Proton transfer in mesoscopic, molecular clusters

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Abstract
Proton transfer within a proton–ion complex embedded in a liquid cluster composed of dipolar molecules is studied. The clusters under investigation are large with dimensions in the nanometer regime and contain tens of solvent molecules. Such mesoscopic clusters possess properties that differ from those of either bulk liquids or small clusters containing a few solvent molecules. The free energy along the reaction coordinate for a quantum proton in a classical cluster solvent is computed using path integral techniques and the results are compared with fully classical treatments of the system. The mechanism of proton transfer as reflected in the solvent structure for given values of the reaction coordinate is examined. The solvent structure that exists in the cluster as a result of the surface forces and finite size gives rise to a coupling between the solvent and the proton–ion complex that differs from that in the bulk. This leads to a mechanism of the transfer process that involves gross solvent density shifts and orientational changes distinct from those seen in bulk polar liquids.