Organic free radicals, electron

Recently McConnell has introduced the technique of labelling proteins lipid membranes, and DNA, with a stable organic free radical, the nitro-xide radical, see reference (24). A measurement of the line shapes of the ESR signal of this label has revealed the rotational times of the molecules to which it is bound. If paramagnetic analogues of the anti-tumour compounds are found then their progress within a cell will be readily followed using ESR. Complexes of all the metals of Table 1 with odd numbers of electrons (d1, d3, d5 and d1) are potentially useful. Thus... 

Another potentially very useful method of studying B 12-enzymes by electron spin resonance has been developed. This method involves attaching a stable organic free radical, in all cases studied so far a nitrox-... 

An exception to this rule arises in the ESR spectra of radicals with small hyperfine parameters in solids. In that case the interplay between the Zeeman and anisotropic hyperfine interaction may give rise to satellite peaks for some radical orientations (S. M. Blinder, J. Chem. Phys., 1960, 33, 748 H. Sternlicht,./. Chem. Phys., 1960, 33, 1128). Such effects have been observed in organic free radicals (H. M. McConnell, C. Heller, T. Cole and R. W. Fessenden, J. Am. Chem. Soc., 1959, 82, 766) but are assumed to be negligible for the analysis of powder spectra (see Chapter 4) where A is often large or the resolution is insufficient to reveal subtle spectral features. The nuclear Zeeman interaction does, however, play a central role in electron-nuclear double resonance experiments and related methods [Appendix 2 and Section 2.6 (Chapter 2)]. 

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... 

In order to identify organic free - radicals present at quantifiable concentrations during the sonication of PCBs, we employed Electron Spin Resonance (ESR) with a spin trap, N-t-butyl-a-phenyl-nitrone (PBN). PBN reacts with the reactive free - radicals to form more stable spin-adducts, which are then detected by ESR. The ESR spectrum of a PBN spin adduct exhibits hyperfine coupling of the unpaired election with the 14N and the (3-H nuclei which leads to a triplet of doublets. The combination of the spin-adduct peak position and peak interval uniquely identifies the structure of a free-radical. 

Senesi and Testini [147,156] and Senesi et al. [150,153] showed by ESR the interaction of HA from different sources with a number of substituted urea herbicides by electron donor-acceptor processes involves organic free radicals which lead to the formation of charge-transfer complexes. The chemical structures and properties of the substituted urea herbicides influence the extent of formation of electron donor-acceptor systems with HA. Substituted ureas are, in fact, expected to act as electron donors from the nitrogen (or oxygen) atoms to electron acceptor sites on quinone or similar units in HA molecules. 

Runge-Morris, M., Wu, N. Novak, R. F. (1994) Hydrazine-mediated DNA damage role of hemoprotein, electron transport, and organic free radicals. Toxicol, appl. Pharmacol., 125, 123-132... 

In 1960 Rex (22) first reported the use of electron paramagnetic resonance spectrometry as a method for demonstrating the presence of stable organic free radicals in humic acid. We felt that this technique might provide useful information about the structure of humic acid which was not readily available by other physical methods, particularly if relations between EPR spectra and chemically modified humic acids could be demonstrated. Our preliminary studies (26) confirmed this presumption. 

The recombination of free radicals can be highly exergonic, since in general a new bond is formed by the formerly non-bonding electrons. In the example of eqn. (4.59) the closed-shell molecule, N02, is then formed in an excited state which can decay through luminescence emission. Some organic free radicals show similar reactions but it must be stressed that in most cases the emission yields are low. 

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