Radiolytic initiation

As an alternative to electrochemical or radiolytic initiation, homolytic dediazoniation reaction products can be obtained photolytically. The organic chemistry of such photolyses of arenediazonium salts will be discussed with regard to mechanisms, products, and applications in Section 10.13. In the present section photochemical investigations are only considered from the standpoint that the photolytic generation of aryldiazenyl radicals became the most effective method for investigating the mechanisms of all types of homolytic dediazoniations —thermal and photolytic —in particular for elucidating the structure and the dissociation of the diazenyl radicals. 

The radiolysis of olefinic monomers results in the formation of cations, anions, and free radicals as described above. It is then possible for these species to initiate chain polymerizations. Whether a polymerization is initiated by the radicals, cations, or anions depends on the monomer and reaction conditions. Most radiation polymerizations are radical polymerizations, especially at higher temperatures where ionic species are not stable and dissociate to yield radicals. Radiolytic initiation can also be achieved using initiators, like those used in thermally initiated and photoinitiated polymerizations, which undergo decomposition on irradiation. 

In related model complex studies, Isied and coworkers, have examined photo-induced (or pulse-radiolytically initiated) electron-transfer processes in which a polypyridine-ruthenium(II) complex is linked by means of a 4-carboxylato,4 -methyl,2,2 -bipyridine ligand and a polyproline chain to a [Co(NH3)5] + or [(-NH-py)Ru (NH3)5] acceptor. Chains composed of from zero to six cis-prolines have been examined. The apparent distance dependence of the electron-transfer rate constant, corrected for variations in the solvent reorganizational energy, seems to exhibit two types of distance dependence, 0.7-1A for short chains and /3 a0.3 A for long chains. A very detailed theoretical analysis of electron transfer in the complexes with four proline linkers has indicated that the electronic coupling is sensitive to conformational variations within the proline chain. ... 

In related model complex studies, Isied and coworkers, have examined photo-induced (or pulse-radiolytically initiated) electron-transfer processes in which a polypyridine-ruthenium (II) complex is linked by means of a 4-carboxylato,4 -methyl,2,2 -bipyridine ligand and a polypro-... 

The group of Isied measured pulse radiolytically initiated LRET from Os(II) to Co(ni) (163) and from Os(II) to Ru(III) (164). The rates and activation parameters were determined. Shanze and coworkers worked with a Ru(II) complex linked through polyproline to a p-quinone (165) and with a Re(I) complex-benzoate pair... 

The use of gamma radiation for the Initiation of free radical chain reactions In liquids offers several advantages over the more conventional methods of initiation. With this technique radical reactions that are not readily accessible by other methods can be studied. Because of the wide temperature range within which radiolytic initiation can be applied this method allows an accurate determination of Arrhenius parameters and therefore can bring better understanding and deeper insight to the factors that control the rates and mechanism of free radical reactions. 

Let us now consider some quantitative aspects of radiolytic initiation. The radiolytic yield, G value, is defined as the number of molecules that react per 100 eV of absorbed energy. 

The advantages and limitations of radiolytic initiation. Radiolytic initiation is particularly useful in kinetic studies of liquids phase free radical chain reactions. The advantages of this technique can best be seen by comparing it with other commonly used methods of initiation, photolysis and thermal decomposition of initiators. 

Finally, side reactions of the absorbent or the initiator may compete with and inhibit the chain reaction under investigation. The products of these side reactions interfere with the determination of the main reaction products. When radiolytic initiation is used the solvent is the main source of radicals and consequently these complications are minimized. 

Radiolytic initiation is not free from limitations and shortcomings. We have previously seen that ions are formed in the initial steps that follow the absorption of gamma radiation. 

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