Determination of Pi-Electron Spin Densities in Aromatic Free Radicals by Linewidth Studies of Electron Spin Resonance Spectra: The 3,5-Difluoronitrobenzene Anion

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Abstract
A method is proposed for the determination of pi‐electron spin densities in aromatic free radicals which is independent of calculations of sigma—pi interactions. It employs linewidth studies of solution spectra of free radicals and is based on the line broadening arising from electron—nuclear anisotropic intramolecular magnetic—dipole interactions. The method is most readily applied to the hyperfine splittings from the nuclei of atoms which have electrons conjugated with the pi‐electron system, such as carbon, nitrogen, or fluorine, rather than to proton splittings, and yields directly the ratios of spin densities on different atoms. Application of the technique has been made to the determination of the spin density on the fluorine atoms in the 3,5‐difluoronitrobenzene anion radical. The fluorine spin density is found to be 1—%2% of the spin density on the nitrogen atom of the nitro group, and within a factor of about two is given by |ρF|=0.003. Although the signs of the fluorine splitting aF and the fluorine spin density ρF were found to be the same experimentally, the data on this radical alone were not sufficient to determine the sign of either ρF or aF. An examination of the data from this and other investigations, however, indicates that ρF is probably negative. It was concluded that it is not yet possible to formulate a satisfactory account of the fluorine splittings in terms of the pi‐electron spin densities, i.e., sufficient data are not available to evaluate reliably the sigma—pi parameters for fluorine.

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