Hydrogen bonding in metal ion solvation: vibrational spectroscopy of Cs +(CH 3OH) 1–6 in the 2.8 μm region

Abstract

The vibrational spectroscopy of cesium ions solvated by methanol has been obtained in the 2.8 μm region, using a tunable infrared laser and a triple quadrupole mass spectrometer. The cluster ions are generated by the collision between the ion and a preformed methanol cluster. The resulting nascent ion cluster is stabilized by evaporative cooling prior to spectroscopic study. The vibrational spectra of the OH stretch, in Cs +(CH 3OH) 1–6, reveal the onset of hydrogen-bond formation with as few as three molecules in the first solvent shell, for a subset of the mass-selected cluster ions. Prominent infrared features have been observed near 3665 cm −1, for methanol molecules individually complexed to the ion, and near 3520, 3415 and 3345 cm −1, corresponding to methanols complexed to the ion and participating in hydrogen bonds. Structural interpretations of these features are made by comparison with vibrational spectra of neutral methanol clusters. There is clear evidence that more than one structural isomer exists for clusters with three to five solvent molecules. The variations in the OH stretching frequency contain far more structural information than previous spectroscopic studies of the CO stretch. Whereas the former experiments revealed the size of the solvent shell and differences between the environments in the solvent shells, structural variations within the first solvent shell are now observable. The balance between electrostatic interactions (ion-solvent) and hydrogen bonding (solvent-solvent) can be examined at the molecular level, which should be extremely useful in developing accurate new, computationally efficient, interaction potentials.