SELECTIVE ION SOLVATION IN MIXED-SOLVENTS - VIBRATIONAL SPECTROSCOPY OF CS+[(CH3)2CO]N(CH3OH)M CLUSTER IONS

Solvation of the Cs+ ion in a binary mixture of acetone and methanol i s studied in mixed cluster ions of the form Cs+[(CH3)2CO]N(CH3OH)M. Vi brational predissociation spectra are recorded for mass-selected clust ers with well-known solvent composition. The pure acetone cluster ions Cs+[(CH3)2CO]N do not absorb in the infrared within the range of our CO2 laser, leaving the CO stretch of methanol as the sole chromophore for excitation. Monte Carlo simulations of the heteroclusters are perf ormed using pairwise additive intermolecular potentials. The infrared spectra indicate that a distinct change in cluster structure takes pla ce in Cs+[(CH3)2CO]N(CH3OH)M cluster ions between a size of N = 3, M = 1 and N = 4, M = 1, as evidenced by a large shift in absorption frequ ency. No other significant change in cluster structure is noted throug h a cluster size of N = 12, M = 1. Spectra of clusters with M > 1 indi cate that the methanol molecules occupy spectroscopically distinct sit es. The simulations suggest that the distinct change in cluster struct ure is due to the formation of a hydrogen bond between an acetone and a methanol molecule before the first solvent shell is completely fille d. A comparison of the spectra of the heteroclusters and those of Cs+( CH3OH)N and the (CH3)2CO-CH3OH dimer support this assertion. The sensi tivity of Monte Carlo simulations to the choice of intermolecular pote ntials is discussed.