TANDEM MASS-SPECTROMETRY STUDIES OF ACETONE AND ACETONE WATER CLUSTER IONS

Ion clusters were formed in a temperature-variable high-pressure ion source from neat acetone and acetone/water mixtures and subjected to tandem mass spectrometry studies-unimolecular and collisionally activated mass-analyzed ion kinetic energy spectroscopy. The predominance of water loss from H+(H2O)(A)l = 3, where A = acetone, suggests that the solvation sphere around H3O+ does not close at l = 3, contrary to the case of acetonitrile or dimethyl ether. The results may be interpreted in terms of suggested ion structures which involve isomerization enroute to dissociation. The virtual absence of H/D scrambling in the collisionally activated dissociation of H3O+(DA)3, DA = acetone-d6, and of D3O+(A)3 means that if enolization takes place, it is a rate-determining step in an irreversible isomerization. The stability of H+(H2O)(A)3 is a dominant factor in the observation of acetone loss from H+(H2O)(A)4.