Electron ejection reaction pathways

From a chemist s viewpoint, the most important act of ionizing radiation (usually X-rays, y-rays or high energy electrons) is electron ejection. Initially the ejected electrons have sufficient energy to eject further electrons on interaction with other molecules, but the electrons ultimately become thermalised and then are able to interact "chemically". We consider first various reaction pathways for these electrons, and then consider the fate of the "hole" centres created by electron ejection. [We refer to electron-gain and electron-loss centres rather than to radical-anions and -cations since, of course, the substrate may comprise ions rather than neutral molecules. 

A remarkably detailed study in which four pathways for excited radical decay are identified involves diphenylketyl radicals in acetonitrileS5 Electron ejection and dissociative OH cleavage reactions are amongst the alternative primary processes involved. 

Electron density calculations are less successful in accounting tor the reactions of benzenes with substituents such as methoxy, and there is strong evidence with these for a different pathway that involves ejection of an electron to form a radical cation (3.7) this is in keeping with the greatly enhanced electron-donor properties of an excited state. Flash photolysis studies support therormation of radical cations for methoxybenzenes on irradiation, and solvated electrons have also been detected in scavenging experiments. Subsequent attack by the nucleophile on the radical cation can then be rationalized by calculations based on this species rather than on the excited state. 

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