Mimicking solvent shells in the gas phase. II. Solvation of K+.

The observed gas-phase coordination number of K+ in K+(H2O)m clusters is smaller than that observed in bulk solution, where the coordination number has been reported to be between 6 and 8. Both theoretical and gas-phase studies of K+(H2O)m cluster ions point to a coordination number closer to 4. In the gas phase, the coordination number is determined by a variety of factors-the most critical being the magnitude of the K+...ligand pairwise interaction. Decreasing the magnitude of the ion...ligand interaction allows more ligands to directly interact with the cation. One method for decreasing the ion...ligand interaction in K+(H2O)m clusters is to systematically substitute weakly bound ligands for the more strongly bound water molecules. The systematic introduction of para-difluorobenzene (DFB) to K+(H2O)m clusters was monitored using infrared photodissociation spectroscopy in the OH stretching region. By varying the ratio of DFB molecules to water molecules present in K+(H2O)m(DFB)n clusters, the observed coordination number of gas-phase K+ was increased to 8, similar to that reported for bulk solution.