Dication−Water Interactions: M2+(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+ (H 2 O) n clusters consisting of an alkaline earth dication (Mg 2+, Ca 2+, Sr 2+, Ba 2+, and Ra 2+ ) 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 Å), 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+ (H 2 O) n clusters. Polarization is largely responsible for the nonclassical bent and pyramidal structures of the di- and trihydrates and for the nonadditive, many-body terms 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.