The formation of iminoxyl and nitroxyl radicals frompara-quinonedioxime

Abstract

The formation of iminoxyl radicals in oxidation of oximes is described in the literature, i -6 We established that solid samples of commercially available paraquinonedioxime reprecipitated from alcohol (methanol or ethanol) give a weak ESR signal with a g-factor of 2.0052, which was primarily assigned to dopant paramagnetic centers. However, an attempt to purify paraquinonedioxime more carefully by vacuum (i0 -2 Tort) sublimation at 60 ~ resulted in quite unexpected phenomena: the intensity of the signal increased sharply to 2.8" 1018 sp/g, and the form of the signal corresponding to triaxial anisotropy became pronounced: gl = 2.0076, g2 = 2.0048, g3 = 2.002t, and a N = 2.6 mT. These parameters are close to those presented for iminoxyl radicals in the solid phase obtained by v-irradiation of polycrystalline oximes. 4 Double sublimation resulted in a subsequent 20-fotd increase in the intensity of the ESR signal. An attempt to perform the third sublimation resulted in the sell-ignition of the sample. The ESR spectrum of a solution of para-quinonedioxime in DMSO exhibits a nitrogen triplet with the constant a~ = 1.07 mT and g = 2.0059, which is typical of aromatic nitroxyl radicals. 7 Calculated per gram of the substance, the concentration of the radicals in a solt, tion of DMSO is twice as high as that in the solid sample. We suppose that the existence of iminoxyl radicals in para-quinonedioxime can be related to its solid-phase oxidation by air oxygen. The amount of the iminoxyl radicals formed can be accumulated only to a certain concentration below which a considerable removal of the radicals from each other still prevents their disproportionation. As indicated above, triple vacuum sublimation is accompanied by ignition of the sample. An attempt to sublime para-quinonedioxime in air also results in its ignition. Perhaps this is associated with the formation of energetically unfavorable iminoxyl radicals in a sufficiently high concentration. The disproportionation of these radicals is accompanied by the release of a great amount of heat. It is most probable that DMSO also oxidizes para-quinonedioxime to iminoxyl radicals, which then add to neutral molecules to tbrrn nitroxyl radicals of the supposed structure (1). This results in the fact that the concentration of the radicals in solution calculated per gram of the substance is somewhat higher than that in the solid state.