The Simplest Free Radicals

INTRODUCTION Every lexicographer knows that the meaning of a word is time dependent, and, in favour of the argument that it is also dependent on latitude and longitude, the perverse amonst us would cite Oscar Wilde's dictum, "England and America are two countries separated by a common language". Certainly the word radical has changed its meaning for chemits who originally thought of it as part of a molecule which remained unaltered through a series of chemical processes. The man the centenary of whose birth we celebrate today inaugurated the 20th century by teaching us that such molecular fragments can exist as separate entities for finite periods, though they may react rapidly with a wide variety of substances, including themselves. Today a free radical is accepted as being either an atom or a group of covalently linked atoms having one or more unpaired electrons. It may be electrically charged, or electrically neutral. On this definition free radicals would include monatomic species such as H•, Li, 0, O., as well as polyatomic species such as Ht, .OH, O, 03, and the more familiar large stable organic radicals like DPPH and galvinoxyl. The mass of the free radical does not alter its essential qualities though it may alter the time scale of the reactions into which it may enter. For this reason, I should prefer to regard the simplest free radical as an electron, which obviously satisfies the basic criterion. If you will bear with me in this further stage of evolution of the meaning of the term 'free radical', I should like first to describe the chemistry of the electron as a reactive entity, and then mention briefly some newer aspects of the chemistry of H, OH and HO2 radicals. This order is appropriate because many of the techniques used for the study of the electron and many of the principles which its behaviour illustrates are equally applicable to other radicals, and in the last half decade or so an immense amount of new information about simple inorganic radicals has been accumulated, which has greatly illuminated some hitherto dark, but important, areas of chemistry. We also recall that the free electron is the simplest chemical entity and by definition also the simplest and most powerful reducing agentf. It is also the species which is exchanged between two partners in a very wide range of chemical reactions, namely redox and exchange reactions between identical groups, or between ions in different valency states, or between molecules. The advances made in recent years have been so numerous that it is impossible, in the space available, to be comprehensive. I shall do no more