Spin-trapping experiments

Morkovnik et al. (1989) found experimentally that the addition of an equimolar amount of 4-morpholino- or 4-dimethylaminoaniline to a suspension of nitrosyl perchlorate in 100 % acetic acid, dioxan, or acetonitrile yields a mixture of the diazonium perchlorate and the perchlorate salt of the amine radical cation, with liberation of gaseous nitric oxide. Analogous results in benzene, including evidence for radicals by ESR spectroscopy and by spin trapping experiments, were obtained by Reszka et al. (1990). 

The reaction of OH radicals with dimethyl sulfoxide in aqueous solution was studied already in 1964 by Norman and coworkers . They used the system Tj -HjOj to produce OH radicals and using ESR/rapid mixing techniques they were able to demonstrate elimination of a methyl radical during the OH induced oxidation. Further studies showed the formation of sulfinic radicals in this reaction either directly or by spin trapping experiments . ... 

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... 

Spin trapping experiments have been performed recently in a fuel cell inserted in the ESR resonator ("in situ" cell), using DMPO and a-(4-pyridyl-l-oxide)-N-ferf-butylnitrone (POBN) as the spin traps [78,82,83], These experiments allowed the separate examination of spin adducts at the anode and cathode sides. 

From the EPR spectroscopist s viewpoint the spin-trap experiment is next to trivial the molecular mass of the radical adduct is small enough to guarantee the molecule to tumble sufficiently rapidly at ambient temperatures in aqueous solution to ensure complete averaging away of any anisotropy in the spin Hamiltonian ... 

Menadione (79) has been shown by Krishna et al. to act as a photosensitizer for thymine degradation. Spin-trapping experiments with several nucleosides showed that the photosensitizations occurred by electron transfer from the substrate to the excited triplet state of menadione to form the cation radical of the substrate and the anion radical of menadione, both of which were detected... 

The success and value of the spin-trapping experiment depend upon a number of features, all of which need to be taken into account when designing the experiment, or interpreting the spectroscopic results. These may be formulated as a series of questions. 

The value of the spin-trapping experiment as a qualitative (and perhaps as a quantitative) probe for radical reactions would be enhanced by two types of quantitative information. The first concerns rates of radical addition to the diamagnetic traps 9 the second involves the rates of decay of the various spin... 

Many of the early reports of spin-trapping experiments were focused on mechanistic investigations, and some of these feature in the early reviews (see p. 4). Unfortunately, it is in this application that inferences drawn may be most suspect. For example, the inability of the method to differentiate between radical trapping on the one hand, and a combination of nucleophile trapping with one-electron oxidation on the other, is a serious shortcoming. An early example of this was the tentative conclusion that acetoxyl radicals were spin-trapped by PBN competitively with their decarboxylation in reactions of lead tetraacetate. In view of the rapidity of the decarboxylation reaction, trapping of acetate ion and subsequent oxidation seems a likely alternative. 

Spin-trapping experiments connected with biological superoxide production have also been used to examine the biochemistry of anti-tumour antibiotics bleomycin (Sugiura and Kikuchi, 1978) and mitomycin C (Lown et al., 1978), as well as the effect of iron concentration on xanthine oxidase reactions (Buettner et al., 1978), and the photochemistry of melanins (Felix et al., 1978). 

The typical spin trapping experiment is designed in the following way. The spin trap is added in low concentration to a solution containing the components of a suspected radical reaction, the reaction is initiated (by addition of a critical component, by raising the temperature of the sample, by photolysis, by electrolysis, by radiation, etc.) and monitored by epr spectroscopy. The appearance and identification of a spin adduct epr spectrum of X—ST is then considered evidence that X was an intermediate in the reaction. 

Eberson and Nilsson (1990). The benzoyl t-butyl nitroxide [9], often detected in spin trapping experiments under oxidizing conditions. 

Table 11 summarizes results of spin trapping experiments where PBN-Nu and other ST-Nu" systems have been oxidized anodically at platinum. Originally, all the reactions were suggested to proceed via Nu radicals (Janzen et al., 1980 Walter et al., 1982), but the fact that PBN is oxidized at a lower potential than Cl-, CNO and CN- (Tables 1 and 5) clearly shows that the faster electrochemical process must be PBN— PBN + at the potentials employed. On the other hand, azide ion is oxidized in a faster reaction than any of the spin traps used and thus azide radical is implicated as being trapped. The Cr 4MePyPBN [17] system is a case where possibly Cl is involved in view of the high pa of this spin trap. 

Photochemical spin trapping experiments are the stock in trade, and the most difficult ones to judge with respect to mechanism because of their high complexity. The method became popular at a time when the effect of light upon molecules was believed to result mainly in homolysis of bonds, principally because of its ready use in combination with epr spectroscopy and... 

If an aqueous solution saturated with oxygen is sonicated hydrogen peroxide formation occurs. This is due to hydroxyl ( OH) and hydroperoxyl ( OOH) radical recombination outside the cavitation bubble (Scheme 4.1). These radicals result from HjO and O2 homolytic cleavage inside the bubble and have been observed by spin trapping experiments (Scheme 4.1) [19]. 

Indirect detection of the acyl radicals in the PFR has been achieved by means of spin trapping with 2-methyl-2-nitrosopropane and subsequent ESR detection of the resulting stable radicals [53]. The ESR technique has also been used for the study of the PFR of diphenyl carbonate but only a line g = 2.005) corresponding to the phenoxy radical could be detected [54], The recorded signal in the spin-trapping experiment consists of a typical sharp triplet with small... 

The critical involvement of alkyl radicals has been established in spin-trapping experiments although deuterium incorporation seems to implicate the eventual protonation of an anionic intermediate... 

Metallic silver was found to react with NBS and provided a convenient high yield route to the silver salt of succinimide (equation 19).280 Spin trapping experiments were performed to show that nitrogen-centred radicals were intermediates in the reaction. 

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