An ESR and ENDOR study of irradiated 6Li-formate

Lithium formate ((6)LiOOCH.H(2)O), 95% (6)Li enrichment, combined with an exchange of crystallization water with D(2)O was investigated. The ESR spectrum of the radiation induced free radicals stable at room temperature consists of a singlet with a narrow line width, 0.92mT. (6)Li has smaller magnetic moment and nuclear spin, which resulted in the narrower line width accompanied with an increase in peak amplitude. In comparison with lithium formate with natural isotopic composition, (6)Li (7.5%, I=1) and (7)Li (92.5%, I=3/2), the sensitivity was increased by a factor of two. With optimised spectrometer settings (6)Li formate had seven times higher sensitivity compared to alanine. Therefore this material is proposed as a dosimeter material in a dose range down to 0.1Gy. The g and the (13)C-hyperfine (hf) tensors of the CO(2)(-) radical anion, major paramagnetic products, were evaluated to be g=(2.0037, 1.9975, 2.0017), and A((13)C)=(465.5, 447.5, 581.3) MHz for polycrystalline samples at room temperature. Furthermore, the (1)H-hf and (6)Li-hf tensors observed for the surroundings of CO(2)(-) by ENDOR technique were in fairly good agreement with DFT calculations. The CO(2)(-) radicals are found to be so stable that the formate is applicable to the ESR dosimetry, because of fully relaxing in a fully relaxed geometrical structure of the CO(2)(-) component and remaining tight binding with the surroundings after the H atom detachment from HCO(2)(-).     

Electron-spin relaxation phenomena in irradiated saccharides detected by pulsed electron paramagnetic resonance spectroscopy

We measured the relaxation times of radicals in saccharides upon γ-irradiation by means of X-band pulsed electron paramagnetic resonance (EPR) spectroscopy. We found that the field-swept signal of irradiated fructose by pulsed EPR showed three to four peaks depending on the dose. The relaxation times (T₁ and T₂) of the side peaks were longer than those of the main peak(s) from each irradiation, indicating that the radicals showing side peaks interact less with the surrounding environment. From relaxation time measurements of several irradiated saccharides, we conclude that T₂ relaxation times decrease with the increasing irradiation dose. In contrast, T₁ relaxation times show no correlation with the irradiation dose.     

ESR spectra of irradiated polyacrylic acid

In view of the interest in acrylic acid co-polymerized with other polymers by radiation to promote enhanced crosslinking, a knowledge of the intermediate reaction products is highly desirable. ESR techniques have been used to study the nature and stability of the radicals formed. Acrylic acid was polymerized by exposure to gamma radiation; the nature of the radicals, their decay at various temperatures, and the effect of inhibitors were investigated.The ESR spectrum at room temperature is similar to that described by earlier workers; its dependence on temperature and impurities, as well as its decay, were investigated. From this work, there is reason to believe that the described triplet also contains a central singlet of ΔH_ms19 G; the addition of impurities to the acrylic acid causes the triplet to decay more rapidly, thus leaving the predominant singlet mentioned above. It was also found that the lines in the outer “wings” of the irradiated specimen disappear on heating; at the same time a new line, possibly a singlet of ΔH_ms131 G, either became more intense or more obvious. Some tentative explanations of this behaviour are advanced.     

ESR study on radical conversions in irradiated low-density polyethylene

ESR studies were carried out on radical conversions by thermal and photochemical mechanisms in low-density polyethylene irradiated mainly with electron beams at ?196°C both in vacuum and in the presence of CO. According to the spectral change, the following radical conversions were elucidated for the samples irradiated in vacuo [eqs. (1) and (1′)] and in the presence of CO [eqs. (2) and (2′)]. From the resemblance of the ESR spectrum observed after direct photolysis of polyethylene to that observed after photo-induced radical conversions of the allylic radicals, it is concluded that an eight-line ESR spectrum observed immediately after photolysis of polyethylene at ?196°C could be attributed more reasonably to the alkyl radicals ? CH₂CHCH₃ than to ? CH₂CH₂ and ? CH₂CHCH₂? .     

ESR study of ascorbic acid irradiated with gamma-rays

The interest in application of high-energy ionizing radiation for stenlization of pharmaceutical products and foodstuffs has led a number of workers to investigate the radiation sensitivity of vitamins. Aside from its use as a vitamin, ascorbic acid (AA) or some derivatives are employed as antioxidants in foodstuffs. The effects of ionizing radiation on AA in simple solutions and in mixture of naturally occurring compounds have been extensively reported in the literature. However, the effects of ionizing radiation on solid AA were reported in few works which described rather dosimetric features of AA No reports, except one, are available describing the characteristic features of the radiolytic intermediates produced after irradiation of polycrystalline AA. Irradiation studies performed on single crystal of AA has led us to reinvestigate our previous work on the radiolytic intermediates produced in irradiated polycrystalline AA. Three radical species, rather than two, having different characteristics were decided contributing to the formation of experimental electron spin resonance (ESR) spectrum of γ-irradiated polycrystalline AA. Spectral parameters of these species were calculated after exhaustive spectrum simulation calculations based on data derived from experimental microwave saturation and dose-response studies.     

Influence of longitudinal relaxation time on double modulation ESR spectra

Extremely narrow lines, observed by the double modulation ESR technique in systems with an inhomogeneously broadened spectrum, are explained in terms of higher-order processes in non-linear conditions. Under proper approximations the shape of these lines is accounted for by the longitudinal rather than the transverse relaxation time.     

ESR study of gamma irradiated 2-methyl nylon3

Gamma radiation-induced radicals of 2-methyl nylon3 (2mN3) have been investigated by electron spin resonance spectrometry. The type of radicals generated, conversion into other radicalic species and their room temperature stability were evaluated. Two types of radicals have been detected. Two different formation mechanisms have been proposed for each radical. As suggested in literature for commercial polyamides, in our system we also observed two different radicals (R1 and R2). Among these radicals, R1 is expected to convert to R2, which is a chemically favorable process.     

ESR study of paramagnetic centres in irradiated platinum complexes

Platinum dichloride complexes with diolefins and phosphines were γ-irradiated at 77K and studied by ESR spectroscopy. Several paramagnetic species were simultaneously produced by electron capture, electron loss and bond homolysis in the pure compounds. An high selectivity in the generation of the reduced species was reached by the irradiation at 77K of the metal complexes in methyltetrahydrofuran solutions. The ESR spectra analysis of the paramagnetic Pt(I) species suggested a SOMO of ?-antibonding character for diolefins complexes and π-out-of-plane character for phosphine complexes.     

An ESR study of free radicals formed in irradiated poly(alpha-methyl styrene) by gamma-radiation

The radiolysis of poly(α-methyl styrene) with γ-radiation has been studied at 77 and 300 K by ESR spectroscopy. The radical intermediates formed at these temperatures have been identified by utilization of the techniques of photobleaching, thermal annealing and simulation. The radical yields were found to be 0.105±0.005 at 77 K and 0.05±0.003 at 298 K.     

Analysis of ESR spectra of fluorocarbon radicals in irradiated tetrafluorethylene

ESR spectra in irradiated tetrafluoroethylene were studied. It was found that during radiolysis at 77 K several types of radicals are formed. The spectra of two types of radicals were established and were preliminarily interpreted as the spectra of fluorine atoms and CF₃-CF₂ radicals.     

ESR study of pressure dependence of free-radical decay in irradiated polystyrene

Radicals giving the usual triplet ESR spectrum have been generated in polystyrene by γ-irradiation at room temperature. The decay of the radicals has been investigated in the temperature interval between 90° and 200° and pressures ranging from 1 to 8000 atm. The effect of pressure on the mechanism of the free-radical decay is discussed. There are two regions of free-radical decay showing different activation volumes: V_I = 11.5 cm³/mole and V_II = 66 cm³/mole. The correlation between molecular motion in the α-relaxation region and radical decay is pointed out.     

ESR spin trap investigations on aqueous glucose solutions irradiated by ultrasound and gamma-rays

Applying 5,5-dimethylpyrroline-N-oxide (DMPO) as the spin trap, the radical adducts produced by ultrasound and γ-irradiation of water and aqueous solutions of d-glucose are compared. In the ESR-spectra obtained for sonolysis and radiolysis of water, H- and OH-adducts are present. In glucose solutions for both types of irradiation the H-adduct is also detected. However, OH-adduct is completely lacking and there is no evidence for the formation of adducts originating from glucosyl radicals. To explain these experimental observations the following mechanism is proposed: the OH- and H-radicals produced by sonolysis or radiolysis in water abstract carbon bond hydrogens from glucose generating glucosyl radicals. This explains the lack of OH-adducts. The glucosyl radicals stabilise by H-transfer to the spin trap. This explains the presence of H-adducts and the lack of glucosyl-adducts. The splitting constants of three additional radical adducts, which are present in minor concentrations as well in water as in glucose solutions, were evaluated. They are supposed to be spin adducts resulting from H-abstraction from DMPO.     

Dielectric relaxation study of gamma irradiated oriented low-density polyethylene

The influence of drawing, gamma irradiation and accelerated aging on the dielectric relaxation of low-density polyethylene has been studied using dielectric loss tangent measurements in the temperature range from 25 to 325 K and in the frequency range from 10³ to 10⁶ Hz. The intensity, position and activation energy of the γ- and β-dielectric relaxations were found to be strongly dependent upon the changes in the microstructure of the amorphous phase induced by uniaxial orientation, oxidation and crosslinking.     

Computer-simulated ESR spectra of irradiated poly(olefin oxides)—effect of glass transition temperature on the ESR spectra

Investigations on the effects of γ irradiation on poly(methylene oxide) (POM) and poly(ethylene oxide) (PEO) have been made employing electron spin resonance (ESR) spectroscopy. The ESR sextet and doublet spectra, recorded for POM and PEO, respectively, on irradition in air at room temperature are broadened as the temperature is lowered and show a reversible change in line shape with temperature. The spectra are analyzed by computer simulation, employing Lorentzian line-shape functions and the least-squares method of total curve fitting. The component spectra are evaluated and are assigned. Superposition of the component quartet, triplet, and doublet spectra, corresponding to the radicals ?H₃, ?H₂O , and O?HO , respectively, together with a singlet due to the radicals ?CH₂ is considered to be the best fit to the observed spectrum for POM. The doublet spectrum recorded for PEO has been assigned to the radicals ?HO . The reversible broadening of the spectra has been associated with the mechanism of molecular motions around the glass transition temperatures of these polymers.     

ESR study of free radicals produced in polyethylene irradiated by ultraviolet light

ESR studies of ultraviolet-irradiated polyethylene (PE) were carried out. Irradiation effects different from those of high-energy radiation are observed. Ultraviolet radiation is absorbed selectively, and especially in carbonyl groups in PE produced by oxidation. Radicals produced were identified as \documentclass{article}\pagestyle{empty}\begin{document}$ \hbox{---} {\rm CH}_2 \hbox{---} {\dot {\rm C}} {\rm H} \hbox{---}{\rm CHO}$\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ \hbox{---} {\rm CH}_2 \hbox{---} {\dot {\rm C}} {\rm H} \hbox{---}{\rm CH}_2 \hbox{---}$\end{document}. Some radicals giving a quintet signal stable at room temperature were also observed but remained unidentified. The radical \documentclass{article}\pagestyle{empty}\begin{document}$ \hbox{---} {\rm CH}_2 \hbox{---} {\dot {\rm C}} {\rm H} \hbox{---}{\rm CHO}$\end{document} undergoes a mutual conversion with the acyl radical: