Gas-phase methanol solvation of Cs+ : Vibrational spectroscopy and Monte Carlo simulation

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Abstract
The solvation of the cesium ion by methanol has been investigated by gas‐phase vibrational spectroscopy and Monte Carlo simulations of small ion clusters: Cs(CH3OH)+N, N=4–25. The solvated ions, generated by thermionic emission and a molecular‐beam source, have considerable amounts of internal energy. After excessive energy is dissipated by evaporation, quasistable cluster ions are mass‐selected for vibrational predissociation spectroscopy using a line‐tunable cw‐CO2 laser. Analysis of the vibrational spectra indicates that the first solvation shell about the cesium ion consists of ten methanol molecules. Larger Cs(CH3OH)+N (N>18) appear to have small clusters of methanol bound to the surface of a solvated ion. Monte Carlo simulations using pairwise interaction potentials at 200, 250, and 300 K have been performed on Cs(CH3OH)+N, N=6–16 and 25. The results from the simulations are consistent with the observed solvent shell size and suggest a significant role for hydrogen bonding in the larger solvated ions (N≥10). Once the first solvation shell is filled, the size of the solvent shell appears to be independent of the number of additional solvent molecules. Gas‐phase solvated ions appear to be extremely useful models for dilute electrolyte solutions.