Fragmentation from radiolysis

The absorption of radiation produces unstable species. Flash photolysis does so by interaction of light with a solute. The transient may be a photoexcited state or a molecular fragment. Pulse radiolysis starts with highly reactive entities formed by dissociation of the solvent (e.g., H, eaq, and HO from H20) and consists of a study of their reactions or of reactive transients derived from them. In either case one monitors the ensuing reactions by luminescence (for excited states), light absorption, or conductivity changes. 

Direct measurement of the reaction of interest is sometimes possible using rapid reaction techniques. In laser flash photolysis, an intense, short-lived pulse of light irradiates the sample and the products are monitored by a variety of techniques, from basic UV/Vis spectroscopy to techniques - such as laser-excited fluorescence -which require a second, analytical pulse of radiation. In pulse radiolysis, a short (1-10 ns) pulse of high-energy (1-10 MeV) electrons irradiates the sample and the decay of the fragments can be analysed in the same way as the fragments from flash photolysis. The equipment for pulse radiolysis is even more complex and costly than that for flash photolysis, and tends to be concentrated in national facilities. 

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 origin of these radical species is also not known. It is often considered that they may result from recoil, either from the original molecule or from fragments of other molecules following collision Perhaps the most commonly assumed, and the most likely, source of free radicals is radiolysis of the target compound The pre-... 

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. 

Typical spectra obtained are shown in Fig. 1.2. Moreover, substantial radiolytically-mediated elevations in the concentration of serum formate, arising from the oxidation of carbohydrates present by OH radical, were also detectable. In addition to the above modifications, 7-radiolysis of inflammatory knee-joint synovial fluid generated an oligosaccharide species of low molecular mass derived from the radiolytic fragmentation of hyaluronate as outlined in the previous section dealing with oxidative damage to carbohydrates. The... 

Degradation products from the diluent (Figure 8.1, Compounds A and B). Compounds A include alkane dimers and fragments of dodecane. Compounds B result from nitration and oxidation of the diluent. These nitration and oxidation reactions arise from the actions of HN03, 02, H20, and their radiolysis products on organic components. 

The principle of the method involves the exposure of a mixture of two compounds (A) and (B) to ionizing radiation when a number of cross-products are formed from the fragments of the parent compounds. If A or B is labeled with tritium prior to irradiation, then the cross-products from the radiolysis will contain tritium in specific parts of the molecule. Ideal systems for the present labeling method are those in which A is an aliphatic molecule and B is aromatic or heterocyclic. Benzene and pyridine are suitable B components since these compounds can be labeled with tritium to very high specific activities by catalytic... 

The unrestricted and free electron transfer (FET) from donor molecules to solvent radical cations of alkanes and alkyl chlorides has been studied by electron pulse radiolysis in the nanosecond time range. In the presence of arenes with hetero-atom-centered substituents, such as phenols, aromatic amines, benzylsilanes, and aromatic sulfides as electron donors, this electron transfer leads to the practically simultaneous formation of two distinguishable products, namely donor radical cations and fragment radicals, in comparable amounts. 

Radical anions of haloaromatic compounds are proposed to be intermediates in different type of reactions. Their fragmentation rates, determined electrochemically [300] or by pulse radiolysis [301] range from lO " s for phenyl halides to 10 s for some halonitrobenzenes. The rate of the reaction for some aryl hahde radical anions is too high to be measured electrochemically, the fragmentation of more stable radical anions such as those of 1-bromo- and 1-iodoanthraquinone [302], p-[303] and m-bromo- [304] and p- [303] and w-chloronitrobenzenes [304] occurs at considerably lower rates and the reaction is favored from their photoexcited state. Aryl halide radical anions may present a-n orbital isomerism depending on the orbital symmetry of their singly occupied molecular orbital [305], a proposal derived from theoretical and experimental evidences [306]. The isomerism is possible... 

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