Influence of solvation on dissociation: Metastable unimolecular decomposition of mixed ammonia—acetone cluster ions

Reactions of proton-bound dimers

Thermal reactions of proton-bound dimers, (CH3CN)2H (+), (CH3OCH3)2H (+), and (CH3COCH3)2H(+), were studied using a selected ion flow tube. Reactions observed include association, switching, and proton transfer. The association channel was observed only for base molecules that had hydrogen bonding protons such as NH3, CH3NH2, (CH3)2NH, and CH3OH. An association-insertion mechaniSoc was proposed in which the central proton of the symmetrically bound dimers is replaced by a protonated base, for example, NH 4 (+) . These reactions are relatively slow, which demonstrates a central barrier along the potential energy surface. Ether-containing dimers do not demonstrate this insertion reaction, except for diethers, for example, CH3OCH2CH2OCH3, which can form stable bicyclic structures. Dimers such as (HCOOH)2H(+), which possess hydrogen bonding protons in the periphery, undergo switching reactions with ammonia and no insertion.

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(+)(H20)(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-d 6, 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(+)(H20)(A)4.