Aspects of mechanism and structure in the formation of muonated radicals

Abstract

Radiolysis effects associated with the implantation of positive muons in liquid samples are reviewed. The major processes to be considered resemble the normal reactions induced by ionising radiation; these are electron ejection and the subsequent chemical reaction of electrons and holes. The thermal or near-thermal reactions are described in terms of a general scheme which includes the various possible routes to the formation of muonium and of muonated organic radicals. The scheme emphasizes the consequences of the extreme dilution of the muon probe, and its outstanding reactivity towards solvents or solutes of even weak basicity, it its role as lightweight proton. The mechanism of radical formation in materials with widely different properties may be predicted from the general scheme. For the few compounds in which the mechanism has been elucidated experimentally this is compared with chemical expectations. Characteristics of the compounds and of their cosolvents which favour or inhibit the different possibilities are discussed in terms of the interplay between reaction mechanism and molecular structure. The balance between competitive processes may be altered by the presence of inert solvent. In protic solvents the balance may also be altered by proton exchange, and indirectly by a modification of reactivities due to hydrogen bonding. Mechanisms involving intermediate muonic cation or radical anion states are established in particular cases. These ionic states may also be the origin of the diamagnetic fraction in certain circumstances.