The arrangement of first- and second-shell water molecules around metal ions: effects of charge and size

Structural features of clusters involving a metal ion (Li+s, Na+, Be2+, Mg2+, Zn2+, Al3+, or Ti4+) surrounded by a total of 18 water molecules arranged in two or more shells have been studied using density functional theory. Effects of the size and charge of each metal ion on the organization of the surrounding water molecules are compared to those found for a Mg[H2O](6)(2+)center dot[H2O](12) cluster that has the lowest known energy on the Mg(2+)center dot[H2O](18) potential energy surface (Markham et al. in J Phys Chem B 106:5118-5134, 2002). The corresponding clusters with Zn2+ or Al3+ have similar structures. In contrast to this, clusters with a monovalent Li+ or Na+ ion, or with a very small Be2+ ion, differ in their hydrogen-bonding patterns and the coordination number can decrease to four. The tetravalent Ti4+ ionizes one inner-shell water molecule to a hydroxyl group leaving a Ti4+(H2O)(5) (OH-) core, and an H(3)O(+)center dot center dot center dot H2O moiety dissociates from the second shell of water molecules. These observations highlight the influence of cation size and charge on the local structure of hydrated ions, the high-charge cations causing chemical changes and the low-charge cations being less efficient in maintaining the local order of water molecules.