A semiclassical approach to concerted proton transfer in condensed media

Abstract

Rate expressions are provided for proton transfer reactions in which a proton is transferred from a donor site to a catalyst molecule “in concert with” proton transfer from a different site on the catalyst to a proton acceptor (bifunctional catalysis). The basis of the description is a semiclassical approach according to which the process is represented by three-well potential energy surfaces spanned by low-frequency nuclear co-ordinates. The minima correspond to localization of a proton on the donor, the catalyst and the acceptor. Each vibrational state of the proton causes a vertical shift of the appropriate surface, and within the semiclassical approach both weak and strong intermolecular interactions are covered.

For strong catalysts, corresponding to low energies of the intermediate state, the two protons are transferred separately. In such cases Brønsted relation and kinetic isotope effect variation with pK_a of the catalyst do not differ fundamentally from those of step-wise individual proton transfers. For weak catalysts the two protons are transferred in a “synchronous” motion. The effect of the catalyst is then reflected in the pre-exponential factor of the rate expressions and gives a small Brønsted coefficient and slow variation of the kinetic isotope effect with pK_a of the catalyst.