Nitrene radicals, from azides

Tlie formation of free radical HN by decomposition of hydrazoic acid has been suggested by a number of authors since 1928 (see Vol. HI, p. 167). This was substantiated by experiments on the decomposition of HNj by the flasli photolysis of Thrush (Vol. Ill, p. 167). The formation of nitrene radicals from azides by flash photolysis was reported simultaneously and independently by Koto (56], Reiser et al. [57—59]. Reiser rationalized the reaction derived from flash pl otolysis of formation of azo compounds (5) ... 

Upon photolysis or pyrolysis of phenyl azide in the gas phase the nascent phenyl nitrene is formed with enough energy to overcome a substantial barrier and ring contract to form cyanocyclopentadiene, the lowest energy isomer of C6H5N [43], In fact laser photolysis of phenyl azide in the gas phase does not produce emission from triplet phenyl nitrene but from the cyanocyclopenadienyl radical instead. 

This indicates that the thermally or photochemically decomposed azide (Figure 4) inhibits the dissolution of the styrene resin into the alkaline developer. The inhibition may be due to the increase of the molecular weight of the styrene resin in the presence of the decomposed azide. Hydrogen abstraction from the polymer by nitrene of the decomposed azide and subsequent polymer radical recombination result in a increase in the molecular weight of the polymer (17). 

These results indicate that vacuum curing occurs through a radical reaction mechanism and is terminated by reaction of the ring-opened epoxy group with the azide group (not nitrene) under exposure. There is a possibility that polymerization initiated by an exposure-induced radical cation may occur. Furthermore, it is thought that reaction products from both the azide and epoxide serve as dissolution inhibitors, because the sensitivity of EAP is almost the same as that of EP, as shown in Figures 1 and 2. 

Chemiluminescence has been used to measure the relative yields of excited ketones formed from self reaction of alkoxyl and alkylperoxyl radical pairs . In the photochemistry of aryl azides a dehydroazepine is detected by time resolved infra red spectroscopy and flash photolysis at room temperature . Singlet and triplet nitrenes and dehydroazepenes have also been detected in the photochemistry of 3- and 4-nitrophenyl azides . Picosecond and nanosecond laser photolysis of p-nitrophenyl acetate in aqueous media produces a triplet state of the -nitrobenzylanion and CO2 after cleavage of the rnr triplet. Absorption, emission, and reaction kinetics of dimethylsilylene produced by flash photolyses of dodecamethylcycloherasilane is another interesting study 2,... 

Very recently Cullin et al. [89b, c] have re-investigated the laser-induced fluorescence spectra produced from phenyl azide and Porter s precursors. Analysis of the rich vibrational-rotational spectra produced in a jet expansion demonstrated that the carrier of the spectrum could not be triplet phenyl nitrene but was more likely cyanocyclopentadienyl radical. 

It is conceivable that the excess vibrational energy imparted to phenyl azide from UV photolysis, combined with the exothermicity of the ensuing isomerization of singlet phenyl nitrene, may produce cyanocyclopentadiene with enough excess vibrational energy to fragment a CH bond to form the fluorescent radical 7, detected by absorption and emission spectroscopy. 

Photochemical and Radical Reactions Most photoreactions in the field of nanotube chemistry serve to the generation of reactive intermediates that attack the nanotube afterward. The functionalizing step itself is rarely photochemical. Examples of such a preparatory step prior to functionalization include the conversion of azides into nitrenes or the radical generation from acyl peroxides, iodoalkanes, etc. In the photochemical reaction of nanotubes with osmium tetroxide, on the other hand, the essential step occurs only under irradiation. 

From the effect of solvent (Table 15) it is evident that the reactions discussed are nitrene reactions hydrogen-rich solvents suppress ring contraction and give rise to solvent dimer (bibenzyl) and/or a yellow nitrene dimer. The structure of the dimer is not known, but one possibility is shown in 144. A similar (colorless) dimer was obtained from 9-phenanthridylnitrene at 500 ° 7). Xhe two dimers formed from 137 and 141 in cyclohexane have nearly identical IR spectra. How could a hydrogen-rich solvent promote dimeriztion There is evidence from aryl azide decomposition in solution that amino radicals are formed first, and these dimerize and dehydrogenate as shown for 1-naphthylnitrene in [Eq. (48)] 82). 

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