Addition of radicals to quinones—II. ESR spectra of radicals derived from fluoranil

In systems consisting of thermally decomposing benzoyl peroxide in alkylbenzenes, fluoranil behaves as a selective spin trap. As a result of charge transfer (CT) complex formation between alkylbenzenes and fluoranil, the latter traps secondary radicals formed via H-atom abstraction by phenyl radicals from the alkyl group of the aromatic solvents. Weaker CT interaction leads to the trapping of phenyl radicals. The kinetic isotope effect in H-atom abstraction leads to the same result.     

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.     

Nature and ESR anisotropy of trapped radicals in UV-irradiated isotactic polypropylene at 4.2 K and 77 K

A comparative ESR study of low-temperature photolysis at 77 and 4.2 K of isotactic polypropylene, preoxidized under controlled conditions, and of 2.6.8 trimethylnonane-4-one as a model compound of the inchain PP ketones ? CH₂COCH₂? is presented. The results show conclusively that the ESR spectra of low-temperature PP photolysis reported in the literature arise from the decomposition of tertiary hydroperoxides, whereas the contribution by the Norrish I photolytic cleavage of ketones is of minor importance; the relative efficiency of radical production at 77 K (λ ≥ 300 nm) by both types of photoactive compound has been measured by ESR and found to be about 15 times greater for the hydroperoxides. The mechanism of solid-state photolysis of PP hydroperoxides is dominated by the nearly complete decay of excited alkoxyls during the photolytic events that yield the chain end radicals ? CH(CH₃)CH₂· by β scission (87.8%) and the inchain radicals ? CH₂C(CH₃)CH₂? by hydrogen abstraction (10.4%). The low yield of methyl radicals detected (1.8%) and their delayed appearance strongly suggest that these species are of secondary origin and that the decomposition of alkoxyls via C? CH₃ scission is of negligible importance. From the ESR anisotropy of the deliberately prepared peroxyradicals ? CH(CH₃)CH₂O₂· and ? CH₂C(CH₃)(O₂)CH₂? evidence of the misalignment of terminal chain segments following scission of alkoxyls at 77 K was obtained. An unexpected result of the measurements at 4.2 K is represented by the identification of a clear signal by H atoms: the H atoms apparently are not trapped in perprotiated matrices, even at 4.2 K, because of their unusually high reactivity toward the hydrogen abstraction assisted by tunneling effects.     

Temperature dependence of ESR spectra of nitroxide spin probe in glassy polymers

Temperature dependencies of ESR spectra of nitroxide spin probes in glassy polymers near and below glass transition temperature were examined in detail. Three temperature ranges, each characterized by specific changes in spectral shape, were defined. (i) In the low temperature range, ESR spectra of nitroxide radical in glassy polymer matrix weakly depend on temperature and remain qualitatively the same. (ii) In the intermediate temperature range, significant changes in the shape of spectra are observed. (iii) A new phenomenon was revealed near and below glass transition temperature: narrowing of linewidths occurs while the ratio of amplitudes of different components varies insignificantly. Analysis of molecular rotational mobility was carried out by means of commonly used empirical approaches. It was shown that widely used formulas and empirical approaches are not applicable for characterization of molecular mobility in glassy polymers. Mechanisms of rotational molecular movements in glassy polymers are discussed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 563–575, 2009     

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 studies of local concentrations of radicals in polyethylene irradiated at 1.5, 4.2 and 77 K

The microwave power saturation behaviour of alkyl radicals in polyethylene irradiated at 1.5, 4.2 and 77 K suggests that the local concentrations of radicals in spurs is considerably higher in the samples irradiated at 1.5 and 4.2 K. Upon annealing the samples at 77 K, the local concentrations became nearly the same as those of the sample irradiated at 77 K. The decrease of the local concentrations at 77 K was larger than that of the total concentrations suggesting an expansion of the initial spur at 77 K.     

An ESR study of polyisobutylene radicals produced by γ-irradiation at 77 K

An electron spin resonance (ESR) study has been made of radicals trapped in polyisobutylene irradiated at 77 K. It is concluded that only the ? C(Me)₂? CH? radical remains after heat treatment above 213 K. This radical shows a very sharp doublet ESR spectrum with hyperfine splitting of about 2.2 mT. A broad doublet ESR spectrum observed without warming, which was previously assigned as ? C(Me)₂? CH? is explained as a mixture of a doublet and a triplet due to the ? C(Me)₂? CH? and radicals. γ-irradiation at 77 K produces these major radical components and other carbon radicals as minor species. It has also been found that a conversion of other radicals into ? C(Me)₂? CH? occurs without decay on annealing below 183 K. Heating in the range 183–213 K causes both conversion and decay of the radicals. On annealing above 213 K, only decay occurs. The ESR spectra of polyisobutylene radicals are easily saturated by microwave power even at 1.6 μW. The mechanism of radiolysis for polyisobutylene is briefly discussed.     

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.     

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.