A method is presented to determine the absolute hydration enthalpy of the proton, ∆H aq °[H + ], from a set of cluster-ion solvation data without the use of extra thermodynamic assumptions. The absolute proton hydration enthalpy has been found to be ∼50 kJ/mol different than traditional values and has been more precisely determined (by about an order of magnitude). Conventional ion solvation properties, based on the standard heat of formation of H + (aq) set to zero, have been devised that may be confusing to the uninitiated but are useful in thermochemical evaluations because they avoid the unnecessary introduction of the larger uncertainties in our knowledge of absolute values. In a similar strategy, we have motivated the need for a reassessment of ∆H aq °[H + ] by the trends with increased clustering in conventional cluster-ion solvation enthalpy differences for pairs of oppositely charged cluster ions. The consequences of particular preferred values for ∆H aq °[H + ] may be evaluated with regard to cluster-ion properties and how they connect to the bulk. While this approach defines the problem and is strongly suggestive of the currently determined proton value, it requires extra thermodynamic assumptions for a definitive determination. Instead, a unique reassessment has been accomplished without extra thermodynamic assumptions, based on the known fraction of bulk absolute solvation enthalpies obtained by pairs of oppositely charged cluster ions at particular cluster sizes. This approach, called the cluster-pair-based approximation for ∆H aq °[H + ], becomes exact for the idealized pair of ions that have obtained the same fraction of their bulk values at the same cluster size. The true value of ∆H aq °[H + ] is revealed by the linear deviations of real pairs of ions from this idealized behavior. Since the approximation becomes exact for a specific pair of oppositely charged ions, the true value of ∆H aq °[H + ] is expected to be commonly shared on plots of the approximation vs the difference in cluster-ion solvation enthalpy for pairs of ions sharing the same number of solvating waters. The common points on such plots determine values of -1150.1 ( 0.9 kJ/mol (esd) for ∆H aq °[H + ] and -1104.5 ( 0.3 kJ/mol (esd) for ∆G aq °[H + ]. The uncertainties (representing only the random errors of the procedure) are smaller than expected because the cluster data of 20 different pairings of oppositely charged ions are folded into the determination.

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A topological enumeration has identified all hydrogen bond arrangements of the (H 2 O) 6 cage, prism, book, chair, and boat frameworks. The 27 chemically distinct H-bond topologies of the cage structure were optimized for geometry and vibrationally analyzed with the PM3 semiempirical method. The structures, which differ only by the arrangement of the H-bonds, have minimized energies falling in a range of 10 kcal/mol and have dipole moments varying from 0 to 11 D. Stability of the structures is correlated with an increase in the number of single donor/single acceptor water (SD-SA) molecules. With structures of the same number of SD-SA water molecules, stability is anticorrelated with the dipole moment. The global minimum-energy structure has been identified and is one of four of particularly stable cage structures related by free hydrogen flipping. The lowest energy structures may interconnect as a result of large-amplitude quantum mechanical motion. The global minimum cage structure is also found to be more stable than the lowest energy, topologically enumerated structures of the prism, book, chair, and boat frameworks.

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