Vibrational and unimolecular dissociation of mixed solvent cluster ions: Na+((CH3)(2)CO)(n)(CH3OH)(m)

The competitive solvation of the sodium ion by acetone and methanol has bee n investigated by vibrational spectroscopy of the C-O and O-H stretching mo des of methanol and by unimolecular dissociation of mass-selected cluster i ons using a tandem mass spectrometer. The onset of hydrogen bonding was det ected by substantial shifts in the C-O (+12 to +16 cm(-1)) and O-H (-200 cm (-1)) stretches, as well as by significant increases in the intensity and w idth of the O-H bands. These onsets were observed when a total of five mole cules were present about the ion. The unimolecular dissociation rates of me tastable ion clusters of composition Na+((CH3)(2)CO)(1-9) and Na+((CH3)(2)C O)(1-8)(CH3OH)(1) were also measured using the same experimental apparatus. A significant increase in rate was observed when seven or more acetone mol ecules were present, suggesting a solvent shell size of six. (C) 1998 Elsev ier Science B.V. All rights reserved.