The Ionic Hydrogen Bond and Ion Solvation. 2. Solvation of Onium Ions by One to Seven H2O Molecules. Relations between Monomolecular, Specific, and Bulk Hydration

Article abstract of DOI:10.1021/ja00317a016

The relation between enthalpies of solvation of onium ions BH⁺ by one water molecule, -ΔH^o_0.1, and by four water molecules, -ΔH^o_0.4, is constant for most onium ions: ΔH^o_0.4/ΔH^o_0.1 is 2.8 +/- 0.1 for all oxonium ions and monoprotonic ammonium and pyridinium ions, and 3.1 +/- 0.1 for polyprotonic ammonium ions.These relations, in conjunction with the correlation between ΔH^o_0.1 and the proton affinity difference ΔPA = PA(B) - PA(H2O), allow the prediction of the total four-molecule specific hydration energy -ΔH^o_0.4 for all onium ions within the experimental accuracy of +/-3 kcal mol^-1.The observed (or predicted) fourfold specific relative hydration energies simulate closely the relative bulk hydration enthalpies for most ions.In other words, for most onium ions differential hydration effects are determined by the specific hydrogen-bonding interactions.Deviations are useful to identify bulk solvation effects.For example, such deviations indicate attenuated bulk solvation of ions with phenyl substituents.