Gas-phase cluster ion vibrational spectroscopy of Na+(CH3OH)(2-7)

The vibrational spectra of Na+(CH3OH)(2-7) in the 2.7-3.0 mu m 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 similar to 3515, 3420, and 3350 cm(-1). The onset of hydrogen-bond formation is nominally first observed for Na+(CH3-OH)(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.