Aminyl radicals recombination

The bulky ter/-butyl substituent loses the free rotation of methyl groups during aminyl radical recombination. This results in the loss of entropy during the formation of the transition state and a decrease in the rate constant. Two isopropyl groups in the corresponding aminyl radical sterically hinder recombination and, therefore, disproportionation occurs. The transition state is rather compact in this case, and its formation is also accompanied by the entropy loss, which explains the low value of kj. 

The effects of uv radiation on V/-nitroso compounds depend on the pH and the medium. Under neutral conditions and ia the absence of radical scavengers, these compounds often appear chemically stable, although the E—Z equiUbrium, with respect to rotation around the N—N bond, can be affected (70). This apparent stabiUty is due to rapid recombination of aminyl radicals and nitric oxide [10102-43-9] formed duting photolysis. In the presence of radical scavengers nitrosamines decay rapidly (71). At lower pH, a variety of photoproducts are formed, including compounds attributed to photoelimination, photoreduction, and photo-oxidation (69). Low concentrations of most nitrosamines, even at neutral pH, can be eliminated by prolonged kradiation at 366 nm. This technique is used ki the identification of /V-nitrosamines that are present ki low concentrations ki complex mixtures (72). 

The most rapid reaction is N—N-dimerization (the rates of reactions A, B, C are related as 1 0.15 0.02 [94], Naphthylaminyl radicals recombine with the formation of N—C-dimers only [95], probably because voluminous naphthalene rings sterically hinder N—N-dimerization. A correlation between the rate constant of hyperfine splitting on the nitrogen atom of the aminyl radical and the rate constant of recombination of substituted ( (YC6H4)2N ) diphenyl-aminyl radicals was observed [95],... 

The literature data on the recombination of diphenylaminyl radicals correlate well (see Table 15.12). The rate constants of recombination of various aminyl radicals measured by flash photolysis vary from 105 to 109 L mol 1 s 1. For substituted diphenylaminyl radicals, a linear dependence was observed between log k9 and rr-Hammett [96] ... 

Rate Constants of Recombination and Disproportionation of Aminyl Radicals in Hydrocarbon Solutions Measured by the Flash Photolysis Technique... 

In solution photochemistry in the presence of acids, the primary process is also the same except that both NND and the aminyl radical are protonated the recombination of the aminium radical and NO to give 295A is too slow to compete with bond scissions174 (Scheme 11). The failure of oxygen to quench nitrosamine photoreactions in either solution (see below) or gas phases under various conditions must also mean a very short lifetime of singlet excited nitrosamines, in agreement with the fast dissociation159,160. 

The rate constant of this reaction was measured by flash photolysis with tetraphenyl-hydrazine as a source of diphenylaminyl radicals [64]. Peroxyl radicals were generated through the photodecay of bis(l,l-dimethylethyl) peroxide followed by the rapid reaction of formed alkoxyl radicals with cyclohexane used as a solvent the reaction conditions were chosen in such a way that [R02 ]o > [Ph2N ]0. Under these conditions, aminyl radicals were destroyed only in the reaction with peroxyl radicals and, hence, the concentration of peroxyl radicals during the measurement of the kinetics of Ph2N consumption varied insignificantly. The rate constant of the recombination of peroxyl and diphenylamine radicals was found to be equal to 6 x 108 L mol 1 s 1 (cyclohexane, 283-303 K). 

Two aminyl radicals combine to form the N—N-dimer with the rate constant proportional to the spin density on the nitrogen atom [96]. This recombination also depends on the steric factor. Thus, diethylaminyl radicals recombine with a rate constant of 109 L mol 1 s 1, while diisopropylaminyl radicals are only able to disproportionate with a rate constant of... 

In the literature, the simultaneous formation of the major NMMO degradation products N-methylmorpholine, morpholine and formaldehyde [20] is always attributed to the disproportionation of the primary aminyl radical 3, as a - not further defined - redox process between two molecules of 3, in which one is reduced to N-methylmorpholine (2) and the other oxidized to N-(methylene)morpholinium cation (17), which upon addition of water, forms morpholine and formaldehyde. Also this disproportionation can be rationalized as a radical coupling reaction which proceeds through recombination of N-centered 3 and C-centered 4, via the ammonium aminal intermediate 16 as the actual recombination product (Scheme 6). The intermediacy of this species was indeed confirmed by isolation from the reaction mixture and full characterization [17]. Compound 16 is quite labile and immediately de-... 

The photoreduction of 9,10-anthraquinone-1,5-disulfonate by 2,2,6,6-tetramethyl piperidine in aqueous media has been studied in the nanosecond and microsecond time domains by use of time-resolved optical and ESR measurements [171]. Electron transfer from the amine to the excited state of the anthraquinone derivative occurs with a rate constant of 5.7 x 10 m s . The aminyl radicals formed via deprotonation of the aminium radicals are long-lived (ca 0.5 ms), because the steric hindrance of these radicals slows down recombination reactions. The aminyl radicals formed in these systems have been characterized by ESR. 

Recombination of Aminyl Radicals. A convenient source of In is tetraphenylhydrazine, Inin. Diphenylamine and oligomeric semidienes were found to be the products of Inin decomposition at T > 400°K 0,4). The kinetics of their formation were studied by Varlamov (5). He found that InH and o-semidienes were primary stable products of... 

The N6-aminyl nucleoside radical of dA (120) has also been produced from a photochemical precursor (121) but not within DNA (Scheme 47). Photolysis (315 it 2 nm) of phenyUiydrazone 121 in the presence of GSH (50 mM) provides a 73% yield of dA. In the absence of excess thiol, the yield of dA is reduced (32%) as expected and is accompanied by the radical recombination product resulting from the dimerization of 120. 

Aliphatic aminyl radicals, as alkyl radicals, enter into disproportionation and recombination reactions. The reaction direction depends on the radical structure. For example, methylaminyl radicals predominantly recombine and diisopropylaminyl radicals disproportionate... 

It is seen that the recombination of aromatic aminyl radicals is not limited by dif-... 

In addition to the stabilization by suitable substituents and the absence of other termination reactions than recombination, it is the strength of the bond formed in the dimerization which is a necessary cofactor for the observation of free radicals by esr spectroscopy. The stability of nitroxides [4] or hydrazyls [5] (Forrester et al., 1968) derives not only from their merostabilized or captodative character but also from a weak N-N bond in the dimer. The same should be the case for captodative-substituted aminyls... 

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