Infrared Spectra of Free Radicals and Chemical Intermediates in Inert Matrices

Abstract

Free radicals have played a significant role in the development of chemistry during the twentieth century. Since Gomberg's (1) preparation in 1900 of the first free radical, triphenylmethyl-and along with it a challenge to the understanding of valence-free radicals have been used increasingly to explain reaction mechanisms. As early as 1918 Nernst suggested (2) that the photo­ chemical reaction of hydrogen and chlorine involved the individual atoms as intermediates. Further important work of Paneth & Hofeditz in 1929 (3) postulated that alkyl free radicals, produced by thermal decomposition of metal alkyls, were responsible for removing a metal film in the famous metal mirror experiments. Following this early work, the appearance of free radical intermediates in the literature became itself a chain reaction. Free radicals were subsequently proposed as reaction intermediates by numerous prominent scientists. Steacie has reviewed much of this early work (4) in his book Atomic and Free Radical Reactions. Recently Pryor (5) has discussed the mechanistic, synthetic, and industrial significance of free radicals in contemporary organic chemistry. Obviously, the validity of a proposed mechanism is improved by the direct observation of the free radical from a related chemical process. Furthermore, free radicals provide unique tests for theories of chemical bonding. The presence of an unpaired electron in a half-filled orbital supplies an excellent opportunity for electronic interaction with neighboring atoms and groups. Knowledge of the vibrational spectrum of simple free radicals provides insight into the structure and bonding of these interesting chemical species. Free radicals are most simply defined as species containing one or two unpaired electrons. Examples include methyl CH3, the chlorine atom CI, and the lithium atom Li, as well as the stable molecules nitric oxide NO and nitrogen dioxide NOz, each of which contains a single unpaired electron. Diradicals, such as triplet CH2, contain two unpaired electrons. In this dis­ cussion we will use the term radical to refer to species containing a single