Radiolysis applications

The application of techniques of pulse radiolysis offers the potential to determine rates of primary radiolysis induced reaction processes. This knowledge can be of great value in the determination of redox processes of Pu ions occurring in a wide variety of aqueous solutions. As a matter of fact, such information is essential to a prediction of the Pu oxidation states to be expected in breached repository scenarios. For an... 

Phenomenological evidence for the participation of ionic precursors in radiolytic product formation and the applicability of mass spectral information on fragmentation patterns and ion-molecule reactions to radiolysis conditions are reviewed. Specific application of the methods in the ethylene system indicates the formation of the primary ions, C2H4+, C2i/3+, and C2H2+, with yields of ca. 1.5, 1.0, and 0.8 ions/100 e.v., respectively. The primary ions form intermediate collision complexes with ethylene. Intermediates [C4iZ8 + ] and [CJH7 + ] are stable (<dissociation rate constants <107 sec.-1) and form C6 intermediates which dissociate rate constants <109 sec. l). The transmission coefficient for the third-order ion-molecule reactions appears to be less than 0.02, and such inefficient steps are held responsible for the absence of ionic polymerization. 

Assigning a mechanism for the formation of products resulting from ionic intermediates is aided by our knowledge of the probable primary ions and the elementary ion-molecule reactions which they may undergo. The second subject to be examined is the applicability of fragmentation patterns and mass spectrometric ion-molecule reaction studies to radiolysis conditions. Lastly, the formation and the chemistry of the ionic species in ethylene radiolysis will be summarized. 

The Application of Electrostatic Fields. The use of electric fields as a diagnostic tool in the radiolysis of gases was first introduced by Essex (13). Certain revisions of the original interpretation were required (6, 37) largely because the importance of ion-molecule reactions had been rediscovered. 

Application to Ethylene Radiolysis. The predominant ions in the mass spectrum of ethylene (1) are ethylene, vinyl, and acetylene ions, which together account for over 85% of the total ionization. A total of 38% of all ions are C2H4+, and since kF(ethylene) = 25.9 e.v./ion pair, the parent ion should be produced with a yield of at least 1.5 ions/100 e.v. absorbed in ethylene. Similar calculations for the probable yields of the other major ions lead to estimates of 0.96 vinyl ions/100 e.v. and 0.94 acetylene ions/100 e.v. Successive dissociations are relatively unimportant in ethylene. 

Considerable support exists for Reaction 18a (35). The application of an electrostatic field during radiolysis of ethylene-nitric oxide (I.P. 9.25 e.v.) mixtures showed no enhancement of the butene yields, consistent with an ionic mechanism. When mixtures of C2D4 and C2H4 are irradiated in the presence of nitric oxide, product butene consists almost entirely of C4H8, C4D4H4, and C4D8—evidence for a molecular association mechanism. 

The results of this study show (99) the involvement of fragments such as Cr(CO) , (3 < jc < 6) which react with CO molecules which come from any of several sources fragmentation of the original molecules, bulk radiolysis of the compound, application of an external atmosphere, or perhaps from intermolecular exchange. It was concluded from the data that diffusion processes are involved and that the relative rates of reaction and of diffusion away are important in determining the height of the annealing plateaus. 

The only respect in which the hot atom chemistry of organometallic compounds has so far been applied to other fields of study is in the area of isotope enrichment. Much of this has been done for isolation of radioactive nuclides from other radioactive species for the purpose of nuclear chemical study, or for the preparation of high specific activity radioactive tracers. Some examples of these applications have been given in Table II. The most serious difficulty with preparation of carrier-free tracers by this method is that of radiolysis of the target compound, which can be severe under conditions suited to commercial isotope production, so that the radiolysis products dilute the enriched isotopes. A balance can be struck in some cases, however, between high yield and high specific activity (19, 7J),... 

Carotenoid radicals — Many of the important oxidations are free-radical reactions, so a consideration of the generation and properties of carotenoid radicals and of carbon-centered radicals derived from carotenoids by addition of other species is relevant. The carotenoid radicals are very short-lived species. Some information has been obtained about them by the application of radiation techniques, particularly pulse radiolysis. Carotenoid radicals can be generated in different ways. "... 

Ogasawara, Ma Application of Pulse Radiolysis to the Study of Polymers and Polymerizations. VoLlQ5,pp. 37-80,... 

Investigations of the kinetics of hole transfer in DNA by means of pulse radiolysis of synthetic ODNs have provided details about the hole transfer process, especially over 1 /is, including the multi-step hole transfer process. Based on the investigation of the kinetics of hole transfer in DNA, development of the DNA nanoelectronic devices is now expected. An active application of the hole transfer process is also desirable from a therapeutical point of view, since hole transfer may play a role in improvement of quantum yield and selectivity of DNA scission during photodynamic therapy. The kinetics of the hole transfer process is now being revealed, although there is still much research to be performed in this area. The kinetics of adenine hopping is another area of interest that should be explored in the future. 

In their 1967 book, Atkins and Symons6a summarized much of the early ESR work on small inorganic radicals, many of which were produced by photolysis or radiolysis of single crystals of the precursor molecules. A good example of the application of the information that can be obtained from such single-crystal studies is the analysis of the spectrum of N03, produced by y-irradiation of KN03 crystals by Livingston and Zeldes.6b Table 4.1 summarizes the results. 

One striking prediction of the energy gap law and eq. 11 and 14 is that in the inverted region, the electron transfer rate constant (kjjj. = ket) should decrease as the reaction becomes more favorable (lnknr -AE). Some evidence has been obtained for a fall-off in rate constants with increasing -AE (or -AG) for intermolecular reactions (21). Perhaps most notable is the pulse radiolysis data of Beitz and Miller (22). Nonetheless, the applicability of the energy gap law to intermolecular electron transfer in a detailed way has yet to be proven. 

AK Pikaev, SA Kabakchi, IE Makarov, BG Ershov. Pulse Radiolysis and its Application. Moscow Atomizdat, 1980, pp 3-290 [in Russian]. 

Many transition metal complexes have been considered as synzymes for superoxide anion dismutation and activity as SOD mimics. The stability and toxicity of any metal complex intended for pharmaceutical application is of paramount concern, and the complex must also be determined to be truly catalytic for superoxide ion dismutation. Because the catalytic activity of SOD1, for instance, is essentially diffusion-controlled with rates of 2 x 1 () M 1 s 1, fast analytic techniques must be used to directly measure the decay of superoxide anion in testing complexes as SOD mimics. One needs to distinguish between the uncatalyzed stoichiometric decay of the superoxide anion (second-order kinetic behavior) and true catalytic SOD dismutation (first-order behavior with [O ] [synzyme] and many turnovers of SOD mimic catalytic behavior). Indirect detection methods such as those in which a steady-state concentration of superoxide anion is generated from a xanthine/xanthine oxidase system will not measure catalytic synzyme behavior but instead will evaluate the potential SOD mimic as a stoichiometric superoxide scavenger. Two methodologies, stopped-flow kinetic analysis and pulse radiolysis, are fast methods that will measure SOD mimic catalytic behavior. These methods are briefly described in reference 11 and in Section 3.7.2 of Chapter 3. 

Application of pulse-radiolysis techniques revealed that the following intramolecular and intermolecular electron-transfer reactions all exhibit a significant acceleration with increasing pressure. The reported volumes of activation are -17.7 0.9, 18.3 0.7, and... 

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