Radiolysis primary processes in the

Electron Spin Resonance Studies of Primary Processes in the Radiolysis of Transition Metal Carbonyls... 

Tripathi GNR (1998) Electron-transfer component in hydroxyl radical reactions observed by time resolved resonance Raman spectroscopy. J Am Chem Soc 120 4161-4166 TsaiT, Strauss R, Rosen GM (1999) Evaluation of various spin traps for the in vivo in situ detection of hydroxyl radical. J Chem Soc Perkin Trans 2 1759-1763 Tsay L-Y, Lee K-T, Liu T-Z (1998) Evidence for accelerated generation of OH radicals in experimental obstructive jaundice of rats. Free Rad Biol Med 24 732-737 Ulanski P, von Sonntag C (2000) Stability constants and decay of aqua-copper(lll) - a study by pulse radiolysis with conductometric detection. Eur J Inorg Chem 1211-1217 Veltwisch D, Janata E, Asmus K-D (1980) Primary processes in the reactions of OH radicals with sul-phoxides. J Chem Soc Perkin Trans 2 146-153... 

Land, E.J., Navaratnam, S., Parsons, B.J., and Phillips, G.O. (1982) Primary processes in the photochemistry of aqueous sulfacetamide a laser flash photolysis and radiolysis study, Photochem. Photobiol., 35 637-642. 

We are fully aware of the extensive review literature concerning the mechanism of primary processes in interaction of the ionizing radiation with matter.5,1, 20 25 However, the most recent of the cited reviews has been written more than a decade ago. The last ten years are marked by intensive development of experimental studies and by the appearance of new theoretical conceptions that change some of the traditional views on primary processes. In this review we discuss the modern ideas concerning the primary radiolysis stage that take into account the latest developments in this direction. 

Nevertheless, with the oldest extraction system in the nuclear field, the TBP-alkane solvent has been studied since the 1950s, and authors have succeeded in proposing a detailed qualitative and quantitative description of the radiolysis. Many authors have focused on the primary process of the mechanism for pure TBP (18, 279-281, 283-287). The formation rate of TBP degradation products has been estimated under several experimental conditions and a degradation scheme proposed with rate constants (9, 311-313). An empirical equation allowing the calculation of the yield of TBP decomposition and degradation product formation in the presence of HN03, Pu(N03)4, and U02(N03)2 has been proposed (9, 11). 

A large number of studies in the last decade have been concerned with primary processes in irradiated polymers. Unstable intermediates have been trapped and identified at low temperatures or detected by pulse radiolysis. In some cases, reactions of these intermediates have been identified and workers have attempted to correlate such reactions with the chemical effects of irradiation observed at room temperature. Some of the conclusions drawn will be summarized below. The elementary processes during irradiation at 77°K will be discussed first. The reactions of charged species, excited states and neutral radicals will be outlined successively. 

Ethylene. The radiolysis and photolysis of ethylene have been studied extensively, and again both dissociation and free radical processes are well established. A review by Meisels (28) and the references therein discuss the mechanisms in detail. In our experiment the detection of ethyl radicals clearly indicates the prior formation of H atoms, a major primary radical in ethylene radiolysis (27). 

Table III also shows that hydrogen and the chlorinated butanes are reduced substantially when ethyl chloride is irradiated in the presence of benzene. The other products are essentially unaffected by this additive. In the radiolysis of certain alkanes (4), benzene, added in small amounts, does not interfere with the fast ion-molecule reactions of primary ionic fragments or with free radical processes, but it will efficiently condense unreactive or long-lived ions in the system. It is reasonable to assume that this is also true for alkyl halide systems and that the reduction in product yields compared with the pure compound upon adding benzene may be attributed to the interception of unreactive ions. Since the rate constants for reactions of the expected primary ions with ethyl chloride are very large (see Table II), the concentration of benzene used in our experiments should not interfere with the initial fast ion-molecule reactions. For ethyl chloride ion-molecule reactions, C4Hi0C1+ is the only unreactive ion of appreciable abundance which is expected in this system at the elevated pressures used in the radiolysis experiments. Thus, the reduced product yields in the presence of benzene additive can be identified tentatively with the removal of this stable ion and the elimination of its resultant neutralization products.

Without going into details to be discussed elsewhere (20), we may point out the main difference between the two approaches as follows While our approach tends to emphasize the contribution gs of slow secondary electrons to the primary yields, Platzman s treatment puts generally more emphasis on the internal energy of the ionized species than we do. Consequently, his absolute values of primary yields are, in general, lower than ours, and a significant part of the observed decomposition is implicitly expected to occur in subsequent physicochemical and chemical stages of radiolysis. In contrast, our approach explains most of the observed decomposition by the primary processes of the physical stage. 

Low energy (55 eV) electron irradiation of multilayers of methanol on silver has been used in conjunction with temperature-programmed desorption of the products as a method to identify radiolysis products. In this case several products such as methoxymethanol were identified . The primary photochemical process in the methanol system is thought to involve the fission of a C—H bond forming a heteroatom stabilized cation such as 6. In addition to methoxymethanol glycolaldehyde (5) or methyl formate are also detected. Apparently the methanol system is unreactive to UV irradiation either at 240 nm or at 355 nm . 

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... 

It has been noted that results of steady radiolysis experiments provide adequate data for separations related problems. The difficulty is that in the absence of kinetic data for the primary process it becomes necessary to repeat this type of experiment for each particular set of concentrations and times. 

Acetylene Ion. No evidence for the contribution of ion-molecule reactions originating with acetylene ion to product formation has been obtained to date. By analogy with the two preceding sections, we may assume that the third-order complex should dissociate at pressures below about 50 torr. Unfortunately, the nature of the dissociation products would make this process almost unrecognizable. The additional formation of hydrogen and hydrogen atoms would be hidden in the sizable excess of the production of these species in other primary acts while the methyl radical formation would probably be minor compared with that resulting from ethylene ion reactions. The fate of the acetylene ion remains an unanswered question in ethylene radiolysis. 

ESR studies and product determination they concluded that the main primary process for radiolysis of both isomers is the dissociation of allylic C—H bonds. The formed hydrogen atoms may add to double bonds or abstract other hydrogen atoms (mainly allylic ones). The ESR spectrum of the radiolysis product at 77 K showed the presence of the cyclo-hexadienyl radical in the case of 1,4-cyclohexadiene, whereas the main intermediate from... 

Despite the present poor understanding of the primary processes, the radiolysis of carbohydrates is a unique way to generate carbohydrate radicals within the carbohydrate matrix. It has enabled some interesting chain reactions to be detected (see Sect. IV,1). Many radical reactions observed in aqueous solution are also observed in the solid state. In addition, there are reactions that are more pronounced in the solid state, and their products can be studied more readily than in the liquid state. 

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