Ionizing radiation and spin trapping

Much work conducted in low-temperature matrices has shown that the primary chemical process induced by y-irradiation is formation of electrons (e ) and positive holes (h+), the latter eventually leading to the formation of radical cations of the component(s) with the lowest ionization potential (Symons, 1997). This means that an added spin trap may be transformed into its radical cation by y-irradiation and thus create conditions for inverted spin trapping, as already described for PBN and DMPO above in experiments designed to study this aspect. 

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Radical cations of PBN and derivatives were generated photolytically and identified from their ESR spectra.34 The radical cations of PBN, DMPO, and 3,3,5,5-tetramethylpyrroline 1-oxide (TMPO) spin traps were detected by EPR spectroscopy after exposure of dilute solutions to ionizing radiation in dry CFCI3 at 77 K.35,36 The same radical cations were detected using matrices containing water and on melting formed the HO radical adducts. 

Hedrick WR, Webb MD, Zimbrick JD (1982) Spin trapping of reactive uracilyl radicals produced by ionizing radiation in aqueous solutions. Int J Radiat Biol 41 435-442 Heelis PF, Deeble DJ, Kim S-T (1992) Splitting of cis-syn cyclobutane thymine-thymine dimers by radiolysis and its relevance to enzymatic photoreactivation. Int J Radiat Biol 62 137-143 Hems G, Eidinoff ML (1958) Effect of X-irradiation on aqueous solutions of adenosine diphosphate. Radiat Res 9 305-311... 

Color centers can be produced in the alkali metal azide by ultraviolet light and ionizing radiation at low temperatures. The phenomenon has been of interest for some time since the defects produced are involved in the process of photochemical decomposition (cf. Chapter 7). In earlier studies [54a, b, c] purely speculative identifications of optical absorption bands with F, V, and aggregate F centers were made by analogy with the alkali halides. The most prominent visible absorption band in each case was attributed to the F center—a defect involving an electron trapped at an azide (N3) vacancy. In the case of NaNa, spin resonance [55] and recent point ion calculations [56] clearly point to the existence of a F center. However, in the case of KN3, spin-resonance studies [54a] point to the existence of molecular centers of type N2 (on low-temperature irradiation) and NJ (on room-temperature irradiation). Infrared absorptions [57] and Raman scattering [58] have been observed in the irradiated alkali azides, which can be correlated with modes associated with these defects. 

Very primary events in the chemical effect of radiations on matter are excitation and ionization of molecules, which result in the formation of neutral free radicals and radical ions. These reactive species play vital roles in the radiation-induced chemical reactions. As they are paramagnetic with an unpaired electron, electron spin resonance (ESR) spectroscopy has been a useful method for elucidating the mechanism of radiation-induced reactions in solid matter where radical species can be trapped temporarily. Since the early days of the chemical application of ESR, this method has been applied very often to the identification and quantification of free radicals in polymers irradiated by radiation [1]. This is probably because, from the view-point of fundamental research, a variety of free radicals are readily trapped in solid polymers and, from the view-point of applied research, these free radicals have close correlation with radiation-induced crosslinking and degradation of polymers. 

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