Nitroxyl photolysis

The formation of diphenylphosphino radicals on photolysis of triphenyl-phosphine, diphenylphosphine, and tetraphenylbiphosphine has been verified. In the case of the reactions of the phosphines, the radicals were trapped with t-nitrosobutane and the resultant nitroxyl radical [Ph2PN(0)Bu ] was identified by e.s.r. The nitroxyl radical has a small P splitting constant, demonstrating that there is no extensive delocalization onto the phosphorus atom. The e.s.r. spectrum of diphenylphosphino radicals, generated by photolysis of tetraphenylbiphosphine in benzene at 77 K, has been observed. When methanolic solutions of the biphosphine or triphenylphosphine are flash-photolysed, a transient species having Amax = 330 nm and which decays by first-order kinetics (A 4 x 10 s )... 

Fig. 27. ESR Spectra of nitroxyl methylcobinamide. (a) Before photolysis and (b) after anaerobic photolysis. In (b) the spectrometer signal level is set much higher than in (a) so that the residual signal intensity is only a few percent of initial intensity...

One other aspect of the photolysis of coordinate spin labeled derivatives is of interest. Nitroxides are good free radical scavengers (123). As a result, when methyl-cobalamin is photolyzed in the presence of a nitroxide, the methyl radical generated will react with the free nitroxide and cause disappearance of the ESR spectrum (123). However, once the nitroxide is coordinated it is no longer susceptible to attack by free radicals. Thus the nitroxyl function is quite well protected from approach by other species. 

In the presence of the nitroxide I, DiPK photolysis yields - in amounts equivalent to the loss of ketone -the two combination products of nitroxyl with the isopropyl and isobutyryl radical respectively (Fig. 3, reaction (6)) ... 

Nitroxyl radicals produced in the reactions of R02 with aminyl radicals react with peroxyl radicals. The latter reaction is considerably slower than the reaction of peroxyl with aminyl radicals, which can be seen from the following data derived by flash photolysis (the photolysis of bis(l,l-dimethylethyl)peroxide in toluene was performed in the presence of... 

Photolysis of benzofurazan N-oxide 292 in chloroform generates the nitroxyl radical 393153 (equation 141), which is formed due to hydrogen abstraction from the solvent... 

The photolysis of methyl nitrite at low temperature in an argon matrix was studied157. The products include formaldehyde, and nitroxyl HNO which also reacts to form N2O and water. The 355-nm photodissociation of gaseous methyl nitrite has been studied by monitoring the nascent NO product using a two-photon laser-induced fluorescence... 

To reveal the mode of dye-DNA binding, we studied quenching of the triplet states of thiaearboeyanine dyes K1 - K4 in solutions and in eomplexes with DNA (aqueous phosphate buffer, pH 7) by the stable nitroxyl radical 4-hydroxy-2,2,6,6-tetramethylpiperidine-l-oxyl (4-hydroxy-TEMPO), iodide ion, and oxygen. To study the properties of the triplet states of the dyes, we used flash photolysis technique. 

The processes of triplet state quenching of dyes K1-K3 and K4 by nitroxyl radical in liquid solutions and in complexes with DNA (cdna = 2.5 x 10 mol l ) were studied by the flash photolysis method. 

Dalby studied the flash photolysis of mixtures of ammonia with nitric oxide and observed the formation of nitroxyl, HNO. He reported that Liu, working in the Ottawa laboratory, did not observe HNO when NH3-O2 mixtures were pho-tolyzed. 

The method of spin-trap has been widely used for detection and identification of active radicals formed during polymerization photolysis, radiolysis and other procedures. This method is based on the reactions yielding nitroxyl radicals e.g.,... 

PMMA, which in itself is stable on exposure to NO2, enters into reactions after previous irradiation by UV light at 293 K [16]. The photolysis of PMMA induces the formation of double bonds as a result of ester group decomposition. The ESR spectrum observed after exposure of samples to NO2 is shown in Figure 2. The spectrum represents the superposition of the signals of two nitroxyl radicals at low frequencies of rotational mobility (10 s < Tc < 10 s) ... 

The formation of nitroxyl radicals during photolysis as well as in the comse of radiolysis of PMMA and AC in the presence of NO is explained by the following scheme ... 

The aliphatic hindered amines (e.g. Ill) fall into the second class [30]. Neither they nor their active transformation products, the stable nitroxyls absorb UV light in the 300-350 nm region and the latter functions as a catalytic trap for alkyl and alkylperoxyl (see Scheme 1.6). The photoantioxidants are ultimately destroyed by attack of alkoxy 1 or hydroxyl radicals (formed by photolysis of hydroperoxides) on the heterocyclic ring. The hindered amines also synergize strongly with the UV absorbers. 

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