Diphenylpicrylhydrazyl , effect

The biradical catalysts described previously for double-base propints (Ref 80) are also effective for hydrocarbon propints. Table 34 shows how p,p,-biphenylene-bis(diphenylmethyl) compares to n butyl ferrocene as a catalyst in a carboxy-terminated polybutadiene. These catalysts are claimed to overcome all of the processing difficulties, chemical stability and volatility disadvantages attributed to catalysts based on ferrocene and carborane derivatives. Another somewhat similar functioning catalyst, the free radical compd, 2,2-diphenylpicrylhydrazyl,... 

There is some evidence in favor ° of the captodative effect, some of it is from ESR studies. However, there is also experimental and theoretical evidence against it. There is evidence that while FCH2 and p2CH are more stable than CH3, the radical Cp3- is less stable that is, the presence of the third F destabilizes the radical. " Certain radicals with the unpaired electron not on a carbon are akso very stable. Diphenylpicrylhydrazyl is a solid that can be kept for years. We have already mentioned nitroxide radicals. Compound 29 is a nitroxide radical so stable that reactions can be performed on it without affecting the unpaired electron (the same is true for some of the chlorinated triarylmethyl radicals mentioned above ). ot-Trichloromethylbenzyl(rer/-butyl)aminoxyl (30) is extremely stable. In... 

These products are thought to result from attack of an intermediate p-nitro-phenyl radical on the solvent. Evidence for this intermediate was obtained by scavenging the radical with diphenylpicrylhydrazyl, halogens, and nitric oxide.(62) However, the presence of the p-nitrophenetole in the products formed in the presence of iodine, which effectively eliminates most other radical products, suggests that another mechanism involving a phenyl carbonium ion may be also operative ... 

An example (1S4) of the Overhauser effect in a nonmetallic system was afforded by diphenylpicrylhydrazyl (DPPH), an organic free radical. For partial saturation of the EPR of the radical, an approximately tenfold enhancement of the proton resonance from the ring protons of the DPPH molecule was observed. The Overhauser effect has also been observed in charcoals (proton resonance) and graphite (O resonance) 1S5), and natural crude oils (1S6). 

Cyclopentadiene undergoes ionic polymerisation by Friedel-Crafts catalysts32. Its polymerisation by y-rays is markedly suppressed by ammonia or amines but much less by diphenylpicrylhydrazyl or oxygen33. This again points to the ionic rather than the free radical nature of the radiation-induced polymerisation. The quenching effect of ammonia was postulated as due to reactions of the type... 

Aromatically substituted enols are easily autoxidized to the keto-hydroperoxide form (4, 5, 6, 10, 11, 12, 16), which, we postulate, would then initiate a radical polymerization. A radical mechanism is proved by the inhibiting effect of quinone and a,polymer yield is directly proportional to the square root of the initiator concentration (7). 

The number of lines in the ESR spectrum of a radical is a clue to its identity. Of further assistance is the intensity pattern in the spectrum. For n equivalent nuclei with 7 = 1/2, the n + 1 lines have intensities proportional to the binomial expansion of order n. Table 13.2 lists the relative intensities up to n = 5. The intensity patterns for sets of equivalent nuclei with 7 > 1/2 are handled in a similar fashion. Figure 13.5 shows the ESR spectrum of diphenylpicrylhydrazyl (DPPH) free radical in benzene. Under low-resolution conditions, the proton hyperfine coupling is not observed and the five-line spectrum with the intensity distribution 1 2 3 2 1 results from the interaction of the unpaired electron with two (effectively equivalent) (7=1) nuclei for nuclei with 7 > 1/2, the intensity distribution is more complex than a simple binomial expansion, and is beyond the scope of this text. 

Similar experiments with SO2 reacted surfaces showed that this had no effect on autohesion. Carley and Kitze (16) have recently speculated on the role of peroxidic groups in discharge treated surfaces. Their analysis of these groups, by reaction with diphenylpicrylhydrazyl (DPHH) and subsequent colorimetry, excluded the possibility of hydroperoxide reactions since these groups could not be detected by ATR. The results presented here must cast some doubt on the validity of their method. 

When steric effects are combined with stabilizing effects such as delocalization, radicals that are very "stable" can be prepared. For example, both galvinoxyl and diphenylpicrylhydrazyl (DPPH) are commercially available free radicals that can be handled with no special precautions (see margin). 

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