Formation and structure of dimer radical cations of fluorinated benzenes in solid matrices

Abstract

Structures of dimeric radical cations of fluorinated benzenes (C₆H₅F⁺ and 1,4-C₆H₄F₂⁺) formed by γ-irradiation in solid halocarbon matrices were investigated by means of EPR and DFT calculations. For both the dimer cations, sandwich type structures, where the two benzene rings are symmetrically overlapped, were suggested, and the ¹⁹F-hyperfine coupling constants of the dimeric cations were found to be smaller than half that of the corresponding monomer cations. This was qualitatively explained in terms of the anti-bonding feature of the C–C bond between the two benzene rings and the C–F bond. DFT calculations performed for the monomer and dimer cations successfully account for this phenomenon. Anisotropic ¹⁹F hyperfine coupling (b) was used as a good indicator for evaluating the distance between the two benzene rings. The formation of the dimer cations strongly depends on the matrices and the concentration of the solutes, suggesting that molecular diffusion in the solid systems is an important factor in the dimerization.