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 +(H 2O)(A) l=3 , where A acetone, suggests that the solvation sphere around H 3O + 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 collissionally activated dissociation of H 3O +(DA) 3, DA acetone- d 6, and of D 3O +(A) 3 means that if enolization takes place, it is a rate-determining step in an irreversible isomerization. The stability of H +(H 2O)(A) 3 is a dominant factor in the observation of acetone loss from H +(H 2O)(A) 4.
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