CATION WATER INTERACTIONS - THE M(-METALS, M=LI, NA, K, RB, AND CS()(H2O)(N) CLUSTERS FOR ALKALI)

Gas-phase binding energies and enthalpies are reported for small M(+)( H2O)(n) clusters consisting of an alkali metal cation (Li+, Na+, K+, R b+, or Cs+) with one to six water molecules. Ab initio molecular orbit al calculations were performed at the RHF and MP2 levels of theory wit h split valence basis sets (3-21G, 6-3+G with effective core potentia ls for the heavier alkali metals). Comparison with higher level calcul ations and with experimentally measured bond dissociation energies sug gests that the RHF/6-31+G method provides a reasonable description of cation-water interactions in the smallest (n = 1-3) clusters. Larger clusters, particularly those that involve water-water hydrogen-bonding interactions, require a correlated treatment at the MP2 level. This s tudy serves to calibrate the RHF/6-31+G and MP2/6-31+G* methods for a pplications to cation-ligand interactions in more extended systems (e. g., the ion-selective binding of crown ethers and cryptands) for which calculations at higher levels of theory are not currently feasible.