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

Kinetic isotope effects on specific acid-catalyzed protonation of ring-substituted α-methoxystyrenes in 50/50 (v/v) H₂O/D₂O were calculated from the yields of the α-CH₃ and α-CH₂D groups at the ring-substituted acetophenone product, determined by ¹H NMR analysis and using φ_H = 0.69 for fractionation of deuterium between L₂O and L₃O⁺. There is a small change in the kinetic isotope effect from 5.6 for protonation of 4-methoxystyrene to 5.0 for protonation of 3,5-dinitrostyrene as the driving force for proton transfer is changed by 9.0 kcal/mol. The intercepts of Arrhenius-type plots of the product isotope effects for protonation of these two styrenes, (A_H/A_D) = 1.00 and 0.95, are consistent with a semiclassical model for proton transfer in which there is minimal tunneling of the transferred hydron through the reaction barrier at T = 278-359 K.