EPR spectra and structure of the radical cations of fluorinated benzenes

EPR spectra for the radical cations of a series of fluorinated benzenes, generated by irradiation with γ-rays in halocarbon solid matrices, have been observed at low temperatures. The spectra consist of a hyperfine structure with axially symmetric anisotropy mainly due to fluorine nuclei. The observed spectra have been analysed by simulation. Ab initio calculations have been conducted for the cation radicals to obtain their optimized geometries. The results reveal that an unambiguous deformation in geometry is brought about by cationization in each case. INDO calculations have been performed for the optimized geometries of these radical cations to calculate the hyperfine couplings. The calculated couplings strongly support the observed ones. The symmetry of the SOMO for the radical cations resembles that of the HOMO of their neutral mother molecules. The deformed geometries of these radical cations suggests that in the process of releasing an electron from an HOMO, those chemical bonds with bonding nature in the HOMO become elongated and those bonds with antibonding nature become shortened. It is concluded that the structure and symmetry of the SOMO of these radical cations are affected not only by the number of substitutions by fluorine but also by the position of substitution.