An ESR Study of the 4,4′-Difluorobiphenyl Cation Radical

The ESR spectra of the 4,4′-difluorobiphenyl cation show resolved second-order splittings and a marked line-broadening attributable to the modulation of the 19F anisotropic hyperfine interactions by the tumbling motion. This dipolar broadening is mainly observed for the lines associated with the triplet state (I=1) of the two equivalent 19F nuclei. When the temperature is lowered, the wings of the triplet due to the two 19F and the downfield second-order components become broad and finally disappear. The dipolar contributions to the line widths are calculated as a function of the correlation time for the molecular motion, τ2. Furthermore, the ESR observation of the 13C hyperfine structures is carried out, and the spin densities on each ring carbon and the fluorine atoms are experimentally determined. From the spin densities thus determined, the parameters in the equation a_F=Q_CF^Fρ_C^π+Q_F(FC)^Fρ_F^π, in which the 19F hyperfine coupling constant is correlated to the π spin densities on the fluorine atom and the adjacent carbon atom, are estimated to be QCFF=39 and QF(FC)F=324 G.