Spin trapping radical intermediate detection

In related work, the reactions of hydrogen peroxide with iron(II) complexes, including Feu(edta), were examined.3 Some experiments were carried out with added 5.5"-dimethyl-1-pyrroline-N-oxide (DMPO) as a trapping reagent fa so-called spin trap) for HO. These experiments were done to learn whether HO was truly as free as it is when generated photochemically. The hydroxyl radical adduct was indeed detected. but for some (not all) iron complexes evidence was obtained for an additional oxidizing intermediate, presumably an oxo-iron complex. 

One way to make the short-lived intermediates amenable to study is to increase their lifetime, usually by irradiation in the solid state and/or at very low temperatures. Then, the intermediates can be detected at the end of the irradiation by ESR or optical absorption spectroscopy. The ESR study of radicals in the solid state is done on single crystals, polycrystalline samples or frozen aqueous solution. In case of polycrystalline samples or frozen aqueous solution the identification of the radicals from the ESR spectra is difficult in many cases and, for better identification, the ESR experiment should be conducted on irradiated single crystals. Later, the method of spin trapping, developed for the liquid phase5, was extended to polycrystalline solids. In this technique the polycrystalline solids are /-irradiated and subsequently dissolved in a solution containing the spin trap. 

Special spin-trapping techniques are also available for the detection of short-lived radicals in both homogeneous and heterogeneous systems. For instance, a-phenyl A-ferf-butyl nitrone (PBN), ferf-nitrosobutanc (f-NB), -(4-pyridyl A-oxidc) A-ferf-butyl nitrone (4-POBN), or 5,5-dimethyl-l-pyrroline A-oxidc (DMPO) can be made to react with catalytic intermediates to form stable paramagnetic adducts detectable by ESR [135], Radicals evolving into the gas phase can also be trapped directly by condensation or by using matrix isolation techniques [139],... 

In the preceding eqnation, the primary anion-radical gives the l-chloro-2,2,2-trifluoroethyl radical. In vivo, this radical was detected by the spin-trapping method (Poyer et al. 1981). Ahr et al. (1982) had presented additional evidence for the formation of the radical as an intermediate in halo-thane metabolism and identified l-chloro-2,2-difluoroethene as a product of radical stabilization. Metabolytic transformations of l-chloro-2,2-difluoroethene lead to acyl halides, which are relevant to halothane biotoxicity (Guengerich and Macdonald 1993). 

In summary, the copper ion transfers an electron from the unsaturated substrate to the diazo-nium cation, and the newly formed diazonium radical quickly loses nitrogen. The aryl radical formed attacks the ethylenic bond within the active complexes that originated from aryldiazo-nium tetrachlorocuprate(II)-olefin or initial arydiazonium salt-catalyst-olefln associates and yields >C(Ar)-C < radical. The latter was detected by the spin-trap ESR spectroscopy. The formation of both the cation-radical [>C=C<] and radical >C(Ar)-C < as intermediates indicates that the reaction involves two catalytic cycles. In the other case, radical >C(Ar)-C < will not be formed, being consumed in the following reaction ... 

EPR spin-trapping technique can be used to detect free radical intermediates, which are free radicals that have a short lifetime to be detected by EPR—for example, OH (Janzen, 1980). The spin-trapping technique is based on the fact that during certain reactions in solution a transient radical will interact with a diamagnetic reagent to form a more persistent radical. The radical product accumulates to a concentration where detection and, frequently, identification are possible by EPR. The key reaction is usually one of attachment.The diamagnetic reagent is said to be a spin trap and the persistent radical product is then the spin-adduct (IUPAC, 1997). 

Jaeger CD, Bard AJ (1979) Spin trapping and electron-spin resonance detection of radical intermediates in the photo-decomposition of water at TO2 particulate systems. J Phys Chem 83 3146-3152... 

ESR spectroscopy has found wide-spread use for the detection of radical intermediates in electrode processes 40 For the same purpose, the newly developed technique of trapping short-lived radicals by nitrones or nitroso compounds 40d-> should be of considerable interest, as should also the chemically induced nuclear spin polarization (CINP) phenomenon 40e-1 be. 

Experimental evidence for the presence of radical intermediates is provided by the identification of expected products from radical rearrangements, by the use of appropriate radical probes and by direct detection by electron spin resonance (ESR). Other mechanistic evidence includes inhibition by radical traps, such as di-t-butylnitroxide (DTBN), TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxy), galvinoxyl and oxygen, and by radical anion scavengers such as p-dinitrobenzene (p-DNB). 

Many of these free radical intermediates have been detected directly with ESR. Others are too reactive to detect directly, but a method to stabilize these free radicals called spin trapping has proved successful. Spin trapping is a technique in which a short-lived reactive free radical (R ) combines with a diamagnetic molecule ( spin trap ) to form a more stable free radical ( radical adduct ) which can be detected by electron spin resonance ... 

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,... 

Two years later (1989), the same authors reported the oxygenation and oxidation chemistry of the Mn-substituted POMs XMnnWn039"- (X = P, Si, Ge, or B) and a2-P2MnnWi706i8-.316 These POMs undergo reversible oxygenation at low temperature in toluene or benzene solution but irreversible oxidation above 22°C. The 02 adduct can be intercepted by the spin trap 5,5-dimethyl-1-pyrroline A-oxide (DMPO). EPR spectra indicate formation of a polyanion-02-DMP0 intermediate that decomposes to the oxidized POM. The nonpolar organic solutions of these POMs catalyze the oxidation of 2,6- and 2,4,6-substituted phenols to benzoquinones or polyphenyl ethers, and a POM-02-phenoxy radical intermediate can be detected by EPR. 

It was previously shown that oxidation produced a radical cation which could be detected by ESR [111, 112] provided the 4 position was fully substituted. If the 4 position contained a hydrogen atom, then the initial oxidation was followed by deprotonation with the formation of a neutral radical which could undergo further oxidation or dimerisation. In-situ spin trapping using PBN demonstrated the presence of radical intermediates in the electro-oxidation of substituted 1,4-dihydropyridines. That the trapped radicals were the deprotonated neutral radical, and not the primary radical cation, was demonstrated by comparison of the ESR parameters with those of the spin adduct produced by electroreduction of A-methylpyridine ion cations, where only the neutral dihydropyridyl radical would be produced. The work of Volke and co-workers clearly demonstrates how spin trapping may be applied to the study of more complex organic electrode reactions and how comparison of ESR spectra generated from different precursors may be used to reveal the nature of the trapped radical. 

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