Interplay between ionic and hydrogen-bond interactions in gas phase cluster ions

The interactions between ions and hydrogen-bonding solvents have been characterized through the study of gas phase cluster ions. Systems including Cs+(CH3OH)(1-6), Na+(CH3OH)(2-7), Cs+(H2O)(1-8), and Cl-(CH3OH)(1-8), have been prepared in an ion cluster apparatus equipped with a triple quadrupole mass spectrometer for size-selection and analysis. A tunable infrared laser is used to excite the O-H stretching modes in the solvents, leading to the vibrational predissociation of the cluster ion. The effects of charge density, solvent type and ion polarity on the onset and extent of hydrogen bonding have been measured as a function of the number of solvent molecules. As the charge density is increased, the onset of hydrogen bonding is delayed to larger cluster sizes, reflecting the ability of smaller ions to disrupt the solvent structure. The solvation of cesium by water and methanol appeared to be identical in regards to the onset and types of hydrogen-bonded structures as a function of size. Multiple structural isomers were observed at a given cluster size once hydrogen bond formation occurred. Rotational structure in the form of resolvable P and R branches for the symmetric O-H stretch and K sub-bands for the asymmetric stretch in H2O, were observed for Cs+(H2O)(1,2). A significant enhancement of the symmetric stretch was also observed for water molecules that did not participate as hydrogen-bonded donors or accepters in Cs+(H2O)(1-5). The solvation of the chloride ion by methanol was completely different from any of the cationic systems. There appears to be only partial coordination of the ion by three solvent molecules, with the ion residing on the surface of the solvent cluster. The rich variety of structures characterized in these experimental studies should prove invaluable in the development of interaction potentials used to describe hydrogen bonds.