Spin-trapping technique, electron studies

Kuruc, J., Zubarev, V.E., Bugaenko, L.T., Macasek, F. 1988. X- and gamma-radiolysis of phosphate esters. Electron spin resonance study by the spin trap technique. J. Radioanal. Nucl. Chem. Lett. 127(3) 177-192. 

The electron paramagnetic resonance (EPR) spin-trapping technique was used to study the photochemical-, thermal-, and electrochemical-initiated decomposition of hydrogenated 1,2-diazocines 16 <1998MRC13>. 

The presence of hydroxylated compounds as reaction intermediates and the detection of hydroxyl radicals through electron spin resonance (ESR) technique lead to the conclusion that in many cases the oxidations are via HO jg-attack (Turchi and Ollis, 1989 1990). Other studies also report the detection of hydroxyl radicals with various techniques such as spin trapping with electron paramagnetic resonance (EPR) (Riegel and Bolton,... 

Use of Electron Spin Resonance Techniques. Electron spin resonance (ESR) studies have been used to examine both activity of antioxidants " and their location within the Uposome . Studies of antioxidant radicals via ESR provide data on the electron delocalization within the antioxidant, which can be correlated with antioxidant activity, although not always with very good agreement with inhibition studies . Spin traps have been themselves examined as potential antioxidants, and have been used to attempt to trap peroxyl species for study . However, trapped peroxyl species are not very stable and carbon-centered radicals have been preferentially trapped, even though in some studies other techniques (e.g. malondialdehyde/thiobarbituric acid, MDA/TEARS-technique) indicate the presence of peroxide species in the sample . Eremy s salt ((K+S03 )2N0 ) has been used in micellar systems to determine rate constants quantitatively for the antioxidants a-Toc and ascorbic acid and their derivatives, because it reacts with them in a way similar to peroxyl radicals and can be used as a spin probe in stop-flow ESR studies . ESR has also been used to monitor the loss of dPPH and galvinoxyl signal intensity... 

Because of interaction between the paramagnetic moment and that from nuclear spin, very small concentrations of free radicals can be detected by their influence on the hyperfine structure of electron spin resonance spectra (ESR). This technique is now much employed in free radical studies. Concentrations of the latter down to 1 in 10 M may be detected by ESR, but many free radicals are not sufficiently long-lived to attain even this concentration. Sometimes spin-trapping techniques are employed whereby highly unstable free radicals are reacted with suitable diamagnetic molecules to form relatively long-lived radical species. 

Carotenoid radical intermediates generated electrochemically, chemically, and photochemically in solutions, on oxide surfaces, and in mesoporous materials have been studied by a variety of advanced EPR techniques such as pulsed EPR, ESEEM, ENDOR, HYSCORE, and a multifrequency high-held EPR combined with EPR spin trapping and DFT calculations. EPR spectroscopy is a powerful tool to characterize carotenoid radicals to resolve -anisotropy (HF-EPR), anisotropic coupling constants due to a-protons (CW, pulsed ENDOR, HYSCORE), to determine distances between carotenoid radical and electron acceptor site (ESEEM, relaxation enhancement). 

Spin trapping is an often-used technique in the study of possible radical production in biological systems (for reviews see Kalyanaraman, 1982 Mason, 1984 Mottley and Mason, 1989), particularly by the detection and monitoring of spin adducts of the hydroxyl and hydroperoxyl ( OOH) radicals in view of their relation to possible damage mechanisms. This is a large area of research which it is not possible to cover in a limited review, and the treatment will therefore be restricted to a discussion of the electron transfer properties of biochemical systems (for a review on the application of the Marcus theory to reactions between xenobiotics and redox proteins, see Eberson, 1985) and... 

Electron spin resonance (ESR) spectroscopy is of application to organic species containing unpaired electrons radicals, radical ions and triplet states, and is much more sensitive than NMR it is an extremely powerful tool in the field of radical chemistry (see Chapter 10). Highly unstable radicals can be generated in situ or, if necessary, trapped into solid matrices at very low temperatures. Examples of the application of this techniques include study of the formation of radical cations of methoxylated benzenes by reaction with different strong oxidants in aqueous solution [45], and the study of the photodissociation of N-trityl-anilines [46],... 

Once deposition is complete and the initial reaction product is trapped in an inert gas matrix, characterization is carried out spectroscopically. Several spectroscopic techniques have been used the most common is infrared spectroscopy, either dispersive or Fourier transform. Raman spectroscopic studies have been carried out as well, but low signal levels have made this approach difficult. When the trapped intermediate is a free radical, electron spin resonance techniques are valuable as well. Finally, a number of researchers are employing electronic spectroscopy, when the species of interest has an absorption in the visible or ultraviolet tegion. 

Low temperatures that allowed one to trap long-lived species and electrons in glasses could be studied using spin resonance techniques. ... 

Zhang et al. [32] have reported the formation of a series of radical cations via the sonolysis of aqueous A-tetraalkyl-p-phenylenediamines. It is suggested that on sonolysis the aqueous solution forms hydroxyl radicals and via a single electron transfer the corresponding alkyl radical cation is produced. In this study both hydroxyl and the jV-tetraalkyl-p-phenylenediamine radicals were identified by ESR techniques. Christman et al. [33] obtained evidence for the production of free radicals in aqueous solutions due to microsecond pulsed ultrasound. Employing spin traps such as 5,5-dimethyl- 1-pyrrolidine-jV-oxide (DMPO) and 4-pyridyl-l-oxide-A-rert-butylnitrone (4-POBN) the ESR spectra obtained provided evidence for the formation of the free radicals OH- and H-. 

There has been a marked increase in the use of a combination of physical methods in most chemical studies. Included within electronic spectroscopy is the application of the relatively-new technique of photothermal lens spectroscopy (PTLS or TLS), and also a return of applications of X-ray photoelectron spectroscopy (XPS). P NMR spin trapping, a recent technique for detecting diamagnetic species, is highlighted as is the synthesis and microwave spectrum of 2-chloroethylphosphine, reported for the first time, and a novel square-wave voltammetric method used for determining organophosphates. 

The formation of phenoxy radicals upon oxidation from substituted phenols has been studied by the ESR technique, using nitrosodurene as spin trap [52]. Chloro(5,10,15,20-tetraphenylporphyrinato)cobalt(III) and its Ti-cation radical were used to promote the reaction. The phenols studied reacted with the cation radical but some of them interacted only with the complex. The reaction is pictured as involving electron transfer from the phenolate, mediated by an axially coordinated chloro ligand. 

Although Otsu et al. [12] have studied the BPO-DMA system by electron spin resonance (ESR) technique and trapped the aminomethyl radical, there is still a lack of direct proof of the above second step, particularly concerning the behavior of the aminium radical salt. We [13] have proposed the aminium radical salt with purple color through this reaction of DMT with CCI4 in the presence of O2 following the displacement reaction as ... 

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