A theoretical study of trivalent lanthanide ion microsolvation in water clusters from first principles

Although metal ion-water clusters Mm+(H2O)n have been widely studied for many singly charged metal ions, thermodynamic and structural studies of di- or trivalent metal ion-water clusters remain relatively rare. We have investigated the structural and thermodynamic properties of Ln3+(H2O)n clusters (Ln3+ = Nd3+, Eu3+, Er3+ and Yb3+) by means of Monte Carlo simulations using newly-developed, polarizable model potentials parameterized on the basis of ab initio calculations for small clusters. We report total cluster enthalpies and stepwise cluster binding enthalpies predicted by our simulations. Our results also indicate that Ln3+ ions exhibit a well-defined interior solvation shell structure. At small cluster sizes (n = 6-12), the first-shell coordination numbers are close to 6 or 7, whereas convergence towards bulk-like coordination numbers seems to be achieved at cluster size n ≥ 24. In contrast, convergence of the thermodynamic properties towards bulk values only occurs at much larger cluster sizes, n ≥ 64.