Free semiquinone radical

The UV-Vis-near-IR absorption spectral change of FcCHCBQH in acetonitrile, on stepwise addition of CF3SO3H (0-100 eq), indicated an interesting proton response, as shown in Figure 13b. The new intense bands appearing at 400 and 456 nm, the molar extinction coefficients, e values, of which were estimated at 4.0 X 10 and 2.5 X lO mol dm cm, respectively, were appreciably similar to those of the free semiquinone radical," " and the weak shoulder band at 600 nm with 8 = 350 mol dm cm" can be attributed to the E2g -> Eiu transition of the ferrocenium ion. The EPR spectrum of the frozen solution of the product at 6.4 K exhibited a well-resolved signal (g = 3.97, = 1.64) attributable to a ferrocenium... 

When catechol was oxidized with Mn04 under aprotic conditions, a semiquinone radical ion intermediate was involved. For autoxidations (i.e., with atmospheric oxygen) a free-radical mechanism is known to operate. 

The decay of amine oxidized by hydroperoxide occurs much more rapidly than free radical generation. Apparently, these reactions proceed by chain mechanism. The diatomic phenols and aryldiamines (QH2) must react with ROOH by the chain mechanism in which the semiquinone radical -QH that reduces hydroperoxide plays the key role. The following chain mechanism can be supposed [122] ... 

Wurster in 1879 had already prepared crystalline salts containing radical cation 23 (equation 12). Subsequently, radical cations of many different structural types have been found, especially by E. Weitz and S. Hunig, and recently these include a cyclophane structure 24 containing two radical cations (Figure 3). Leonor Michaelis made extensive studies of oxidations in biological systems, " and reported in 1931 the formation of the radical cation species 25, which he designated as a semiquinone. Michaelis also studied the oxidation of quinones, and demonstrated the formation of semiquinone radical anions such as 26 (equation 13). Dimroth established quantitative linear free energy correlations of the effects of oxidants on the rates of formation of these species. ... 

Many antioxidants quoted as potential protective agents against free-radical-induced DNA damage have more than one phenolic group. Their chemistry is, therefore, also of some interest in the present context. The semiquinone radicals, derived from hydroquinone by one-electron oxidation or from 1,4-benzoqui-none by one-electron reduction, are in equilibrium with their parents (Roginsky et al. 1999), and these equilibria play a role in the autoxidation of hydroquinone (Eyer 1991 Roginsky and Barsukova 2000). Superoxide radials are intermediates in these reactions. 

In some cases radical cations may undergo cycloadditions with an acceptor derived intermediate without prior proton transfer. This is observed especially for radical cations without sufficiently acidic protons, although it is not limited to such species. For example, the photoreaction of chloranil with 3,3-dimethylindene results in two types of cycloadducts [141]. In the early stages of the reaction a primary adduct is identified, in which the carbonyl oxygen is connected to the p-position of the indene (type B) in the later stages this adduct is consumed and replaced by an adduct of type A, in which the carbonyl oxygen is connected to the a-position. CIDNP effects observed during the photoreaction indicate that the type B adduct is formed from free indene radical cations, which have lost their spin correlation with the semiquinone anions. 

Vivo, 1955 Fukuzumi et al., 1975). Radical formation at pH 6 apparently depends on both the concentration of hydroquinone and the amount of oxide. This is in accord with the studies of Fukuzumi et al. (1975) and Ono et al. (1977) at pH 9, in which the formation of radicals obeys first-order kinetics with respect to both phenol concentration and the amount of Mn oxides. The presence of semiquinone radicals indicates that the reduction of hausmannite involves a one-electron transfer process. The radical concentration initially increases, but then decreases simultaneously with the consumption of dissolved O2 (Fig. 8-12). Once O2 is depleted, the concentration of free radical gradually increases again. The rapidly generated semiquinone anion radical is apparently slowly oxidized by dissolved O2 in solution. The radical becomes more abundant at relatively high concentrations of hydroquinone. Oxide suspensions containing high concentrations of hydroquinone have insufficient capacity to oxidize hydroquinone to quinone completely, resulting in the accumulation of the semiquinone radicals. 

In some situations substrate-derived free radicals occur by non-enzymatic reactions following product formation. This phenomenon seems to be common in enzyme reactions involving either two-electron oxidation of a hydroquinone or two-electron reduction of a quinone. In each case a mixture of quinone and hydroquinone is produced which is in chemical equilibrium with semiquinone radicals. 

In 2007, Dellinger et al. (27A27) conducted and reported on the formation and stability of resonance stabilized free radicals of the type hypothesized by Pryor and his associates in the particulate phase of MSS. They concluded that the commonly observed free radicals in the particulate phase of MSS were not a surface associated semiquinone and were more likely an intrinsic, polymeric radical with a delocalized electron. The EPR signal observed by Pryor in the alcohol extract of the particulate phase of MSS may be from an extracted and autooxidized hydroquinone, not a particulate-phase-associated semiquinone radical. The semiquinone radical was observed in the particulate-phase MSS collected below 400°C and has a five-line spectrum with g 2.006. Semiquinone radicals were formed in the particulate phase of MSS only after aging. 

The radicals are persistent and are observed in stored samples for several months. As in the case of EPR spectra of solid cigarette tar, these semiquinone radicals are considered as a dynamic mixture of hydroquinone (QH2), semiquinone (QH ) and quinone (Q). The quantitative measurements (with standard the stable free radical DPPH) showed that the free radical concentrations were in the range of lO -lO spins/g. This range of spins/g is comparable to the solid tar of cigarette smoke, collected on a Cambridge filter, extracted with benzene and dried under reduced pressure) (Squadrito et al. 2001 Valavanidis et al. 2005a, b). 

Binding studies combined with ESR spectroscopy have revealed the mechanism of the interaction of metal ions with melanins which indicates the formation of a chelate complex between di- and trivalent diamagnetic metal ions and o-semiquinone radical centers on the pigment polymer (85, 124). This interaction often results in an increase of total free radical concentration. Furthermore, the binding capacity varies for melanins of different origins with the number of reactive sites (230). 

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