Interpretation of the gas-phase solvent deuterium kinetic isotope effects in the S(N)2 reaction mechanism: Comparison of theoretical and experimental results in the reaction of microsolvated fluoride ions with methyl halides.

We carried out a comprehensive ab initio calculation and transition-state theory analysis of the solvent and secondary deuterium kinetic isotope effects in the SN2 reactions of microsolvated fluoride ions with methyl halides. Water, methanol, and hydrogen fluoride were used as solvents, and the results are compared with recent experiments. Kinetic isotope effects were dissected into contributions from translations, rotations, and different vibration modes, and the validity of such analysis is also discussed. Excellent agreement was found for some reactions, whereas the agreement was poor for other reactions. We showed that the deviation between theory and experiments is related to the reaction kinetics; a faster reaction produced a kinetic isotope effect that was systematically larger (less inverse) than the calculated value. In addition, we also found that the magnitude of the deviation is proportional to the reaction efficiency. We rationalize the disagreement as a failure of the transition-state theory to model barrierless reactions, and we propose a very simple scheme to interpret these findings and predict the deviation between experimental and theoretical values in those reactions.