Oxygen and Hydrogen Isotopic Preference in Hydration Spheres of Chloride and Sulfate Ions

Abstract

Abstract Molecular orbital calculations were performed to estimate the 18O/16O and D/H isotopic reduced partition function ratios (RPFRs) of water molecules around chloride and sulfate ions. Cl-(H2O) n and SO4 2-(H2O) n clusters with n up to 120 were considered as models of aqueous solution containing those anions. The calculations indicated that the decreasing order of the 18O preference over 16O in the primary hydration sphere is: (bulk water) > SO4 2- > Cl-. That is, water molecules in the primary hydration spheres of those anions are expected to be depleted in the heavier isotope of oxygen relative to water molecules in bulk, and the degree of the depletion is larger for the chloride ion than for the sulfate ion. Similar tendency was also observed for the isotope preference of hydrogen. No such isotopic preference was observed either for oxygen or for hydrogen in the secondary and outer hydration spheres.