An S N2-Model for Proton Transfer in Hydrogen-Bonded Systems

A new mechanism of proton transfer in donor–acceptor complexes with long hydrogen bonds is suggested. The transition is regarded as totally adiabatic. Two closest water molecules that move synchronously by hindered translation to and from the reaction complex are crucial. The water molecules induce a shift of the proton from the donor to the acceptor with simultaneous breaking/formation of hydrogen bonds between these molecules and the proton donor and acceptor. Expressions for the activation barrier and kinetic hydrogen isotope effect are derived. The general scheme is illustrated with the use of model molecular potentials, and with reference to the excess proton conductivity in aqueous solution.