Ring opening reaction of protonated polyfluorinated benzenes in the gas phase. A theoretical MNDO study

The MNDO calculations of protonated polyfluorobenzenes [Ph‐F n ]H + indicate the possibility of a relatively free migration of the hydrogen proton with energy barriers of 125–145 kJ mol −1 . At a higher degree of substitution ( n ) the protonation of the ipso carbon atom occupied by fluorine becomes energetically feasible, along with analogous migrations of fluorine, which, however, are energetically the most advantageous (Δ E a ∼ 230 kJ mol −1 ). In addition to bridged fluoronium ions, relatively stable cyclic intermediates were also found, which make possible a rearrangement to the difluoromethylenecyclopentadienyl cation and thus the elimination of CF 2 observed in collision‐induced dissociation mass spectra.