Magnetic Field Dependence of ¹⁹F Nuclear Spin Polarization. The System Tetrafluorobenzoquinone-Tetrafluorohydroquinone

The magnetic field dependence of the ¹⁹F polarization observed in the reaction of tetrafluorobenzoquinone with tetrafluorohydroquinone in benzene has been studied. The results can be interpreted in terms of three different polarization mechanisms. The effects observed at fields below 100 G can be ascribed to a radical-pair mechanism involving S-T_± mixing. At fields of 1–20 kG two different contributions have been identified which can be observed separately under appropriate experimental conditions. One contribution showing a maximum near 4 kG is assigned to a biradical adduct between the triplet quinone and the solvent, benzene. The other contribution shows a monotonical increase with magnetic field strength. This effect is assigned to the triplet-Overhauser mechanism in the reaction of the triplet quinone with the hydroquinone.