Two stable isomers of the benzenium ion are identified by CID MS, the most stable of which appears to be the face-centred TC complex.Arenium ions are important intermediates in electrophilic substitution reactions and have long been of great interest.' The benzenium ion, C6H7+, is the simplest of this class. Although difficult to make in solution Olah et aE.1,2 showed this species to be the CJ complex I1 using NMR spectroscopy. The benzonium form I11 is inferred to be present as the transition state for intramolecular proton transfer around the ring at ca. 40

The author is indebted to Irving Sunshine for his encouragement to proceed with the collection and analysis of the data and to Halle Landesman for her capable technical assistance.

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