Contributions of C 3 H 6 O +· ions with the oxygen on the middle carbon to gas phase ion chemistry

The numerous ways in which studies of C 3 H 6 O +ċ ions with the oxygen on the second carbon have added to our knowledge of gas phase ion chemistry are reviewed. The enol form of this ion ( 1 ) first attracted interest during early investigations of the mechanism of the McLafferty Rearrangement and later in characterizing the double McLafferty Rearrangement. Next, it was found that 1 isomerizes to the higher energy acetone ion ( 2 ). This discovery sparked studies of the relative stabilities of ionized and neutral enol and ketone species. It also led to the discovery that 1 → 2 surmounts a substantial barrier, and that 2 dissociates faster than the excess energy deposited in it by the isomerization can become randomly distributed; i.e., dissociation following 1 → 2 is nonergodic. This fact is manifested by a more abundant loss of the methyl formed by isomerization and a greater associated translational energy release. Propene oxide and methyl vinyl ether ions also appear to ionize to 2 and to dissociate nonergodically. Methane is eliminated from 2 through a methyl–acetyl ion complex. Characterization of this reaction by photoionization mass spectrometry, ab initio theory, and RRKM calculations helped to establish that complex‐mediated alkane eliminations are generally confined to a region just above threshold. At higher energies, because attractions between ions and nonpolar neutrals are weak, simple dissociation is too fast for complex‐mediated H‐transfer to compete with it. Studies of the collision‐induced dissociations of 2 demonstrate that the first electronically excited A state of 2 is very long‐lived and efficiently releases its energy into translational energy when the ion collides with a neutral at low impact energies. Finally, ion–molecule reactions of 2 and acetone‐containing ion clusters in the gas phase are described. © 2000 John Wiley & Sons, Inc., Mass Spec Rev 19: 38–61, 2000