads to a (non-equilibrium) kinetic isotope effect depending on and T. At T=300 K the example yields k_H/k_d=1.345(=5 · 10^–4),=1.409 (=10^–5) and=1.414–k_H^/(eq)/k_D^(eq)(10^–6).