Irradiated, electron spin resonance

Electron irradiation Electron spin resonance Electrostatic painting Ellipsometry Elongation at break... 

It is usually difficult to establish whether the polymerization is free radical or ionic in initiations by irradiation. Electron spin resonance measurements often give signals, but this does not prove that the free radicals responsible for these signals actually start the polymerization. Chemical experience is used in many cases If a monomer polymerizes cationically only in solution, then the polymerization in the crystal cannot be free radical, and vice versa. 

Despite the fact that natural elemental sulfur contains 0.75% of the isotope [4] with a nuclear spin of 7 = /2 no NMR spectra of elemental sulfur have ever been reported. Such spectra are however well-known for compounds containing just one or two sulfur atoms [5]. Electron spin resonance spectra of irradiated elemental sulfur samples and of quenched sulfur vapor have been reviewed elsewhere [6-8]. 

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]. 

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). 

Electron paramagnetic resonance (EPR), also known as electron spin resonance (ESR), has rightly been called the NMR of unpaired electrons . A sample of a radical (generally in solution) is irradiated with a beam of microwave radiation and the proportion of the radiation that is absorbed is then determined. 

Kazansky, Pariisky, and Voevodsky (273) investigated the results of y-irradiation of silica gel using electron spin resonance techniques. Hydrogen atoms are formed and kept in the structure at —196°. Recombination starts at — 150 to — 120°. Irradiation of evacuated samples... 

The electron spin resonance spectra were run in nitrogen-saturated solutions of aromatic compound ca. 10" m) and nucleophile (0-05-0-1 M) in the solvent(s) indicated. Irradiation in the cavity was effected with a high pressure mercury arc. Electrolysis was performed with the platinum cathode in the cavity, tetraethyl-ammonium perchlorate as electrolyte and electric currents of 10-250 /lA. 

Thermolysis of aryl chloro diazirine (18) in the presence of acetone and a trapping agent such as A -phenylmaleimide gave rise to cycloadducts such as 41. The unstable adduct hydrolyzed during purification resulting in synthesis of bicyclic hemiacetals 42 and 43 as a mixture of endo and exo adducts in 37 and 8% yield, respectively. The exclusive generation of the singlet carbene was confirmed by low-temperature electron spin resonance (ESR) study of the irradiated diazirine. 

One of the standardized methods, electron spin resonance (ESR) technique, permits identification of food that contains a hard, dry matrix, e.g., bone. When food containing bone is irradiated, free radicals are produced and trapped in the crystal lattice of the bone, which can be detected by ESR spectroscopy [137]. Thermoluminescence of contaminating minerals for detection of radiation treatment of, e.g., spices and dried fruits can be successfully applied [138, 139]. Another standardized method that has been developed for identification of irradiated fat-containing foods is the mass-spectrometric detection of radiation-induced 2-alkylcyclobutanones after gas-chromatographic separation [140]. The... 

Fig.5 First-derivative electron spin resonance spectra found immediately after /-irradiation of samples of 20 mg/mL DNA in 7M LiBr with various loadings of MX. The dashed spectra are simulations made by linear least-squares fits of the benchmark functions (Fig. 3a and b) to experimental spectra. The spectra clearly show that MX increases in relative amount to the DNA anion radical with increased loading of MX. At the lowest loading of MX (228 bp/1 MX) these fits suggest that 8.7% of the electrons are found on MX, whereas at the highest loading (23 bp/1 MX) 59% are captured by MX with the remainder on DNA. The fraction of electrons captured by MX increases with time [7a]. Reprinted with permission from the J. Phys. Chem. Copyright (2000) American Chemical Society...

Ag-NaA zeolites Silver clusters Silver clusters, formed in X-ray irradiated samples at 77 K and annealed at 280 K, were characterized by electron spin resonance spectroscopy 556... 

Table I. Electron Spin Resonance Study of Irradiated Polystyrene...

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. 

Evidence for such stable free radicals has been obtained from electron spin resonance measurements. A sample of the 77-23 PVC-styrene, which had been exposed to 0.8 megarad of gamma-radiation at room temperature, displayed resonance peaks comparable to 3 X 10 8 mole per gram of free radicals (compared with a diphenyl picrylhydrazyl standard). When an identical sample was heated for 10 minutes at 75°C. following irradiation, the free radical population had fallen below detection limits. Heating evidently destroyed or decreased the free radical content by reaction or termination. 

Fig. II Electron spin resonance (ESR) spectra of PTFE powder after irradiation...

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 sign of the dominant charge photocarriers in most of the polymers investigated has been found to be positive. The photosensitivity increase under ultraviolet irradiation might be connected with the photoionization of polymer molecules and the creation of local, positively charged centers acting as photoelectron traps. This mechanism seems to be confirmed by the electron spin resonance method. 

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. 

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