Trapped radicals, structure

The rate of oxidation/reduction of radicals is strongly dependent on radical structure. Transition metal reductants (e.g. TiMt) show selectivity for electrophilic radicals (e.g. those derived by tail addition to acrylic monomers or alkyl vinyl ketones - Scheme 3.89) >7y while oxidants (CuM, Fe,M) show selectivity for nucleophilic radicals (e.g. those derived from addition to S - Scheme 3,90).18 A consequence of this specificity is that the various products from the reaction of an initiating radical with monomers will not all be trapped with equal efficiency and complex mixtures can arise. 

Based on data from competition experiments, trapping of vinyl radicals occurs via a cr-type intermediate, which is lower in energy than the alternative jt-radical structure [55, 56], Stabilization of cr-radicals via hyperconjugation is small, which causes vinyl radicals to be more reactive than e.g. the methyl radical. /Z-Isomerization of a strained cr-vinyl radical proceeds with a rate constant k 3 x 108-1010 s-1 to provide the thermodynamically most favorable geometry [56],... 

Figure 1 Free radical structures, parent compounds, and stable end products for the various components of DNA (a) deoxyribose, (b) guanine, (c) adenine, (d) thymine, and (e) cytosine. Panel (f) shows trapping of the electron and hole by proton transfer in the GC base pair in duplex DNA.

Other researchers have experimentally observed heterogeneity in crosslinked polymers by studying radical concentrations and environment with ESR [101-106], Knowledge of the structure and reactivity of trapped radicals is especially important when considering the long term physical and mechanical properties of dental polymers. Kloosterboer et al. [106] has studied the structure of trapped acrylate radicals while Hamielec and coworkers [102-105] have... 

The production of the radicals was carried out by reduction of NMMO with Fe(II) or by oxidation of N-methylmorpholine with Fe(III), at different temperatures. Other reductants and oxidants gave nearly identical results. The similar outcome of NMMO reduction and NMM oxidation was already a clear indication that the structure of the radical intermediates was correctly proposed - apart from the definite proof by the chemical structure of the trapped radical intermediates. 

Reactive free radicals also react with the nitrogen of nitroso groups, forming a nitroxide one atom closer to the trapped radical than is the case with nitrone spin traps. This results in ESR spectra containing more chemical structural information. While nitroso spin traps provide radical identification, the resultant adducts are often less stable than those derived from nitrone traps. In particular, nitroso traps are unreliable for oxygen-centered radicals even in vitro. 

Narayana PA, Bowman MK, Kevan L. (1975) Electron spin echo envelope modulation of trapped radicals in disordered glassy systems Application to the molecular structure around excess electrons in y-irradiated lOM sodiiun hydroxide alkaline ice glass. J Chem Phys 63 3365-3371. 

Vitamin E and BHT are radical inhibitors, so they terminate radical chain mechanisms by reacting with radicals. How do they trap radicals Both vitamin E and BHT use a hydroxy group bonded to a benzene ring—a general structure called a phenol. 

Electron-resonance spectroscopy was used for identifying the radical obtained on x-irradiation "" and 7-irradiation of crystalline glycolic acid. The evidence supports the structure H0CHC02H for the trapped radical, with only very slight indications of the presence of another radical. One possible step in the formation of the (carboxyhydroxymethyl) radical is as follows. 

Nitrosobenzenes may function as spin traps the main advantage of nitrosobenzenes as compared to nitrones is that the radical is added to the nitrogen rather than to the carbon as in the nitrones this gives more direct information on the structure of the trapped radical the main disadvantage is the rather narrow potential window available [169,170]. 

The rate of decay of trapped radicals in polymers depends on the nature of the radical, the chemical structure and crystallinity of the polymer, the temperature, the linear energy transfer and eventually the dose rate. 

V.W. Bowry and K.U. Ingold, Kinetics of nitroxide radical trapping. 2. Structural effects, J. Am. Chem. Soc. 1992, 114, 4992M996. 

Two examples of radical inhibitors that are present in biological systems are vitamin C and vitamin E. Like hydroquinone, they form relatively stable radicals. Vitamin C (also called ascorbic acid) is a water-soluble compound that traps radicals formed in the aqueous environment of the cell and in blood plasma. Vitamin E (also called a-tocopherol) is a water-insoluble (hence fat-soluble) compound that traps radicals formed in nonpolar membranes. Why one vitamin functions in aqueous environments and the other in nonaqueous environments should be apparent from their structures and electrostatic potential maps, which show that vitamin C is a relatively polar compound, whereas vitamin E is nonpolar. 

Vitamin C traps radicals formed in aqueous environments (Section 9.8). It is an antioxidant because it prevents oxidation reactions by radicals. Not all the physiological functions of vitamin C are known. What is known, though, is that vitamin C is required for the synthesis of collagen, which is the structural protein of skin, tendons, connective tissue, and bone. If vitamin C is not present in the diet (it is abundant in citrus fruits and tomatoes), lesions appear on... 

Spin trapping with PMNB was applied to the radicals derived from initiator decomposition (formula 3) and their subsequent reactions with the model compounds (formula 5). Both initiator radicals could be trapped and identified. When model compounds were present during UV-irradiation, new radicals were identified from the ESR spectra. For dihydrocyclopentadiene (DHCPD) only one trapped radical was found and for ethylidene norbornane (ENB) two radicals. By comparison with computer simulated ESR spectra, it is concluded that the radicals of these model compounds are all allyl radicals (formula 8 and 9) formed by hydrogen abstraction from the models. Radical (8 a) has two stereoisomers but they have closely the same ESR spectra when trapped and cannot be separated. Radical (8 b) has two resonance structures (shift of double bond in the ethylidene group) but only one radical (8 b) is trapped, probably due to steric hinderance for trapping the methin radical. The DHCPD radical (formula 9) has two steric forms because the two allylic hydrogens are not identical. Once they are formed, the spin trap can only approach from one side and only one of the steric forms is trapped as shown in the ESR spectrum. 

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