Electron Spin Resonance Studies of Semiquinones and the C–F Bond

The electron spin resonance spectrum has been observed for a solution of tetrafluoro p-benzosemiquinone and a value, aF=4.14±0.08 gauss, obtained for the fluorine hyperfine splitting. It is concluded that this splitting is caused by the σ—π configuration interaction proposed for protons attached to an aromatic free radical. The results show that the fluorine orbital in the C–F bond contains about 5% s character. The relationship between the hyperfine splitting constants and the contact contribution to the electron coupling of nuclear spins is commented upon. The electron spin density ρC at a carbon atom in an aromatic free radical is related to aF, for a fluorine bonded to the carbon, by the approximate relation aF≅−40ρC. Data are presented which can be explained by assuming the semiquinone free radicals produced by air oxidation of hydroquinones in alcoholic KOH dimerize or otherwise react in concentrated solution to give radicals with oxygen linkages, while dibenzosemiquinone radicals are produced from quinones under reducing conditions. An X-band spectrometer was used; a brief description is given of it and of an electronic sweep unit for the electromagnet.