Detection by electron spin resonance

Trapped electrons were identified by ESR at very low microwave power in polyethylene (Marlex and Alathon) after 7-irradiation at —198°C (Fig. 26) [141]. The spectroscopic splitting factor was found to be g = 2.0022 0.0002. 

Similarities between the signal assigned to trapped electrons in hydrocarbon glasses and in polymers appear immediately the singlet is quite narrow and readily bleached by near infrared and it saturates at low power in both cases. Saturation, however, appears at about 0.025 mW with a hydrocarbon glass such as 3-methylhexane whereas it begins to be operative at 0.004 mW with polyethylene. These three characteristics allow unequivocal discrimination between trapped electrons and polymeric free radicals. These last species, indeed, are not saturated at a power of 0.1 mW, do not disappear under IR illumination and produce much broader signals. The trapped electron is thus weakly bound to the matrix. 

EFFECT OF CRYSTALLINITY ON TRAPPED ELECTRON YIELDS IN 7-IRRADIATED HYDROCARBON POLYMERS [142] 

Polymer Percent Crystallinity Initial G(et )a (Electrons 100 eV) Maximum Observable Concentration of Trapped Electronsb (electrons g l) 

The initial G(e ) values and the maximum concentration of (e ) in various hydrocarbon polymers were reported later [142] and are given in Table 9. They were found to increase with the degree of crystallinity of the polymer. These findings support the earlier suggestion that electrons are trapped in crystalline regions of hydrocarbon polymers. 

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The intrinsic defects include paramagnetic and diamagnetic species (24,27,28). The paramagnetic defects have received the most study because they are readily detectable by electron spin resonance (esr) spectrometry. Paramagnetic defects that have been identified by esr include the center, 6i the... 

Issa et al. [34] used 2,3-dichloro-5,6-dicyano-p-benzoquinone for the spectropho-tometric determination of primaquine and other antimalarials. The drugs were determined in tablets by a spectrophotometric method based on the reaction with 2.3-d ich loro-5.6-dicyano-p-benzoquinone and measurement of the absorbance at 460 nm. The reaction occurred fastest in methanol and acetonitrile to yield a radical anion, which was detected by electron spin resonance. The color attained its maximum intensity after 5 min and remained stable for at least 1 h. The absorbance versus concentration curve obeyed Beer s law in the concentration range 1-4 mg per 100 mL. The recovery was 99.9-102.6%. 

Kosaka, H., Imaizumi, K., and Tyuma, 1. (1981). Mechanism of autocatalytic oxidation of oxyhemoglobin by nitrite an intermediate detected by electron spin resonance. Biochim. Biophys. Acta 702, 237-241. 

Yarlett N, Rowlands CC, Evans JC, Yarloett N, Lloyd D (1987) Nitroimidazole and oxygen radicals detected by electron spin resonance in hydrogenosomal and cytosolic fractions from Trichomonas vaginalis. Mol Biochem Parasitol 24 255-261... 

As a means of identifying the potent oxidant another approach was made. By the use of certain organic compounds one can infer the presence of a free radical by converting a highly unstable one such as OH to a much more stable compound which can accumulate in solution, rendering its detection by electron spin resonance possible. To measure the presence of OH the compound DMPO (5,5-dimethyl l-pyrroline-N-oxide) has been used The characteristic adduct of OH with... 

The basic process whereby radicals are formed remains obscure. The energy available per quantum of radiation is very large and in many ways it is surprising that molecular breakdown, as detected by electron spin resonance, is both simple and remarkably selective. 

Experimental evidence for the presence of radical intermediates is provided by the identification of expected products from radical rearrangements, by the use of appropriate radical probes and by direct detection by electron spin resonance (ESR). Other mechanistic evidence includes inhibition by radical traps, such as di-t-butylnitroxide (DTBN), TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxy), galvinoxyl and oxygen, and by radical anion scavengers such as p-dinitrobenzene (p-DNB). 

Almeida et al. studied the oxidation of dipyridamole in acetonitrile and ethanol by voltammetric and spectroscopic methods [58]. The electrochemical oxidation of dipyridamole in acetonitrile and ethanol at high and low scan speeds was similar, and exhibited two reversible voltammetric waves corresponding to consecutive one-electron reactions. The Ei/2 values were 120 and 400 mV in acetonitrile, and 470 and 680 mV in ethanol. The release of one electron produced a radical cation which was detected by electron spin resonance spectrometry. The electrochemical oxidation of dipyridamole was accompanied by a decrease in fluorescence intensity at 470 nm, and significant changes in the ultraviolet-visible absorption... 

Tt is well known that the presence of precipitated polymer can influence the course of polymerization. In bulk acrylonitrile polymerization the effects are most dramatic and have been the subject of many studies. The literature on this subject has been reviewed by Bamford et al. (4) by Thomas (29), and by Peebles (23). Under conditions where the system becomes heterogeneous owing to precipitation of small particles of polymer, a protracted acceleration period is observed at the start of polymerization, and the final rate is found to depend on the 0.8 power of the concentration of free radical initiator. Unusual post-polymerization effects are observed in photoinitiated polymerization of acrylonitrile, owing to the presence of trapped radicals which can be detected by electron spin resonance. None of the detailed mechanisms proposed to... 

Kadirov MK, Bosnjakovic A, Schlick S (2005) Membrane-derived fluorinated radicals detected by electron spin resonance in UV-irradiated Nation and Dow ionomers effect of counterions and H202. J Phys Chem B 109(16) 7664-70... 

Many of these free radical intermediates have been detected directly with ESR. Others are too reactive to detect directly, but a method to stabilize these free radicals called spin trapping has proved successful. Spin trapping is a technique in which a short-lived reactive free radical (R ) combines with a diamagnetic molecule ( spin trap ) to form a more stable free radical ( radical adduct ) which can be detected by electron spin resonance ... 

Trimethylenemethane (TMM) is a highly reactive species that has only been detected by Electron spin resonance (ESR)... 

The term free radical is often used in the context of a reactive intermediate, as in the case of polymerization of vinyl monomers, but the same structure (unpaired electron) can and does exist in a kind of immobilized environment. For example, a bulk-polymerized (monomer and initiator only in the polymerization system) poly(methyl methacrylate) (PMMA) contains an appreciable number of free radicals that can be detected by electron spin resonance (ESR) [1]. When the polymerization system becomes highly viscous toward the end of the bulk polymerization, gel formation occurs and immobilizes the growing end of free radical chain growth polymerization, preventing recombination of two free radical ends of growing chains. 

On semiconductors many different states of adsorbed dioxygen can be visualized, O2 (bridged), bound di-radical—O—0-, terminally and laterally n-bonded O2, O2", Oz, 0, 04 , O, 0 , 0, etc., some of which have been detected by electron spin resonance. The reactivities of few of these have been investigated. 

Paramagnetic species, generated in the vapor phase in a crossed-beam experiment by irradiation with 1 Mev. He ions, have been trapped at 77 °K. and detected by electron spin resonance (ESR). This paper describes the results obtained from irradiated methyl-, ethyl-, and tert-butyl alcohol, acetone, and ethylene. Trapped electrons together with the radicals CH2OH, CHsCHOH, and (CHa)2C(OH)CH2- and (CH i).tC are formed in methyl-, ethyl-, and tert-butyl alcohol respectively. Ethyl radicals are formed from ethylene. Acetone gives rise to CHjCOCH, and CHS radicals and appears to form trapped electrons in the deposit. The results are compared with the radiation chemistry of these systems in the solid and vapor phase. 

Reactions of lithium alkyls are generally considered to be carbanionic in nature, but in reactions with alkyl halides free radicals have been detected by electron spin resonance.32 Lithium alkyls are widely employed as stereospecific catalysts for the polymerization of alkenes, notably isoprene, which gives up to 90% of 1,4-cA-polyisoprene numerous other reactions with alkenes have been studied.33 The TMED complexes again are especially active not only will they polymerize ethylene but they will even metallate benzene and aromatic compounds, as well as reacting with hydrogen at 1 atm to give LiH and alkane. 

Several authors have found a direct relationship between the y phase or Cr + concentration detected by electron spin resonance and the catalytic activity of chromia-alumina catalysts. This relationship is illustrated by the data of Bukanaeva et al, (133), shown in Fig. 20 for... 

Spin label A substance that has an unpaired electron detectable by electron spin resonance and that is used as a chemical label. 

Table2.6 Relative free radical intensities detected by electron spin resonance (ESR) after the plasma treatments...

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