Change with temperature of the ESR spectra of peroxy radicals trapped in irradiated polytetrafluoroethylene

The ESR spectra of peroxy radicals in irradiated powders and oriented samples of polytetrafluoroethylene (PTEE) have been measured with a K-band spectrometer, and the principal values and directions of the g tensor were determined both at room temperature and at 77°K. In contrast to the spectra of the usual peroxy radicals, those trapped in γ-irradiated PTFE exhibited an ESR spectrum apparently having a larger principal value for g⊥ than for g∥ when measured at room temperature, although the normal principal values were observed at 77°K. As for the directions of the principal axes, g∥ was directed along the chain axis at room temperature and was perpendicular to the chain axis at 77°K. From the temperature change of the g tensor and the line shapes in the oriented samples, it is shown that the observed temperature change of the spectra is due to rapid rotation at room temperature around the chain axis rather than around the C? O bond axis. Assuming this, the apparent principal values of the g tensor at room temperature were calculated from the g tensor obtained at 77°K. for the rigid state, and the results are in good agreement with observations at room temperature. A structure for the peroxy radicals is also proposed. In addition, the spectral line shape function for the uniaxially oriented samples has been derived.     

ESR spectra of phosphorus-containing tetrazonyl radicals

Conclusion The oxidation of the products of the reaction of dialkylphosphorylacetaldehydes with aryldiazonium salts gives tetrazonyl radicals.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1168–1169, May, 1987.     

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 spectra of anion radicals of phenazine derivatives

The ESR spectra are reported for electrolytically generated radicals. The hyperfine structure is found to be due to primary interaction of the unpaired electron with two nitrogen atoms, the quintet being then split by the ring protons. A quantum calculation of the spin density agrees satisfactorily with the results.     

ESR spectra of fluorine-containing radicals of phosphorus and arsenic

ESR spectra observed at 120 K in γ-irradiated solid solutions of PF₅ in neopentane and AsF₅ in SF₆ are ascribed to the radical ions PF₅^? and AsF₅^? in which the unpaired electron interacts strongly with the central nucleus and with four equivalent ¹⁹F nuclei. Hyperfine interaction with the fifth ¹⁹F nucleus was not resolvable in either case. The spectra were quite distinct from those of the tetrafluorides PF₄ and AsF₄, thus eliminating the possibility that they were due to tetrafluoride radicals undergoing rapid intramolecular ¹⁹F exchange.     

ESR spectra and the behavior of substituted indophenoxyl radicals

ESR spectra and structure are compared for radicals formed from substituted indophenols by oxidation (substituents CH₃, i-C₃H₇, t-C₄H₉, t-C₅H₁₁, cyclohexyl). The unpaired electron interacts with the nitrogen and with the hydrogen in the ortho and meta positions of both benzene rings. A kinetic equation of first order with respect to the indophenol applies to the reaction of the latter with benzoyl peroxide. The loss of indophenoxyl radicals in benzene obeys an equation of second order in the radical concentration. The rate constants for radicals with various substituents indicate the radical stability, which falls greatly in going from ones with ortho-t-alkyl substituents to ones with less highly branched groups.     

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:      

ESR study of free radicals in UHMW-PE fiber irradiated by gamma rays

ESR spectra of the trapped radicals in an ultra-high molecular weight polyethylene (UHMW-PE) fiber irradiated by gamma rays showed well-resolved hyperfine splitting at room temperature since the c-axis of the crystallites is aligned with the fiber direction and the radicals are trapped in crystallites. The alkyl radical (?CH₂?^?CH?CH₂?) was the major product after irradiation in vacuum and in air at room temperature. Some of the alkyl radicals converted to allyl radicals (?CH₂?^?CH?CH=CH?) and polyenyl radicals (?CH₂?^?CH?(CH=CH)_n?CH₂?) during storage in vacuum. Upon storage in air atmosphere, the alkyl radicals decayed by reaction with oxygen. Of particular interest is the very slow decay rate of the alkyl radical trapped in UHMW-PE fiber, the half-life is 26 days in vacuum, and 13 days in air at room temperature, which is about 1/30 and 1/100 of that reported for high density polyethylene (HDPE), respectively. The extremely long lifetime of the alkyl radical is supposed to be caused by the large size of crystallites in UHMW-PE fiber. The rate of radical decay was accelerated by annealing at elevated temperature.     

ESR spectra of radicals at a low-temperature X-radiolysis of phosphates

ESR spectroscopy at 77 K has been used for the investigation of radicals formed in the low-temperature X-radiolysis of tri-n-butylphosphate /TBP/, di-n-butylphosphate /DBP/, di /2-ethylhexyl/phosphoric acid /DEHPA/, tri-p-tolylphosphate /TTP/, di-n-butyl-n-butylphosphonate /DBBPN/. Besides the phosphate radicals also hydrogen atom stabilization in the matrix has been observed. In the presence of oxygen peroxy radicals occur as well. The structure of the radicals and reaction mechanisms are discussed.     

The numerical calculation of ESR spectra of randomly oriented quartet radicals

The ESR spectra of randomly oriented quartet radicals were calculated with the aid of an analytical formula. A line broadening through Lorentz lines was explicity taken into consideration. The experimental spectrum of an alternant hydrocarbon radical was simulated with sufficiently good agreement.     

Change with temperature of the ESR spectra of methacrylic acid radicals

In order to elucidate the structure of methacrylic acid radicals, the change with observation temperature of the ESR spectrum of free radicals trapped in solid methacrylic acid γ-irradiated at ?196°C was studied. Below ?80°C, we found a 9-line spectrum, which is similar to the ordinary 9-line spectrum observed in irradiated poly(methacrylic acid) or poly(methyl methacrylate), but which differs in the stronger intensity of the so-called 4-line component. Our 9-line spectrum changes reversibly into a 13-line spectrum above ?80°C. With broad-line NMR measurements of methacrylic acid, it was found that there is such an unusual crystalline transition around ?30°C that the line width is narrower in the lower-temperature region (phase II) than that in the higher-temperature region (phase I). The change of the ESR spectrum can be interpreted in terms of the exchange of the two β-protons due to the hindered oscillation around the C_α? C_β bond of the single radical ···C_βH₂C_α(CH₃)COOH if one assumes the gradual change of the hindering potential barrier caused by the crystalline transition and the lower barrier in phase II. The modified Bloch treatment gave the hindering potential barrier to be 7.2 kcal/mole in phase I and 1.5 kcal/mole in phase II. The difference between our 9-line spectrum and the ordinary one with the very weak 4-line component comes from the difference of the surrounding matrix.