Model calculations of isotope effects in proton transfer reactions

Abstract

An electrostatic charge-cloud model, including an empirical repulsive potential is used to calculate the properties of initial, final and transition states for the reaction X^–H⁺+ Y^δ–→X^–+ Y^δ–H⁺, where X^– and Y^δ– are spherical charge distributions. The real and imaginary frequencies thus derived are used to calculate hydrogen isotope effects as a function of δ and hence of the energy change in the reaction. It is concluded that the observed variation of isotope effects cannot be accounted for in terms of the real vibrations of the transition state, but are primarily determined by the tunnel correction. The model also accounts for the dependence of isotope effects upon steric hindrance.