The cluster-pair approximation (CPA) utilizes cluster and bulk thermodynamic data in estimating the hydration enthalpy and free energy of the proton. Here the CPA is analyzed from a quasi-chemical theory (QCT) perspective. The statistical mechanical analysis shows that a key assumption of the CPA leaves open the possibility of shifts between the proton values obtained by extrapolation of small cluster data and the actual large-system values. Numerical tests of the CPA are performed using the QCT, polarizable classical simulations (AMOEBA model), and quantum chemical calculations. The results indicate a small shift in the proton hydration free energy and a larger shift in the enthalpy relative to the previously accepted values. The resulting hydration entropy is consistent with the Marcus value. The electrochemcial surface potential of water obtained in the analysis is roughly -0.4 V, but it is emphasized this widely discussed quantity can only be uniquely defined for single ions in modest-sized water clusters.

Analysis of the hydration thermodynamics of the proton

248th National Meeting (2014)

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