PROTON-TRANSFER BETWEEN WATER-MOLECULES - A THEORETICAL-STUDY OF SOLVENT EFFECTS USING THE CONTINUUM AND THE DISCRETE CONTINUUM MODELS

Proton transfer between two water molecules both in the gas phase and in solution has been studied at the HF/6-311G** level. For the gas phase we have characterized the reduced potential energy surface. Proton transfer in solution has been modeled with a pure continuum model and with a mixed discrete-continuum model where the two water molecules between which the proton is transferred are surrounded with four more water molecules and a continuum. Both in the gas phase and in solution the proton transfer between the two water molecules presents a double-well potential with a barrier of 0.30 kcal/mol in the gas phase and, in solution, 1.54 kcal/mol in the continuum model and 0.99 kcal/mol in the mixed discrete-continuum model. The reaction path, within the reaction-solvent equilibrium hypothesis, is nearly coincident for both phases, and the transition structure hardly changes. Differences between the pure continuum model and the discrete-continuum model are rationalized by means of a double effect played by the discrete solvent molecules: an electrostatic effect equivalent to the continuum model and a charge delocalization effect which increases the proton acceptor character of the water molecules. By using the discrete model, it is also shown that solvent parameters can play an important role in the description of the proton transfer.