Oxyaminoxyl radicals

The features of initiation of free radical reactions in polymers by dimers of nitrogen dioxide are considered. The conversion of planar dimers into nitrosyl nitrate in the presence of amide groups of macromolecules has been revealed. Nitrosyl nitrate initiates radical reactions in oxidative primary process of electron transfer with formation of intermediate radical cations and nitric oxide. As a result of subsequent reactions, nitrogen-containing radicals are produced. The dimer conversion has been exhibited by estimation of the oxyaminoxyl radical yield in characteristic reaction of p-benzoquinone with nitrogen dioxide on addition of aromatic polyamide and polyvinylpyrrolidone to reacting system. The isomerisation of planar dimers is efficient in their complexes with amide groups, as confirmed by ab initio calculations. 

Essentially the same spectrum was observed for p-benzoquinone. In tbe spectrum shown in Figure 5.2, it is possible to distinguish the signals of three types of radicals. The triplet signal, characterised by the nitrogen coupling constant =2.82 mT and g = 2.0053, corresponds to oxyaminoxyl radicals [70, 87] arising from the addition of NO to the double bonds of the quinines ... 

Oxyaminoxyl radicals with similar ESR parameters have heen detected in reactions of NO with phorone [70] in which it is believed that these radicals are produced by... 

In this connection, the analogous structure of oxyaminoxyl radicals in TBQ seems the most preferable. 

Obviously, iminoxyl and acyl(alkyl)aminoxyl radicals cannot be formed by the direct interaction of NO with the double bonds of the quinines, but these radicals are produced as a result of thermal conversions of primary oxyaminoxyl radicals most likely because of the strong angular strain arising from their bicyclic structure. Oxyaminoxyl radicals can decompose, giving alkoxyl radicals, which subsequently break down, leading to the destruction of the quinine structure with the formation of acyl radicals containing nitroso groups ... 

The detection of some molecular products using IR spectra confirms the free-radical processes taking place in p-benzoquinones under the action of NO [86]. Two new bands at 1560 cm and 1340 cm" are due to the asymmetric stretches of NO groups in nitro compounds observed in the IR spectrum of TBQ exposed to NO. The formation of nitroso groups is also traced by the band at 1120 cm", indicating the presence of the nitroso trans-(X mtrs [92]. Evidence for the conversion of oxyaminoxyl radicals according to (Equations 5.144-5.146) is provided by the appearance of the absorption band at 1704 cm", which can be assigned to the carbonyl stretch in a, 3-unsaturated aldehydes. 

Oxyaminoxyl radicals of p-benzoquinones are sensitive to temperature a stationary concentration of the radicals varies reversibly with changes in temperature in an atmosphere of NO. The corresponding temperature dependence is given in Figure 5.3. 

At first at T < 300, the radical concentration increases and attains a maximum at 12 °C, but then it decreases with further lowering of temperature. Oxyaminoxyl radicals quickly disappear at room temperature within several minutes after pumping out NO2 from the samples. The mechanism of decay involves recombination of oxyaminoxyl radicals with the resulting formation of N O and two molecules of TBQ. The increase... 

Figure 5.4 Dependencies of initial rates of accumulation of oxyaminoxyl radical at 20 °C on the partial pressures of NO and...

Based on the kinetic data obtained, the formation and decay of oxyaminoxyl radicals can be presented by the following scheme ... 

The observed ability of iminoxyl radicals XIV to form unstable compounds with NO allows choosing between the structures XXb and XXc having approximately identical ESR spectra (Table 7.1). As shown previously (Section 5.5), interaction of the conjugated double bonds of p-benzoquinone with NO results in the formation of appropriate oxyaminoxyl radicals. Based on this fact, one can believe that the radicals XTV of structure XXc, having a system of conjugated double bonds similar to those in p-benzoquinone, are capable of recombining with NO ... 

On exposure of BQ to nitrogen dioxide, the formation of radicals I of oxyaminoxyl type[15] takes place by the following scheme [13] ... 

The diamagnetic (PD) is capable of generating nitrogen-containing radicals in specific reaction with system of the connected double bonds of p - quinines (Q) [42]. On exposure of BQ to nitrogen dioxide, the formation of radicals I of oxyaminoxyl type [43] takes place by the following scheme ... 

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