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 pha se 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 be tween which the proton is transferred are surrounded with four more wa ter molecules and a continuum. Both in the gas phase and in solution t he proton transfer between the two water molecules presents a double-w ell 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 t he mixed discrete-continuum model. The reaction path, within the react ion-solvent equilibrium hypothesis, is nearly coincident for both phas es, and the transition structure hardly changes. Differences between t he pure continuum model and the discrete-continuum model are rationali zed by means of a double effect played by the discrete solvent molecul es: an electrostatic effect equivalent to the continuum model and a ch arge delocalization effect which increases the proton acceptor charact er of the water molecules. By using the discrete model, it is also sho wn that solvent parameters can play an important role in the descripti on of the proton transfer.