<sup>18</sup>O/<sup>16</sup>O and D/H Isotopic Preference in Hydration Spheres of Alkali Metal Ions

With the final goal set at theoretical elucidation of experimentally observed isotope salt effects, molecular orbital calculations were performed to estimate the ¹⁸O/¹⁶O and D/H isotopic reduced partition function ratios (RPFRs) of water molecules around lithium, sodium, and potassium ions. As model water molecules in the ith hydration sphere of the cation in aqueous solutions containing that cation, we considered water molecules in the ith hydration sphere that were surrounded by water molecules in the (i + 1)th hydration sphere in clusters, M⁺ (H₂O)_n (M = Li, Na or K; n up to 100). The calculations indicated that the decreasing order of the ¹⁸O preference over ¹⁶O in the primary hydration sphere is: Li⁺>; (bulk water) ≥ Na⁺>; K⁺. That is, water molecules in the primary hydration spheres of the Li⁺, Na⁺, and K⁺ ions are, respectively, enriched, slightly depleted, and depleted in the heavier isotope of oxygen relative to water molecules in bulk. No such preference was observed for hydrogen isotopes in any hydration sphere or for oxygen isotopes in the secondary and outer hydration spheres.