Electron spin resonance , free system

In certain systems ascorbic acid free radical on the 2- and 3-positions may be an intermediate in the antioxidant function, but it is a short-lived intermediate (18). An electron spin resonance flow system was used to study scavenging of a nitrosating agent by ascorbic acid, and a total spin free radical, which is the same as that produced by radiolytic oxidation (48), was determined (47). 

Electron donor-acceptor complexes, electron transfer in the thermal and photochemical activation of, in organic and organometallic reactions, 29, 185 Electron spin resonance, identification of organic free radicals, 1, 284 Electron spin resonance, studies of short-lived organic radicals, 5, 23 Electron storage and transfer in organic redox systems with multiple electrophores, 28, 1... 

Force-field methods, calculation of molecular structure and energy by, 13,1 Free radical chain processes in aliphatic systems involving an electron-transfer reaction, 23, 271 Free radicals, and their reactions at low temperature using a rotating cryostat, study of, 8. I Free radicals, identification by electron spin resonance, 1, 284... 

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. 

The electron spin resonance of pyropolymers has been reviewed by Singer (44). Broadly speaking, heat treatments up to 650° C increase the number of free radicals present but above this temperature the number of unpaired spins decreases. This has been related to the increase in size of the crosslinked system (45) but it has been shown that this effect is almost entirely due to the effect of oxygen on the polyene/aro-matic n bonding system (43). 

Differences owing to warming rate are also to be expected on the basis of electron spin resonance studies (37) of protein and peptide systems which indicate that the free radicals observed on — 196°C. irradiation differ from those observed on room temperature irradiation and that the types observed at — 196°C. are transformed to the form observed at room temperature on wanning. Since some of the low temperature free radicals have half-lives of the order of days at — 196°C., long holding times and/or slow... 

Electron spin resonance was first applied to coal during the 1950s (Ingram et al., 1954 Uebersfeld et al., 1954) as a method for the determination of free-radical species in coal. Since that time, electron spin resonance has been used to compare the data for coals of different rank and to explore the potential for relating the data to the various carbon systems as well as offering valuable information about aromaticity (Toyoda et al., 1966 Retcofsky et al., 1968, 1978 Petrakis and Grandy, 1978 Kwan and Yen, 1979 Khan et al., 1988 Thomann et al., 1988 Nickel-Pepin-Donat and Rassat, 1990 Bowman, 1993 Sanada and Lynch, 1993). 

Electron-spin resonance (ESR) indicated free radicals in a brown polymer (mean ca 1 kDa by field-desorption MS) from a glucose-4-chloroaniline model system.185 Such free radicals (see Chapter 2) could be a source of visible colour. Exposure of the melanoidin to nitric oxide, a radical-trapping agent, diminished the ESR signal by 48% and changed the colour to red-brown. 

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... 

Free radicals may be reaction intermediates in biological systems in more situations than are presently recognized. However, progress in detecting such species by Electron Spin Resonance has been relatively slow. ESR is a very sensitive technique for free radical detection and characterization. It can be used to investigate very low concentrations of radicals provided they are stable enough for their presence to be detected. 

Electron spin resonance (ESR) or electron paramagnetic resonance (EPR) spectroscopy has developed at an outstanding pace since its discovery in 1945 (Zavoiskii 1945), so that at present the technique is very well understood in its many aspects. In wood chemistry, ESR has become an essential tool for the study of the structure and dynamics of molecular systems containing one or more unpaired electrons, i.e., free radicals. ESR has found applications as a highly sensitive tool for the detection and identification of free radical species in lignin and lignin model compounds (Steelink 1966, Kringstad and Lin 1970). A recent literature review of free radicals in wood chemistry is available (Simkovic 1986). 

In recent years the state of the art in ESR instrumentation has matured enough that the technique is being used more and more by photochemists to elucidate the structure and reactivity of free radicals and organic triplet molecules in photochemical systems. Indeed, a renewed and increasing interest in the field of free radical chemistry during the last decade can be attributed mainly to the rapid development of electron spin resonance... 

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