Radiolytic reactions

Considerable interest has been reported in the radiolytic reactions of radiosensitiz-ing nitroimidazoles such ns Metronidazole, 2-methyl-5-nitro-l//-imidazole-1-ethanol (52). Again loss of the nitro function as nitrite appears to be one of the principal events. The formation of nitrite from /-irradiation of the Ni(II) complex of the imidazole 52 arises by hydroxy radical attack to form the radical anion. This either eliminates nitrite or undergoes a four-electron reduction to a hydroxylamino derivative68,69. 

In the usual situation the radiolytic reaction determines the rate of initiation and R, is given by... 

Y is either a solvated electron (displaced electron formed during the radiolytic reaction) or the product of the electron having reacted with some compound in the reaction system [Allen et al., 1974 Hayashi et al., 1967 Kubota et al., 1978 Williams et al., 1967]. If Y is an electron, the propagating carbocation centers are converted to radical centers that subsequently undergo reaction with some species in the reaction system to form molecular species. The termination rate is given by... 

Other than water, protein is the major constituent of meat averaging nearly 21% in heef or chicken meat, with fat varying fiom 4.6 to 11.0% in beef and fiom 2.7 to 12.6% in chickoi. The principal radiolytic reactions of aqueous solutions of aliphatic amino acids are reductive deamination and decarboxylation. Alanine yields NH3, pyruvic add, acetaldehyde, propionic acid, CO2, H2, and ethylamine (6). Sulfur-containing amino adds are espedally sensitive to ionizing radiation. Cysteine can be oxidized to cystine by the hydroxyl radical or it can react with the hydrated electron and produce... 

Vacuum ultraviolet photolysis of acetylene results in formation of triplet C2, as evidenced by its characteristic emission.139 Presumably, triplet acetylene is first formed and decomposes to C2 and H2. Saturated hydrocarbons undergo radiolytic reactions, but the relative importance of excited states versus ionized states has not yet been established with any certainty. 

Careful reviews by Raes (1985) and Raes et al. (1985) leave unanswered the question of the role of humidity, and of acid or organic vapours, in modifying the diffusivity of decay product ions. By comparison with the mobility in normal air of decay product and ordinary atmospheric small ions, the diffusivity of decay product small ions is probably 2 to 3 x 10-6 m2 s-1. For neutral atoms, or possibly oxide molecules, most measurements give D in the range 5 to 8 x 10-6 m2 s-1, except where radiolytic reaction products or reactive trace gases are present in sufficient concentration to form intermediate ions. 

In fact, the rate of formation of the carbonium ions from the 8-decay is simply proportional to the first power of the concentration of the tritiated compound in the system. On the other hand, the rate of formation of labeled radiolytic products varies with a higher power of the concentration of tritiated molecules which determines, in the first place, the intensity of the 3-radiation (and therefore the total rate of the radiolytic reactions), and in the second place, the probability that the radiolytic processes affect, in particular, a tritiated molecule. 

These considerations show that the rate of formation of tritiated products via radiolytic reactions can be reduced, in principle, to an insignificant fraction of the rate of formation of the labeled products from the reactions of the tritiated decay ions, simply by choosing a sufficiently low concentration of the multilabeled compound. 

Since the decay of a monotritiated molecule can produce only unlabeled carbonium ions, any tritiated product other than the starting material detected in the blank experiments could be formed only via radiolytic reactions. The blank runs carried out in all the systems investigated showed that the radiation damage of the sample does not represent a significant source of tritiated products, when compared to the reactions of the labeled decay ions, provided that the specific activity of the system was kept below 0-1—0-5 Curies per mole of gas. 

When people consider confinement effects, they consider mainly an increase in the encounter probability inside a single pore and therefore, expect an acceleration of the reaction. Such in-pore acceleration has been quantified by Tachiya and co-workers for diffusion-limited reactions through the so-called confinement factor [see Eq. (11.58) in Ref. 40]. From this treatment, confinement effects are expected to disappear when the reaction radius is less than one tenth of the confinement radius. Considering the reaction radii of radiolytic species, no acceleration by confinement should be expected for pore diameter larger than 10 nm. For smaller pore size, acceleration of the recombination reactions within spurs would be critical in the determination of radiolytic yields in the nanosecond time range. However, the existence of such an acceleration of radiolytic reactions has not been suggested in the nanosecond pulse radiolysis of zeolites and has still to be assessed using picosecond pulse radiolysis. 

Ionic reactions in ethyl chloride have been studied by both mass spectrometric and radiolysis techniques. The radiolysis mechanism advanced on the basis of our experimental observations indicates that the major radiolytic reaction mode in this system is excited neutral molecule decomposition. While the role of ionic reactions in the radiolysis therefore appears to be relatively minor, it was possible to establish a good correlation between the predictions of the mass spectrometric studies with respect to ionic intermediates and the participation of such ions in the radiolytic reaction scheme. These results emphasize the advantages of combining the techniques used here to obtain a complete description of the reactive system. 

The ozonide ion is paramagnetic with one unpaired electron and is apparently bent ( 100° O—O 1.2A) (cf. C102). There is evidence that 03 occurs as a reaction intermediate in the decomposition of H202 in alkaline solution24 and in radiolytic reactions.25... 

Post-irradiation reduction of cerium (IV) in concentrated sodium nitrate solutions was previously reported by Pikaev, Glazunov, and Yakubovich (20) in pulsed electron irradiations of 0.8IV sulfuric acid containing 0.5M sodium nitrate and 2 X 10"4M cerium (IV). They attributed the post-irradiation reaction to intermediate formation of pernitric acid, originally suggested by Allen (1) as an intermediate in the radiolysis of nitric acid solutions. Furthermore, they ruled out nitrous acid since it has been shown previously that the non-radiolytic reaction between cerium (IV) and nitrous acid goes to completion very quickly under the same conditions. However, they presented no evidence to refute the... 

Since we have obtained evidence for nitrous acid as the intermediate which causes post-irradiation reduction of cerium (IV) for 00Co gamma experiments, nitrous acid must also be the intermediate which causes the post-irradiation reaction in the pulsed electron experiments of Pikaev, Glazunov, and Yakubovich (20). Their failure to reach this conclusion may be because of non-uniform energy absorption which caused cerium-(IV) concentration gradients in their irradiated solutions and made difficult the duplication of identical conditions for the non-radiolytic reaction between cerium (IV) and nitrous acid. Energy absorption in the Ghorm-ley-Hochanadel (9) source, which we used, is essentially uniform. 

In the case of the carbonyl compounds, the reactive center of the radiolytic reactions is the carbonyl group. The radiolytic reactions in carbonyls include formation in ketones of carbon monoxide and breakage of the C-H bond adjacent to the carbonyl, in carboxylic acids of carbon dioxide, and in esters of carbon monoxide and carbon dioxide. The products formed during radiolysis of acetone and methyl acetate are listed in Table 11.7. 

Irradiation of PP in inert atmosphere results in similar yields for chain scission and crosslinking [Geymer, 1973]. Because of scission, the intrinsic viscosity of PP initially decreases. At higher doses, >100 kGy, gelation caused by crosslinking has been observed [Black and Lyons, 1959 Kondo and Dole, 1966]. Two factors contribute to this effect (i) the presence of trace amounts of hydroperoxides in the PP, which get consumed in various radiolytic reactions, e.g. ... 

The chemistry of "hot" intermediates pre-thermalized charges, highly excited solvent states, energetic fragments (such as "hot" H atoms). What is the nature of these states What role do they play in radiolysis How do they relax and react What happens to the heat dissipated in radiolytic reactions What is the mechanism for vibrational deactivation of the products Could the heat be... 

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