Nitroxide radicals stability

The presence of /3-hydrogen in the nitroxide radical may lead to disproportionation reactions. In spin-trapping experiments, N-t-butyl-a-phenyl nitrone yields rather unstable spin adducts. This type of radical can be stabilized by coordination to Nin. The Ni11 complex with N-oxy-A-r-butyl-(2-pyridyl)phenylmethanamine (923) reveals a distorted octahedral geometry with antiferromagnetic interactions between the unpaired electrons of the metal ion and the radical spins.00... 

When the lifetime of the radicals is very short and direct ESR detection is not an option, spin trapping is used to detect radicals at ambient temperatures. The method is based on the scavenging of radicals, P, by a spin trap, leading to the formation of a spin adduct with higher stability in most cases, this adduct is a nitroxide radical. 

Nitroxide radicals are widely used as spin labels in biology, biochemistry and biophysics to gain information about the structure and the dynamics of biomolecules, membranes, and different nanostructures. Their widespread use is related to an unusual stability, which allows researchers to label specific sites and to detect the most informative EPR parameters (g and hyperfme tensors) that are very sensitive to interactions with the chemical surroundings. Figure 2.1 collects all the radicals used in the following to illustrate the different aspects mentioned in the preceding section. 

The latter compound and its substituted derivatives readily react with polyvinyl acetate radicals. Substituents in the aromatic ring were found to influence the reactivity of the hydroxylamine slightly p = — 0-16 0-04 (Simonyi et al., 1967a). In comparing this value with those of phenols, the decreased substituent effect can be rationalized by considering two factors. First, the 0—H bond is more remote from the aromatic ring in hydroxylamine than in phenol. Second, the stability of phenyl nitroxide radicals is higher than that of phenoxy radicals. It is... 

Although nitroxide radicals do not seem to exist in naturally biological systems, they are relatively non-toxic, and survive as such for considerable periods after administration. This property makes them important as spin-labels in biological systems, ESR spectroscopy being an ideal technique for studying their behaviour. However, in these studies their radical nature is not significant chemically it is the presence of an unpaired electron, and the remarkable stability of these species that is utilized. However, nitroxides can act as electron donors and acceptors, and we compare them here with ascorbate radicals (Section 1.4). 

Nitroxide radicals continue to be those most widely studied by NMR. This stems from their relatively high stability, high solubility in a wide variety of solvents, and their well resolved NMR spectra when measured in concentrated solutions in which spin exchange is sufficiently rapid. H and/or data have been reported for some aryl... 

As mentioned above, intermolecular interactions are the interactions that a stable molecule experiences in the presence of other molecules (not necessarily of the same type) [1], We consider as stable molecules any aggregate of atoms having a long enough stability and lifetime, on a chemical scale. This definition also considers as stable molecules, open-shell radicals whose chemical structure makes them long-living, like those found in molecular magnets (for instance the family of the nitronyl nitroxide radicals). 

In Table III, the positive interaction between IMZ stearate (6) and the hydroxybenzoate stabilizer UV-Chek AM-340 is shown. This degree of lifetime enhancement was not achieved in any of the HALS compounds synthesized in which the hydroxybenzoate group was present as an intramolecular substituent. The proximity of the hindered phenolic group may in some way affect generation of the nitroxide radicals from the amine nitrogen. 

Synthesis and properties. The intrinsic stability of nitroxide radicals is enhanced by bulky substituents in the immediate vicinity of the NO group, as in di-t-butyl nitroxide radical (Forrester and Thomson, 1964 Rozantsev and Sholle, 1971). Phenyl nitroxide radicals, such as phenyl t-butyl nitroxide and diphenyl nitroxide, are known to decompose readily even in solid form by bimolecular attack of the NO oxygen on the p-carbon of another radical (Forrester and Thomson, 1964). 

Jeschke G, Schlick S (2006) Spatial distribution of stabilizer-derived nitroxide radicals during thermal degradation of poly(acrylonitrile-butadiene-styrene) copolymers a unified picture from pulsed ELDOR and ESR imaging. Phys Chem Chem Phys 8 4095 103... 

The monomers used in anaerobic adhesives and sealants generally contain at least one free-radical stabilizer, such as hydroquinone or />-methoxyphenol. It was found that ben-zoquinone, naphthoquinone, and similar compounds provided improved shelf stability without retarding the anaerobic cure [56]. It was also found that anaerobic formulations could be stabilized with a stable nitroxide free radical such as di-/-butyl nitroxide (LIV) [57]. The use of a soluble metal chelating agent such as tetrasodium EDTA (V) was found to be an effective method of stabilizing an anaerobic formulation against small amounts of metal contamination [58]. 

The same argument also provides an explanation for the stability of nitroxide radicals such as TEMPO, which we briefly mentioned in Chapter 2 (Section 2.5). 

The synergism exhibited by the ternary mixture of a-tocopherol, ascorbic acid and phospholipids has been shown to be due to the stabilization of a-tocopherol, on the basis of ESR studies with methyl linolenate oxidized at 90°C to detect the free radicals of a-tocopherol and ascorbic acid. Evidence was obtained by this technique for the formation of nitroxide radicals (R-N-0 ) in the presence of phosphatidylserine or phosphatidylethanolamine or soybean lecithin and oxidized methyl linolenate. However, as pointed out earlier (Section C), the synergistic activity of this ternary mixture may be derived from antioxidant products formed from the phospholipids at elevated temperatures by the Maillard browning reaction (Chapter 11). 

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