Computations on quantum-mechanical tunnelling in proton-transfer reactions in solution

Abstract

Experimental data on six proton-transfer reactions are fitted, with the aid of computer programmes, to the theoretical equations for quantum-mechanical tunnelling through parabolic energy barriers (with ΔH°= 0). Values for the widths and heights of energy-barriers calculated for various reactions are compared. A correlation is noted between the ΔpK of the reaction, which is related to the symmetry of the transition state, and the difference in barrier-heights for H⁺- and D⁺-transfer, which is related to the zero-point energies of the vibrations involving H or D in the transition state. When the ΔpK is zero, indicating a symmetrical transition state, the difference in barrier-heights reaches a maximum value of around 1.1 kcal mole^–1, indicating that the zero-point energies in the transition state are small.