Radical characteristics

More recently the use of esr techniques has allowed the unequivocal identification of free radicals in radiolyses. It will therefore be convenient, in this section, first to discuss this evidence and then to summarise some of the other radical characteristics, e.g. structure, spectra, reactivities, which have been deduced from systems where the supposition of free radical processes provides the simplest explanation of results. 

Another technique involving photolysis which is often effective involves exposure of a solid containing a trace of some photosensitive compound which absorbs in a region of lower energy than the onset of absorption by the solvent . Typically, dilute solid solutions of hydrogen peroxide in a wide range of solvents can be photolysed with a medium-pressure mercury arc. Early studies of this sort, described in Section III, involved the use of alcohols and it was shown that radicals characteristic of the alcohols were trapped. No evidence for trapped hydroxyl radicals was forthcoming and it was concluded that a secondary reaction occurred such as ... 

Effect of O2 and NO. These are radical inhibitors and scavenge the CH radicals, removing the source of C2H6, CH4 and the minor products. 02 has two unpaired electrons and NO is an odd electron molecule. This means that these unpaired electrons can pair up with the unpaired electron on radicals to produce a single bond. This removes the radical characteristics, which are dependent on the presence of at least one unpaired electron. The yields will then drop to zero. This is observed and confirms the central role of CH in the mechanism. 

Free Radical Characteristics and Reactions in Weathered Wood. Wood, wood fiber components, and isolated lignin contain certain amounts of free radicals that are detectable by ESR spectroscopy (SS, S9). Unexposed green wood with 69% moisture content (in dark and in vacuo) was found (77a) to contain no free radicals. A trace amount of free radicals may be produced in the presence of oxygen, and most of these free radicals are generated in wood during mechanical preparation (90) as well as in wood exposed to electromagnetic irradiation. ESR studies revealed that wood interacts readily... 

Alkyl halides that do not readily undergo nucleophilic attack often proceed by radical mechanisms . Reactions of IrCl(CO)(PMe3)2 showed radical characteristics and the scheme below was suggested ... 

A laser-flash examination of the photochemical behaviour of ketoprofen (73) in water has shown that deprotonation occurs with decarboxylation following as a second step to yield the benzylic anion (74). Interestingly this anion exhibits both anionic and radical characteristics. In another study of compound (73)... 

When ks,i46 > ks,i45 and substance X is not quickly lost by any termination reaction, the ozone decomposition flux according to cycle 1 becomes larger than it follows only from reaction (5.145). Reaction (5.147) is generally faster than (5.146) because of the stronger radical characteristics (with the exception of hydrogen atoms Table 5.14). The species H and OH are natural (in contrast to halogens and nitrogen species) constituents of the stratosphere (but with lower concentrations... 

Comparison of ESR spectra of radicals with various chain length at 150 °C. Dimeric model radical, model radical with Pn = ioo, and radicals in a radical polymerization (propagating radical). Characteristic lines were enlarged on the right hand side. 

In this most likely multistage reaction, the carboxylic acids and gaseous oxidative products are produce by peroxy radicals. Characteristic of the radiation-induced oxidation is the significant peld of carboxylic adds and low yield of hydroperoxides. The chain reaction involving hydroperoxides may be more important during the thermal oxidation. 

Due to the free radical characteristic of Tempamine nitroxyl radicals, the radical incorporated polyglycolide (PGA) must exhibit an EPR spectrum that has the characteristic of nitroxide. Figure 2 is such an EPR spectrum of TAM-PGA. 

Because the conversion of TAM nitroxyl radical (i.e., chemical reduction of nitroxyl radical) to hydroxypiperine structure in an acidic environment would remove the free radical characteristic of TAM, EPR could not detect the presence of hydroxypiperine and the spin numbers of TAM would continue to decrease with time. The extent of this conversion and hence the number of spins of TAM would depend on the strength of acidity of the medium. Figure 5 illustrates such an effect of pH on the peak intensity of EPR spectra of TAM nitroxyl radicals in glycolic acid media. As the pH of the media decreased from 7.44 to 4.0 (EPR spectrum b) and 3.0 (EPR spectrum d), the EPR spectra peak intensities were reduced accordingly. This would suggest that more TAM nitroxyl radicals would be reduced to hydroxypiperine at a lower pH and fewer TAM nitroxyl radicals would remain in an acidic medium and exhibit weaker EPR signal. 

Pacansky, J., Brown, D.W., and Chang, J.S., Infrared spectra of the isobutyl and neopentyl radicals. Characteristic spectra of primary, secondary and tertiary alkyl radicals, /. Phys. Chem., 85, 2562, 1981. 

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