Ion–water interaction potentials for alkali metal cations and halide anions

Empirical potential parameters for ion–water interactions have been constructed by three simple methods and successfully used for describing ion hydration. In the first two methods, we have computed the range parameter (ρij) by fixing the energy parameter at a particular value, in a potential of the form EREP =Aij exp(−Rij/ρij). For this purpose, we used either the experimental ion–water distances or one of the experimental enthalpies (ΔH0, 1). In the third method, we have developed a simple correlation scheme by which both the parameters (Aij and ρij) can be computed. Although the parameters obtained by all the three methods seem to describe the gas phase hydration energies equally well, the last method seems to be the best as it allows the computation of both the parameters. These procedures are useful in reproducing the experimental energetics from the experimental ion–water distances and vice versa without requiring parameters taken from other sources. The consistency of one can be checked by the other, independently. The range parameter for the repulsive interaction has been shown to be the most sensitive of all the parameters and hence plays a significant role in deciding the magnitudes of the energies and geometries for the hydration of the ions. Parameters derived from scattering experiments have also been used to describe the gas phase hydration energies but they are found to be unsuccessful in producing consistent results. The major result of the present investigations is that a simple correlation scheme can be developed for describing all interactions. This type of analogy is extremely helpful especially for explaining ion–ion, ion–atom, atom–atom, atom–molecule, molecule–molecule, or ion–molecule interactions.