Electron spin resonance intermediate radicals

By using this technique acrylamide, acrylonitrile, and methyl acrylate were grafted onto cellulose [20]. In this case, oxidative depolymerization of cellulose also occurs and could yield short-lived intermediates [21]. They [21] reported an electron spin resonance spectroscopy study of the affects of different parameters on the rates of formation and decay of free radicals in microcrystalline cellulose and in purified fibrous cotton cellulose. From the results they obtained, they suggested that ceric ions form a chelate with the cellulose molecule, possibly, through the C2 and C3 hydroxyls of the anhy-droglucose unit. Transfer of electrons from the cellulose molecule to Ce(IV) would follow, leading to its reduction... 

Evidence indicates [28,29] that in most cases, for organic materials, the predominant intermediate in radiation chemistry is the free radical. It is only the highly localized concentrations of radicals formed by radiation, compared to those formed by other means, that can make recombination more favored compared with other possible radical reactions involving other species present in the polymer [30]. Also, the mobility of the radicals in solid polymers is much less than that of radicals in the liquid or gas phase with the result that the radical lifetimes in polymers can be very long (i.e., minutes, days, weeks, or longer at room temperature). The fate of long-lived radicals in irradiated polymers has been extensively studied by electron-spin resonance and UV spectroscopy, especially in the case of allyl or polyene radicals [30-32]. 

Although Ce(IV) oxidation of carboxylic acids is slow and incomplete under similar reaction conditions , the rate is greatly enhanced on addition of perchloric acid. No kinetics were obtained but product analysis of the oxidations of -butyric, isobutyric, pivalic and acetic acids indicates an identical oxidative decarboxylation to take place. Photochemical decomposition of Ce(IV) carbo-xylates is highly efficient unity) and Cu(ll) diverts the course of reaction in the same way as in the thermal oxidation by Co(IIl). Direct spectroscopic evidence for the intermediate formation of alkyl radicals was obtained by Greatorex and Kemp ° who photoirradiated several Ce(IV) carboxylates in a degassed perchloric acid glass at 77 °K in the cavity of an electron spin resonance spectro-... 

X-Ray irradiation of quartz or silica particles induces an electron-trap lattice defect accompanied by a parallel increase in cytotoxicity (Davies, 1968). Aluminosilicate zeolites and clays (Laszlo, 1987) have been shown by electron spin resonance (e.s.r.) studies to involve free-radical intermediates in their catalytic activity. Generation of free radicals in solids may also occur by physical scission of chemical bonds and the consequent formation of dangling bonds , as exemplified by the freshly fractured theory of silicosis (Wright, 1950 Fubini et al., 1991). The entrapment of long-lived metastable free radicals has been shown to occur in the tar of cigarette smoke (Pryor, 1987). 

The electron spin resonance (ESR) spectra of free radicals obtained by electrolytic or microsomal reduction of several potential antiprotozoal 1,2,5-oxadiazoles were characterized and analyzed. Ab initio MO calculations were performed to obtain the optimized geometries, and the theoretical hyperfine constant was carried out using Zerner s intermediate neglect of differential overlap (ZINDO) semi-empirical methodology. DFT was used to rationalize the reduction potentials of these compounds <2003SAA69>. 

Felby, C., Nielsen, B.R., Olesen, P.O. and Skibsted, L.H. (1997b). Identification and quantification of radical reaction intermediates by electron spin resonance spectrometry of laccase-catalyzed oxidation of wood fibres from beech (Fagus sylvatica). Applied Microbiology and Biotechnology, 48(4), 459 64. 

Endo et al. investigated the reductive decomposition of various electrolytes on graphite anode materials by electron spin resonance (ESR). In all of the electrolyte compositions investigated, which included LiC104, LiBF4, and LiPFe as salts and PC, DMC, and other esters or ethers as solvents, the solvent-related radical species, which were considered to be the intermediates of reductive decomposition, were detected only after prolonged cathodic electrolysis. With the aid of molecular orbital calculation, they found that the reduction of salt anion species is very difficult, as indicated by their positive reduction enthalpy and that of free solvent (A/4 — 1 kcal mol ). However, the coordination of lithium ions with these solvents dramatically reduces the corresponding reduction enthalpy (A/ —10 kcal mol ) and renders the reaction thermodynamically favored. In other words, if no kinetic factors were to be considered, the SEI formed on carbonaceous anodes... 

The first intermediate to be generated from a conjugated system by electron transfer is the radical-cation by oxidation or the radical-anion by reduction. Spectroscopic techniques have been extensively employed to demonstrate the existance of these often short-lived intermediates. The life-times of these intermediates are longer in aprotic solvents and in the absence of nucleophiles and electrophiles. Electron spin resonance spectroscopy is useful for characterization of the free electron distribution in the radical-ion [53]. The electrochemical cell is placed within the resonance cavity of an esr spectrometer. This cell must be thin in order to decrease the loss of power due to absorption by the solvent and electrolyte. A steady state concentration of the radical-ion species is generated by application of a suitable working electrode potential so that this unpaired electron species can be characterised. The properties of radical-ions derived from different classes of conjugated substrates are discussed in appropriate chapters. 

Mason RP, Holtzman JL. 1975. The mechanism of microsomal and mitochondrial nitroreductase. Electron spin resonance evidence for nitroaromatic free radical intermediates. Biochemistry 14 1626. 

Progress in photochemistry could only be made following progress in spectroscopy and, in particular, the interpretation of spectra in at least semiquantitative terms, but history has shown that this was not enough. The arrival of new methods of analysis which permit determination of small amounts of products, the development of flash photolysis, nuclear magnetic resonance, and electron spin resonances which can yield valuable information about the natures of intermediate excited states, as well as of atoms and radicals, all have permitted the photochemist to approach the truly fundamental problem of photochemistry What is the detailed history of a molecule which absorbs radiation ... 

The types and reactions postulated for reactive intermediates in the radiation chemistry of polyethylene are reviewed. Ultraviolet spectroscopy is an important tool in complementing data obtained from electron spin resonance studies. Finally, the kinetics of growth and decay of the allyl and polyenyl free radicals as inferred from ultraviolet spectra are discussed. 

The reactive intermediates mentioned above are initially ions and excited molecules and subsequently may be free radicals. Many ions are probably formed on irradiating PET, as judged by the large concentration of spins detected at —196°C. by electron spin resonance (ESR), but nothing is known directly about their chemical structure or reactivity. Any chemical role of excited molecules is equally a matter of conjecture. In these circumstances, the influence of dose rate will be discussed by reference to free radicals. Eventually, when more quantitative experimental data are obtained, the adequacy of free radical reactions may be better assessed, and the role of ions and excited molecules brought into perspective. 

ESR Spectroscopy. Electron Spin Resonance spectroscopy is an important technique for investigating the role of radical intermediates in radiation chemistry. The technique has been used widely for many years in the study of radicals occurring in irradiated solid polymers (.6,7). However, by their very nature, such species are reactive and may only exist in low concentration. The identification of these species can also be a problem since in the majority of polymers the environment of the radicals leads to broad, unresolved ESR spectra, which makes detailed spectral analysis difficult. In recent years, many of these problems of sensitivity and resolution have been reduced by more sensitive and stable ESR spectrometers and by development of new methods of data handling and manipulation. 

The partition of the dimerization yields at 77 °K. into RPD and DD yields is shown in Table VIII. DD was obtained at 20°C. from the yields in the presence of added I2 and at 77 °K. by utilization of the efficiency factor equal to 2.3. Note the low yields at 20°C. for products of secondary radical combinations. We postulate that the intermediate excited primary radicals formed after neutralization in the UPD mechanism at 77 °K. may rearrange to secondary radicals before the final cross bond is formed. Evidence for this postulate is indicated in Table VIII by the observation that the n-C12 and 5-MeCu yields appear to be almost entirely due to the DD mechanism. Such isomerization of primary to secondary alkyl radicals in the solid phase at —196 °C. has recently been observed by electron spin resonance.62... 

Ionic processes of monomers, nitroethylene, n-butylvinylether and styrene, in organic glass matrices of 2-methyltetrahydrofuran, 3-methyl-pentane and n-butylchloride irradiated by y-rays at 77° K, are studied by observing the electron spin resonance spectra of trapped electrons and ion radicals formed from the solute monomers. The primary ionic intermediates are the trapped electrons and their counterpart, cation radicals of matrix molecules. However, in 2-methyltetrahydrofuran glass, the anionic processes of solute monomers resulting from the trapped electrons proceed selectively. On the contrary, only the cationic processes proceed selectively in n-butylchloride glass. Both processes are able to occur in 3-methylpentane glass. 

Schuchmann MN, Naumov S, Schuchmann H-P, von Sonntag J, von Sonntag C (2005) 4-Amino-3Ff-pyrimidin-2-one ("isocytosine") is a short-lived non-radical intermediate formed in the pulse radiolysis of cytosine in aqueous solution. Radiat Phys Chem 72 243-250 Schulte-Frohlinde D, Hildenbrand K (1989) Electron spin resonance studies of the reactions of OH and SO4 radicals with DNA, polynucleotides and single base model compounds. In Minisci F (ed) Free radicals in synthesis and biology. Kluwer, Dordrecht, pp 335-359 Schulte-Frohlinde D, Behrens G, Onal A (1986) Lifetime of peroxyl radicals of poly(U), poly(A) and single- and double-stranded DNA and the rate of their reaction with thiols. Int J Radiat Biol 50 103-110... 

Further studies verified the intermediate formation of free radicals, as demonstrated by the electron-spin resonance spectra obtained during autooxidation of cellulose,75 and hydrogen peroxide was identified as a byproduct in the autooxidation of D-glucitol. Similar oxidations of cellulose in the presence of alkenic monomers afforded graft copolymers. The autooxidation of cellulose and of the cello-oligosaccharides was shown to be more extensive in the presence of transition-metal cations. 

Among the techniques available to study free radicals or radical ions in solution, electron spin resonance (ESR) stands out as a technique with sufficient resolution to provide detailed information about the identity of the intermediate in question. In an external magnetic field the unpaired electron of such a species can adopt either of two spin orientations, parallel or antiparallel to the field H0. The two orientations are of slightly different energies and transitions between them can be stimulated by applying radiation of a frequency satisfying the resonance condition in Eq. (14) ... 

Jaeger CD, Bard AJ (1979) Spin trapping and electron-spin resonance detection of radical intermediates in the photo-decomposition of water at TO2 particulate systems. J Phys Chem 83 3146-3152... 

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