Dication-water interactions: M(2+)(H2O)(n) clusters for alkaline earth metals M=Mg,Ca,Sr,Ba, and Ra

Gas-phase structures, binding energies, and enthalpies are reported for small M(2+)(H2O)(n) clusters consisting of an alkaline earth dication (Mg2+, Ca2+, Sr2+, Ba2+, and Ra2+) with one to six water molecules. Ab initio molecular orbital calculations were performed at the RHF and MP2 levels of theory using split-valence basis sets (6-31+G* with effective core potentials for the heavier alkaline earth metals). The water molecules in these clusters coordinate the dications in highly symmetric arrangements that tend to enhance electrostatic charge-dipole interactions while minimizing ligand-ligand repulsions. Comparisons of the calculated structures and binding energies to higher level treatments reveal fairly reasonable agreement. The optimized M-O distances are slightly long (by 0.02-0.03 Angstrom), and binding energies are somewhat weak (by 1-3 kcal mol(-1) per ligand). Natural energy decomposition analysis emphasizes the importance of polarization effects in the M(2+)(H2O)(n) clusters. Polarization is largely responsible for the nonclassical bent and pyramidal structures of the di- and trihydrates and for the nonadditive, many-body tens that contribute importantly to the binding energies. This study serves, in part, to calibrate the RHF/6-31+G* and MP2/6-31+G* approaches for applications to dication-ligand interactions in more extended systems (such as the ion-selective binding of crown ethers) for which calculations at higher levels of theory are not currently feasible.