DICATION-WATER INTERACTIONS - M(2-EARTH METALS M=MG,CA,SR,BA, AND RA()(H2O)(N) CLUSTERS FOR ALKALINE)

Gas-phase structures, binding energies, and enthalpies are reported fo r small M(2+)(H2O)(n) clusters consisting of an alkaline earth dicatio n (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 eff ective core potentials for the heavier alkaline earth metals). The wat er molecules in these clusters coordinate the dications in highly symm etric arrangements that tend to enhance electrostatic charge-dipole in teractions while minimizing ligand-ligand repulsions. Comparisons of t he calculated structures and binding energies to higher level treatmen ts reveal fairly reasonable agreement. The optimized M-O distances are slightly long (by 0.02-0.03 Angstrom), and binding energies are somew hat weak (by 1-3 kcal mol(-1) per ligand). Natural energy decompositio n analysis emphasizes the importance of polarization effects in the M( 2+)(H2O)(n) clusters. Polarization is largely responsible for the nonc lassical 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 i nteractions in more extended systems (such as the ion-selective bindin g of crown ethers) for which calculations at higher levels of theory a re not currently feasible.