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

The vibrational spectra of Na + (CH 3 OH) 2-7 in the 2.7−3.0 μm region have been measured using a LiNbO 3 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 + (CH 3 OH) 4, with more extensive hydrogen bonding observed for Na + (CH 3 OH) 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 + (CH 3 OH) 1-5 and (CH 3 OH) 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.