Gas phase solvation of Na+ with methanol

Cluster ions of the form Na+(CH3OH)N, N=3–25 have been studied using experimental and theoretical methods. The cluster ions were prepared in a molecular beam by combining a thermionic alkali ion emitter with a continuous expansion of methanol in argon, and were found to contain a substantial amount of internal energy. A cw CO2 laser was used to record the vibrational spectrum of mass-selected cluster ions in the 1020–1060 cm−1 region. Information on the stepwise solvation of the sodium ion by methanol is gained by comparing changes in the spectra as a function of cluster size. The first solvation sphere of the sodium ion is occupied by six methanol molecules. Further solvent shell structure is seen for N>6, including evidence of methanols resembling ‘‘bulk’’ solvent. The microscopic structure was investigated by Monte Carlo simulations of Na+(CH3OH)N, N=6–24. Radial distribution functions display clear minima that indicate the spatial extent and occupation numbers of solvation shells. No hydrogen bonding takes place between methanol molecules in the first solvent shell but is instrumental in determining the framework of the rest of the cluster ion. The internal energies of the cluster ions were estimated using time-of-flight measurements and calculations within the evaporative ensemble formalism.