Kinetic isotope effect and tunnelling in the proton transfer reaction between 2,4,6-trinitrotoluene and 1,1′,3,3′-tetramethylguanidine in dimethylformamide solvent

Abstract

The proton transfer reaction between 2,4,6-trinitrotoluene and 1,1′,3,3′-tetramethylguanidine in dimethylformamide solvent shows a large primary deuterium isotope effect, kH/kD = 24.3 at 0 °C and 16.9 at 20 °C. The enthalpy of activation difference (ΔHD≠ − ΔHH≠) = 2.6 ± 0.4 kcal mol−1 and the entropy of activation difference (ΔSD≠ − ΔSH≠) = 3.4 ± 1.3 cal mol−1 K−1. This isotope effect, when fitted to Bell's equation, indicates that there is a considerable contribution to this reaction from tunnelling of the proton through the potential energy barrier.