ESR study of some iminoxy free radicals generated from phosphorylated oximes and dioximes

The new type of phosphoniminoxy free radicals was generated electrochemically or by chemical oxidation of oximes. It was established the strong difference of ESR parameters for isomeric Z and E forms of the radicals. The spin distribution and its correlation with electronic and spatial characteristics of molecules as well as with intramolecular interactions are discussed.     

理疗超声空化及其自由基的ESR检测

本文利用5,5-dimethy1-1-purrolinel-oxide(DMPO)自旋捕集剂及电子自旋共振(ESR)技术研究证明,理疗级超声辐照可在自然空气饱和的水溶液中导致^·OH及^·H自由基的形成。认为这是超声瞬态空化过程产生局部高温高压的结果。当超声辐照时间取3min时,发现超声阈值空化强度在0.537 W/cm²-0.632 W/cm²间。随着辐照声强的增加,^·OH自由基产量在声强为1 W/cm²-2 W/cm²间迅速增长,大约在3 W/cm²以上,产量将不再增加。·OH产量D与辐照声强τ的关系可近似用拟合方程D=8.1(√相似文献   

ESR and ENDOR of free radicals in X-irradiated barbituric acid

Free radicals from dihydrate and dideuterate crystals and from anhydrous powder of barbituric acid X-irradiated between 77 K and room temperature have been studied by ESR and ENDOR spectroscopy. Four different species were characterized. In dihydrate crystals, small amounts of molecular anions were found to be formed in addition to the major radical which results from elimination of a hydrogen from the C(₅)-methylene group. The α-hydrogen coupling in the latter as well as its weak intra- and intermolecular proton interactions were investigated in detail by ENDOR. The cation of barbituric acid was observed in dideuterate crystals between 77 K and 130 K. Isothermal decay curves indicated its transformation into the deuterated analog of the R₂Ċ(₅)-H fragment upon warming. The dose-yield relationship of dihydrate and anhydrous powder suggested a stabilization of this radical via interaction with the crystal water. Both the transformation and the stabilization mechanisms were supported by ENDOR-findings on weakly coupled protons.     

ESR spectroscopics study of radicals formed from methoxycarbonylcholine picrate hemihydrate

Abstract

The electron spin resonance of γ-irradiated single crystals of methoxycarbonylcholine picrate hemihydrate, C₇H₁₆NO₃ ⁺ · C₆H₂N₃O₇ ^? · ½ H₂O has been observed and analyzed for different orientations of the crystal in the magnetic field. The crystals have been investigated between 70 and 350 K. The spectra were found to be temperature independent and the radiation damage centers are attributed to – ?[Obar]OCH₃ and –CH₂CH₂O? radicals. The g and hyperfine coupling constants were found to be almost isotropic with an average, g = 2.0060, a _H1 = 4.4G for –CH₂CH₂O?, g = 2.0050, a _H2 = 3.5G for –CH₂CH₂O? and g = 2.0045, a _H = 3.5 G for –?[Obar]OCH₃. These values indicate a long-range coupling between the unpaired electron and H protons.     

Investigation of free radicals induced by light ions in CR-39 using ESR spectroscopy

In previous investigations the formation of radicals induced by γ-irradiation in CR-39 samples has been studied by electron spin resonance (ESR) spectroscopy. On the other hand, all attempts fail to detect radicals in samples irradiated by particles (p, and neutrons). The present work succeeded in detection of radicals produced by charged particle radiation in CR-39 (TASTRAK). The detectors were irradiated with protons of 7 MeV and -particles of 3 MeV with fluences in the order of 10¹¹–10¹² particles/cm². The observed ESR signal is composed of at least four different resonance lines for each of which we determined the amplitudes, g-factors, lifetimes and the G-value. The amplitude of the total ESR signal decreases rapidly with a short and a longer lifetime. About 5 days after irradiation the ESR intensity decreased to an undetectable value. The lowest detectable dose (LDD) is of the order of 1 kGy.     

Application of ESEEM to study the structure of free radicals

Manifestations of the hyperfine interaction of paramagnetic particles in their electron spin echo signal decay were comprehensively discussed. Mechanochemically activated calcium gluconate was studied using electron spin echo modulation phenomenon and electron-nuclear double resonance techniques and quantum-chemical calculations. Three possible structures are obtained for free radicals in calcium gluconate. To specify the structure of radicals, further investigations are needed.     

Formation of muonium substituted free radicals

SR experiments with mixtures of benzene with cyclohexane, 2,3-dimethyl-1,3-butadiene and carbontetrachloride allow the conclusion that the direct muonic precursor of the muonic cyclohexadienyl radical is thermal muonium. In neat benzene it has a life time of 10 ps and disappears by the addition reaction. Muonium is formed within about 1 ps by combination of the positive muon with an electron near the end of the track of the thermalizing muon.Support by the Swiss National Foundation for Scientific Research and by the Swiss Institute for Nuclear Research (SIN) are gratefully acknowledged.     

Proton ENDOR of dimethylnitramine free radicals

Electron nuclear double resonance (ENDOR) spectra of free radicals produced by ultraviolet (UV) photolysis of polycrystalline and single crystal dimethylnitramine (or N-methyl-N-nitromethanamine) [DMN; (CH₃)₂NNO₂ were studied atca. ?30°C. Results suggest that multiple radical species are formed during UV photolysis of DMN, perdeutero-DMN-d₆, and¹⁵N-labeled DMN. Proton ENDOR spectra are consistent with assignment of a cation radical (CH₃)₂NNO ₂ ⁺ as the major DMN radical species. Proton hyperfine coupling anisotropy, which is observed from the ENDOR spectra, is attributed to inequivalence of the two DMN methyl groups.     

Double quantum transitions in free radicals

The observation of double quantum transitions on three kinds of free radicals (DPPH, DPANO, ultramarine) at temperatures of liquid nitrogen is described. Signals having an absorption and emission character were observed in the 12-5 MHz–18 MHz frequency band in the direction parallel to the external magnetic fieldH ₀ and their dependence on different parameters was studied.Bloch equations were solved for two mutually perpendicular high-frequency fields and expressions were found for the components of dynamic susceptibility in theH ₀ direction. The experiments agree with theory.The possibility of using double quantum transitions for measuring relaxation times is discussed.

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(DPPH, DPANO, ) . 12,5MHz–18MHz H ₀, ; . H ₀. . .

     

Formation and stabilization of persistent free radicals

We demonstrate that stable and relatively unreactive “environmentally persistent free radicals (PFRs)” can be readily formed in the post-flame and cool-zone regions of combustion systems and other thermal processes. These resonance-stabilized radicals, including semiquinones, phenoxyls, and cyclopentadienyls, can be formed by the thermal decomposition of molecular precursors including catechols, hydroquinones and phenols. Association with the surfaces of fine particles imparts additional stabilization to these radicals such that they can persist almost indefinitely in the environment. A mechanism of chemisorption and electron transfer from the molecular adsorbate to a redox-active transition metal or other receptor is shown through experiment, and supported by molecular orbital calculations, to result in PFR formation. Both oxygen-centered and carbon-centered PFRs are possible that can significantly affect their environmental and biological reactivity.     

The electronic spectra of conjugated free radicals

Ensemble averaging schemes are applied within the PPP model to calculate excitation spectra of free conjugated hydrocarbon radicals and further, more general forms of the pairing and particle-hole properties for conjugated hydrocarbons are shown to be valid.     

Dipolar broadening of electron spin resonance lines of free radicals: Spin-trapping studies of free radicals in cigarette smoke

Electron spin resonance spectra of spin-trapped free radicals from tobacco smoke have been examined. The cigarettes studied were standard IRI research cigarettes supplied by the University of Kentucky Tobacco and Health Research Institute. Substantial changes in the spectra were observed upon aging of the samples. Aging for several days resulted in at least two remaining long-lived spin-adducts. For the experimental configuration employed the efficiency of the spin-trapping apparatus was determined to be about 47%. Utilizing measurement of the broadening of the magnetic resonance lines as function of the amount of smoke passed through the phenyl-t-butyl nitrone spin-trapping solution, a concentration of about 1 × 10¹⁸ free radicals per cigarette puff was deduced. The method employed in this determination was tested with a number of stable free radicals in several solvents.     

Study of the influence of grain size on the ESR angular response in alanine radicals

The simulation of electron paramagnetic resonance (EPR) spectra of radicals potentially due to alanine radiolysis has been done. Combining theoretical spectra in varying proportions allows computing different alanine spectra. The small changes in specific regions of experimental spectra correspond to weak variations in the proportion of radicals.As the study of several parameters, such as ESR angular response of powder or time after irradiation, is not possible in simulated spectra, experimental analyses have been carried out for the ESR angular response dependence on grain size in various combinations, corresponding to an available external surface. The stabilization of the angular response is associated with a spatial reorganization of radicals. It seems that available surface and radical proportions are linked. Predicted values of angular response calculated from pure powders do not correspond to experimental ones. Weak changes in spectra seem to confirm that these variations may be interpreted as a transformation of one radical into another.     

Identification of free radicals in irradiated MA-AMPS copolymer

Methacrylamide-2-acrylamido-2-methylpropanesulphonic acid (MA-AMPS) (80:20) is subjected to n -irradiation and the generated free radicals are identified by the electron spin resonance (ESR) technique. The ESR spectrum "obser"ved for MA-AMPS (80:20) has shown a complex line shape, indicating the presence of more than one free radical. Computer simulations are employed to unravel the radicals responsible for the ESR spectrum. Radical "identification has been done with the magnetic parameters employed during computer simulation. The observed ESR spectrum of the copolymer is simulated to be a superposition of component quartet, quintet and singlet spectra. The component spectra are assigned to ～CH 2 - . (CONH 2 )-CH 2 ～ or ～CH 2 - . H-CH 2 ～, . H 3 radicals, "respectively. The possibility of formation of such radicals in the sample material has been discussed.