A simple method to determine kinetic deuterium isotope effects provides evidence that proton transfer to carbon proceeds over and not through the reaction barrier

A fast, simple, simple, and general method is described for determining primary kinetic isotope effects (KIEs) on proton transfer in hydroxylic solvents and its application for obtaining primary deuterium isotope effects on protonation of ring-substituted [alpha]-methoxystyrenes X-1 over a broad range of temperature and thermodynamic driving force. The theoretical calculations that reproduce Bronsted parameters and KIEs have to potential to provide a realistic description of the transition state for the proton reactions.