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

The competitive solvation of the sodium ion by acetone and methanol has been investigated by vibrational spectroscopy of the C-O and O-H stretching modes of methanol and by unimolecular dissociation of mass-selected cluster ions using a tandem mass spectrometer. The onset of hydrogen bonding was detected 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 width of the O-H bands. These onsets were observed when a total of five molecules were present about the ion. The unimolecular dissociation rates of metastable ion clusters of composition Na+((CH3)(2)CO)(1-9) and Na+((CH3)(2)CO)(1-8)(CH3OH)(1) were also measured using the same experimental apparatus. A significant increase in rate was observed when seven or more acetone molecules were present, suggesting a solvent shell size of six. (C) 1998 Elsevier Science B.V. All rights reserved.