Hydrogen from radiolysis

The structures under consideration are indicated in Inserts VI and VII. Such radicals are usually secondary products of radiolysis, formed, for example, by extraction of hydrogen from a R2C(H)-precursor. However, closely related radicals can be formed, for example, by electron-loss from vinyl- or allyl- derivatives (66,67) or from substituted aromatic cations (68,69) [see, for example, VIII-X]. 

Hart (1952, 1954) studied the oxidation of formic acid by the radiolysis method. In the presence of oxygen, hydroxyl radicals abstract hydrogen from HCOzH. Both the carboxyl radicals and formyl radicals are formed. These radicals undergo oxygen addition and subsequently dissociate ... 

Early on, the differences in the products from radiolysis of water were noticed. It was found, as mentioned above, that the radiolysis of pure water seemed to lead to almost no damage. If there were impurities in the water, radiation damage would occur. However, irradiation by high LET radiation would clearly lead to the formation of hydrogen. Further experiments showed that if one irradiated a sealed sample with high-LET radiation, and then the sample was irradiated with a low-LET source, the gas formed by the radiation would then disappear and it would appear as if there were no long-term decomposition. These data were part of the reason that A. O. Allen proposed the theory of radiation chemistry where back reactions occurred. ... 

Irradiation of saturated aliphatic compounds typically results in imsaturation, polymerization, and isomerization. The radiolysis of cyclohexane illustrates all three of these processes. If the radicals are very energetic, cyclohexene can be formed by the abstraction of hydrogen from a cyclohexyl radical either by a hydrogen atom or by another cyclohexyl radical. If the radicals become thermalized, recombination of radicals can occur to give bicyclohexyl. A less frequent process is rearrangement, followed by hydrogen atom capture to yield methylcyclopentane. 

Although the radiation yield of hydrogen from irradiated polymers became no concern from the point of view of safety, it remained the object of interest from the point of view of mechanisms of radiation induced chemical reactions. In analogy to water radiolysis, the yield of hydrogen become of interest if radiations of higher LET values were used for irradiation of polymers [19], These are proton beams and alpha radiation, which can suggest different yields of hydrogen, and they really do. 

Radiolysis of cyclohexane in the presence of 20 mmoFL of N O resulted in the reduction of the H yield by about one third [Scholes and Simic, 1964]. Later, Asmus et al. [1970] estimated the contribution of ionic mechanisms to the radiolytic production of hydrogen from hydrocarbons, to be about one third. These observations suggest that the recombination of electrons with the cations contributes to the formation of H in reactions such as (R-5) and (R-7). In the presence of N O, the reactions (R-5) and (R-7) could be modified resulting in reduced yield of hydrogen because of the occurrence of (R-16) and the following types of reactions [Foldiak, 1981] ... 

Steady-state radiolysis was used to study the efficiency of tirapazamine radicals in abstracting hydrogen from alcohols or sugars, by measuring the chain length in reaction sequences such as ... 

Table II lists the relative rate constants for hydrogen abstraction from alkanes and alcohols by alkyl and aryl radicals. As can be seen, the relative rates of abstraction of primary, secondary, and tertiary hydrogens from alkanes by the various radicals are about the same, with the exception of the radiolysis values for the H atom. A similar observation is made...

Krylova and Dolin [319,320] investigated the radiolysis of water adsorbed on silica gel induced by y-radiation ( Co). The radiolysis products, H2 and CO2, formed as a result of the energy transfer from the solid adsorbent silica gel to the molecules of the adsorbed water. The oxidation component of radiolysis is spent on the oxidation of trace amounts of organic compounds present on the sample surface carbon dioxide results. Radiolysis was investigated in relation to the conditions of the prehminary treatment of Si02, the amount of adsorbed water, and the intensity of the radiation. The formation of hydrogen from the surface OH groups of sihca gel was also possible. A likely mechanism of radiolysis was described. 

In Fig. 47 the proportion of various types of radicals is presented indicating the radiolysis route. The evolution of hydrogen from irradiated polystyrene depends on the position of bond scission 22.3 % result from a-position, 39.2 % come out from 3-position and 38.5 % have the origin from benzene ring [60C1]. 

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