Electrons, free radicals produced

Smaller B, Remko J R and Avery E C 1968 Electron paramagnetic resonance studies of transient free radicals produced by pulse radiolysis J. Chem. Rhys. 48 5174-81... 

The reaction mechanisms of plasma polymerization processes are not understood in detail. Poll et al [34] (figure C2.13.6) proposed a possible generic reaction sequence. Plasma-initiated polymerization can lead to the polymerization of a suitable monomer directly at the surface. The reaction is probably triggered by collisions of energetic ions or electrons, energetic photons or interactions of metastables or free radicals produced in the plasma with the surface. Activation processes in the plasma and the film fonnation at the surface may also result in the fonnation of non-reactive products. 

Breaking a bond to a primary hydrogen atom m propene requires less energy by 42 kJ/mol (10 kcal/mol) than m propane The free radical produced from propene is allylic and stabilized by electron delocalization the one from propane is not... 

Examples include luminescence from anthracene crystals subjected to alternating electric current (159), luminescence from electron recombination with the carbazole free radical produced by photolysis of potassium carba2ole in a fro2en glass matrix (160), reactions of free radicals with solvated electrons (155), and reduction of mtheiiium(III)tris(bipyridyl) with the hydrated electron (161). Other examples include the oxidation of aromatic radical anions with such oxidants as chlorine or ben2oyl peroxide (162,163), and the reduction of 9,10-dichloro-9,10-diphenyl-9,10-dihydroanthracene with the 9,10-diphenylanthracene radical anion (162,164). Many other examples of electron-transfer chemiluminescence have been reported (156,165). 

Although all sources of reactive free radicals which have been tried initiate the polymerization of unsaturated monomers, the converse of this statement, namely, that all initiators are free-radical-producing substances, is not true. Thus, strong acids (in the Lewis sense) such as AICI3, BF3, and SnCL, which are characterized by a strong affinity for a pair of electrons, bring about rapid polymerization of certain monomers. These polymerizations also proceed by chain mechanisms. The propagating center is, in this case, a positively... 

J. Kroh, B.C. Green, and J.W.J. Spinks, Electron paramagnetic resonance studies on free radicals produced by T(3-particles in frozen H2G and D2G media at liquid nitrogen temperature. Can. J. Chem. 40, 413-425 (1962). 

If XO is an undoubted historical pioneer among free radical-producing enzymes, whose capacity to catalyze one-electron transfer reactions opened a new era in biological free radical studies, NADPH oxidase is undoubtedly the most important superoxide producer. This enzyme possesses numerous functions from the initiation of phagocytosis to cell signaling, and it is not surprising that its properties have been considered in many reviews during last 20 years [56-58]. 

The reaction of the diene with a free radical produces an allyl radical having unpaired electron delocalised over more than one carbon atom. The allyl free radical can undergo 1, 2, or 1, 4 addition. 

Buettner, G. R., KeUey, E. E., and Bums, C. P., 1993, Membrane hpid free radicals produced from LI 210 murine leukemia ceUs by photofrin photosensitization an electron paramagnetic resonance spin trapping study. Cancer Res. S3 3670-3673. 

It has been previously reported [21, 22] that metal colloids are formed by radiochemical reactions in water/alcohol solutions, in which the reduction of metal salts takes place by solvated electrons and free radicals produced under UV or y-ray irradiation. Ichikawa et al. have applied this photoreduction method to the surface-mediated reaction of metallic ions and succeeded in synthesizing metal/aUoy nanowires in the constrained cavities of mesoporous supports such as FSM-16 and MCM-41 [18-20, 23-25]. The adsorbed water and alcohol work not only as solvents in the nanoscale silica void space but also as a source of reducing species for metallic ions to metals under UV-vis and y-ray [11, 18, 19] irradiation. The results indicate the dense formation of Pt nanowires inside the charmels of mesoporous supports, such as FSM-16, which act as the templates. In fact, no any Pt wire is observed on the external surface of FSM-16 or amorphous silica surface. Short wires, 10 nm long, are also observed as a minor species in the samples in the initial stage of UV and y-ray irradiation. 

The data support evidence from electron spin resonance studies by Henriksen 22,23,24,25) and Gordy 16) showing that the radicals present in irradiated proteins at —80° to — 196°C. are different from those found on room temperature irradiation. They found that the free radicals produced at — 196°C. are relatively stable at that temperature, but on warming to room temperature these free radicals both react with each other and... 

There is definite evidence that ions, electrons, free radicals and excited molecules are produced in systems exposed to ionising radiation. Qualitatively, it has been possible precisely to identify some of these intermediates in selected systems. Quantitatively it has been possible to separate out the yields arising from each of these products in only a limited number of systems and much more work needs to be done. More information is also desirable on the inter-relationship of these species in a particular system. A vast amount of kinetic data characterising some of these intermediates has also been accumulated. 

Decarboxylation of acids is of special interest, since the free radicals produced may combine with iodine, heteroaromatic bases or electron-deficient alkenes affording useful products in clean reactions. 

Forest and co-workers claimed that the first oxidation product in vivo (as in vitro) is the free radical produced by loss of an electron they observed small amounts of a metabolite with the properties of a free radical in the urine of some patients treated with chlorpromazine. 

It is generally accepted that thermal cracking is a free radical chain reaction (a free radical is an atom or group of atoms with an unpaired electron). Free radicals react with hydrocarbons and produce new hydrocarbons and new free radicals ... 

In the first study 2,2-dimethoxy-2-phenyl acetophenone was photolysed at 366 ran in n-butylvinyl ether in the presence of di-p-tolyl iodonium hexafluorophosphate as oxidising salt. The free radicals produced in the photolysis were transformed into cationic active species for the polymerisation of the vinyl ether by the electron transfer to the iodonium ion. In the second report, various radical sources were photolysed in the presence of the monomer and silver hexafluorophosphate, the latter acting as one-electron oxidant. 

Equations (54) and (55) clearly show that the rate of polymerization depends on the monomer concentration, the quantum yield of free-radical formation, and the reactivity of the free radicals produced after the photoinduced electron transfer process. 

Secondly there are direct techniques, notably electron spin resonance spectroscopy (ESR), in which the free radicals produced by the fracture of covalent bonds are directly observed, both in respect of their chemical nature and their number. Much of this review is orncemed with the results of ESR studies and this technique is therefore treated at some length below. One little used technique for the direct assessment of free radicals produced by n chanical means is that of Pazonyi et al and Salloum and Eckert They dropped various polymers in an ethanolic solution of diphenyl picryl hydrazyl, a chemical indicator, and determined the free radical concentration in the cut surfaces by colorimetrie measurements of the colour change. This method is subject to soixm uncertainty on account of possible side reactions. 

The effects of irradiation on many different types of substances are now studied at CHAM (Analytical Chemistry Department at the University of Louvain, School of Pharmacy) [2] and in other groups [3]. All the most advanced analytical tools are explored to detect the traces and to study radiosterilization Electron Spin Resonance (ESR) is the most sensitive technique and the spectra were found to be specific to free radicals produced in irradiated... 

Debije M.G., Bernhard W.A., Thermally stable sites for electron capture in directly ionized DNA free radicals produced by the net gain of hydrogen a C5/C6 of cytosine and thymine in crystalline oligodeoxynucleotides. J. Phys. Chem. A, 2002,106,4608-4615. 

Reactive oxygen species (ROS) are by-products of aerobic metabolism ( Figure 3-2). Under physiological conditions, it is estimated that up to 1% of the mitochondrial electron flow leads to the formation of superoxide (OJ.), the primary oxygen free-radical produced by mitochondria. Interference with electron transport can dramatically increase O2. production. ROS are toxic to cells. 

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