Spin trapping of radicals formed upon irradiation of organobromine compounds with low-energy X-rays

Abstract

Solutions of several organobromine compounds in methanol or benzene were irradiated with Mo-Kα, β X-rays (Ē= 17.79 keV). In contrast with γ-rays, such low-energy photons interact predominantly (ca. 89–95 %) with bromine causing Auger cascades and eventually the formation of highly excited moieties, which are possible precursors of free radicals. The irradiation was followed by the detection of free radicals through spin-trapping and EPR spectroscopy. Radicals from both the solvent and the solute were identified and quantified. The yield of radicals normalized to unit energy deposited by secondary electrons was studied as a function of the solvent and the compound structure. In benzene, this yield is very low, indicating that the organobromine moieties largely escape fragmentation, since their excess energy is easily channelled into collective excitation of the π-electrons of benzene molecules. In methanol two factors contribute to a more abundant formation of radicals: (a) the lack of a ‘deactivation’ mechanism of excited moieties which now tend to decay more freely, yielding aryl radicals and (b) the in situ radiolysis of methanol, yielding additionally methoxy and hydroxymethyl radicals. The in situ radiolysis follows the high-LET pattern.