D/H and 18O/16O Isotopic Reduced Partition Function Ratios ofWater Molecules around a Sodium Ion

Abstract

With the final goal set at theoretical elucidation of experimentally observed isotope salt effects, molecular orbital calculations were performed to estimate the D/H and 18O/16O isotopic reduced partition function ratios (RPFRs) of water molecules around a sodium ion. As model water molecules in the ith hydration sphere of the sodium ion in sodium ion-bearing aqueous solution, we considered water molecules in the ith hydration sphere that were surrounded by water molecules in the (i+1)th hydration sphere in clusters, Na+(H2O)n (n up to 100). The calculations indicated that the 18O/16O RPFR in the primary hydration sphere is slightly smaller than that of bulk water while the D/H RPFR is practically the same as that of bulk water, and that the influence of the existence of the sodium ion is limited to the primary hydration sphere.