Radiolytically generated solvated

Packer and Richardson (1975) and Packer et al. (1980) made use of the fact that electrons can be generated in water by y-radiation from a 60Co source (Scheme 8-29) to induce a free radical chain reaction between diazonium ions and alcohols, aldehydes, or formate ion. It has to be emphasized that the radiolytically formed solvated electron in Scheme 8-29 is only a part of the initiation steps (Scheme 8-30) by which an aryl radical is formed. The aryl radical initiates the propagation steps shown in Scheme 8-31. Here the alcohol, aldehyde, or formate ion (RH2) is the reducing agent (i.e., the electron donor) for the main reaction. The process is a hydro-de-diazoniation. 

They may be formed by the solvation of radiolytically generated secondary electrons after these undergo thermalization 106, 123). This has been demonstrated in water (66, 83, 84) in alcohols 117) and in ethylenediamine 12). The presence of long-lived trapped electrons has been demonstrated in radiolyzed alkaline ice 71, 86), and their behavior was found to be analogous to elq. 

Since its first modem application in the late fifties, radiolysis has been extensively employed for generating transient ion-dipole complexes in the gas phase and to investigate their reactivity in the absence of the perturbing effects of solvation and ion pairing. The growing impact of the radiolytic technique in the gas-phase... 

Finally, solute radical ions can be generated by light-induced, one-photon or multiphoton ionization of their parent compounds (Chaps. 5 and 16). This approach is particularly useful in the ultrafast studies of short-lived, unstable radical ions that aim to unravel their solvation, recombination, reaction, and vibrational relaxation dynamics of the primary charges (see, e.g., Chap. 10). Whereas the time scale of radiolytic production of secondary ions is always limited by the rate with which the primary species reacts with the dispersed parent molecules, light-induced charge separation can occur in <100 fsec. There are many studies on photoionization of solute molecules in liquid solutions we do not intend to review these works. 

Although the mechanism of the photo-induced generation of mono- and bimetallic metal clusters, except for the photographic application (Section 20.6), has been studied with considerably less detail than for the radiolytic route, some stable clusters, mostly of noble metals (Ag, Au, Pt, Pd, Rh), have also been prepared by UV excitation of metal ion solutions [129-141]. Generally, halides and pseudo-halides counter anions are known to release, when excited, solvated electrons, which reduce the metal ions up to the zerovalent state. Oxalate excitation yields the strong reducing carbonyl radical COO [30]. Photosensitizers are likewise often added [142]. Metal clusters are photo-induced as well at the surface of photo-excited semiconductors in contact with metal ions [143,144]. 

Examples of the chemical fate of the reactants generated radiolytically in specific media are described in this chapter and, as one would expect, in many cases the reactions involve the gain or loss of a single electron. This is always true for the solvated electron which can only react by transfer into a vacant orbital of an acceptor ... 

Photochemical colloid syntheses fall into two categories the reduction of metal salts by radiolytically produced reducing agents such as solvated electrons and free radicals, and the photolysis of photolabile metal complexes. Again, the essence of these preparative procedures lies in the generation of zerovalent metal under conditions which prevent, or at least retard, the formation of bulk metal predpitates. 

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