A Simple Method To Determine Kinetic Deuterium Isotope Effects Provides Evidence that Proton Transfer to Carbon Proceeds over and Not through the Reaction Barrier

摘要

We report a fast, simple, and general method to determine primary kinetic isotope effects (KIEs) on proton transfer in hydroxylic solvents, and its application to obtain primary deuterium isotope effects on protonation of ring-substituted a-methoxystyrenes X-1 (Scheme 1) over a broad range of temperature and thermody-namic driving force. These data provide evidence that hydron transfer at X-1 proceeds by passing over a reaction barrier, as opposed to tunneling through the barrier and insight into the relationship between the primary kinetic isotope effect and the transition state structure. Proton transfer at carbon is an apparently simple reaction, whose profound importance in chemistry and biology has led to interest in providing a theoretical description of the reaction coordinate profile. Kinetic isotope effects on proton transfer at carbon can be predicted by theory and determined by experiment. Advances in theory and computational power have been incorporated into recent theoretical studies on KIEs, but there has been little effort to extend classic experimental studies using modern experimental methods. Studies of this type are needed to provide a reality check for theory.