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 (ΔE_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 CF₂ observed in collision-induced dissociation mass spectra.