Evaluation of Hydration Enthalpies of Monatomic Cations by Considering Both Long-Range and Short-Range Interactions IchiedaNobuyuki KasunoMegumi BanuKhaleda KiharaSorin NakamatsuHirohide 2003 The standard hydration enthalpy, Δ<i>H</i>°<sub>hyd</sub>, of a monatomic cation was calculated as the sum of (1) the enthalpy due to the long-range interaction between a hydrated ion and bulk water, Δ<i>H</i>°<sub>LR</sub>, (2) the enthalpy due to the short-range interaction between the ion and water molecules in the first hydration shell, Δ<i>H</i>°<sub>SR</sub>, and (3) the enthalpy due to the ligand field stabilization of an ion, Δ<i>H</i>°<sub>LF</sub>, which arises for a transition-metal ion. Δ<i>H</i>°<sub>LR</sub> was estimated on the basis of the Born theory assuming the radius of the hydrated ion as the interatomic distance between the ion and the oxygen atom of a water molecule in the first hydration shell,<i> r</i><sub>M</sub><sub>-</sub><sub>O</sub>, determined experimentally. Δ<i>H</i>°<sub>SR</sub> was evaluated on the basis of the donor−acceptor interaction between an ion and a water molecule coordinating to the ion, which was evaluated by the molecular orbital calculation of a monohydrated cluster of an ion combined with the Mulliken population analysis. Δ<i>H</i>°<sub>LF</sub> was calculated on the basis of the crystal field theory. Hydration enthalpies of 48 monatomic cations thus calculated agreed well with those observed experimentally.