Gas-Phase Cluster Ion Vibrational Spectroscopy of Na+(CH3OH)2-7

Abstract

The vibrational spectra of Na+(CH3OH)2-7 in the 2.7−3.0 μm region have been measured using a LiNbO3 optical parametric oscillator and a triple-quadrupole molecular beam apparatus. The cluster ions are formed by the impact of a sodium ion with a preformed methanol cluster. Frequency shifts of the methanol O−H stretch are used to identify both the onset of hydrogen-bond formation and the nature of the hydrogen-bonded species in the cluster ions, permitting the microscopic characterization of the solvation process. Methanol molecules associated with the cluster ion, but not acting as proton donors in a hydrogen bond, absorb in the 3660−70 cm-1 region, while bands due to hydrogen-bonded species have been observed at ∼3515, 3420, and 3350 cm-1. The onset of hydrogen-bond formation is nominally first observed for Na+(CH3OH)4, with more extensive hydrogen bonding observed for Na+(CH3OH)5-7. An assessment of the influence of the sodium ion on the methanol vibrational frequencies can be made through comparison with the spectra of Cs+(CH3OH)1-5 and (CH3OH)2-3. The effect of the electrostatic interaction, between either the sodium or cesium ion and the methanol solvent, is primarily reflected by the onset and extent of hydrogen-bond formation as opposed to the magnitude of vibrational spectroscopic shifts.