Perturbed equilibria and statistical energy redistribution in a gas-phase S(N)2 reaction

The relative translational energy of one set of reactants in the gas-phase equilibrium between chloride ion and chloroacetonitriie Cl-35(-) + (ClCH2CN)-Cl-37 reversible arrow Cl-37(-) + (ClCH2CN)-Cl-35 is varied without affecting the energy distribution of the other set. The steady-state ratio of ionic isotopes changes so that there is an increase in the concentration of energized reactants. The ratio provides a measure of the translational energy dependence of the rate constant for the bimolecular nucleophilic substitution (S(N)2) reaction. The observed energy dependence is indistinguishable from that predicted by Rice-Ramsperger-Kassel-Marcus theory, suggesting that the increased translational energy redistributes statistically in the collision complex.