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 chloroacetonitrile 35 Cl − + 37 ClCH 2 CN ⇌ 37 Cl − + 35 ClCH 2 CN 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.