Reaction Mechanisms of Gaseous Organic Cations. 20. Reactivity of Ionized 3-Phenylisoxazol-5-(4h)-One

The chemistry of ionized 3-phenylisoxazol-5(4H)-one, which is relatively slow reacting in the gas phase, has been investigated by analysis of the mass-analyzed ion kinetic energy spectrum (MIKES), kinetic energy release, appearance energy, exact mass measurements and D, 15N, and 13C labeling. Appropriate experiments show that the N,O-heterocyclic radical cations under study undergo unimolecular dissociations leading to the formation of benzoyl and formanilinium cations through pathways which involve phenyl migration. PhCO+ fragment ion formation must occur through a reaction channel akin to a thermochemical process. This implies a deep skeletal reorganization similar to an electrophilic substitution onto the aromatic ring and excludes the occurrence of photochemical-like activation giving rise to isoxazole-oxazole ring isomerization. The decomposing ions possess, therefore, only a small excess energy, and have to follow only low-lying reaction channels.