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

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 often methanol molecules. Larger Cs(CH ₃OH)_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.