SITE OF PROTONATION OF FLUOROBENZENES BY GASEOUS BRONSTED ACIDS

The protonated fluoro-, difluoro- and trifluoro-benzenes fragment in l ow energy collision-induced dissociation (CID), in part, by eliminatio n of HF. With D3O+ as the reactant ion the MD+ ions formed fragment by loss of HF and DF in a ratio consistent with D+ addition to the aroma tic ring and equilibration of the added deuterium with the ring hydrog ens prior to fragmentation with an isotope effect favouring HF elimina tion over DF elimination. With CO2D+ and CD5+/C2D5+ as the reagent sys tems, considerable enhancement of DF elimination is observed, particul arly at low collision energies. This result is rationalized by direct D+ addition to the fluorine and fragmentation of the fluorine-deuterat ed species by specific elimination of DF, in addition to deuteration o f the ring, as was observed with D3O+ reagent. The results indicate ab out 12-15% fluorine deuteration of fluorobenzene in the stable ions sa mpled at 250-degrees-C ion source temperature. The percentage of stabl e fluorine-deuterated species increases markedly with decreasing ion s ource temperature. The extent of fluorine deuteration in the stable MD + ions is greater for the di- and tri-fluorobenzenes, reaching about 5 0% for o-difluorobenzene and the trifluorobenzenes with CO2D+ as reage nt. Substantial differences also are observed in the high energy CID m ass spectra with changing reagent ion, consistent with significant flu orine deuteration when thermochemically possible. In high energy CID, the fluorine-deuterated species fragments by elimination of DF while t he ring-deuterated species fragment preferentially by elimination of H , H-2 and deuterium-containing equivalents. It is concluded that the p rotonation of fluorobenzenes by Bronsted acids under CI conditions is kinetically controlled and involves interaction of the reagent ion wit h the local C-F bond dipole.