Electron resonance of gaseous free radicals

Abstract

At this meeting we shall discuss the application of electron spin resonance to the study of short–lived free radical intermediates in chemical reactions. Many such studies have now been described, but they are almost entirely confined to reactions in the liquid phase. When my colleagues and I began working on this subject almost two years ago, only five short–lived free radicals in the gas phase had been detected by electron resonance methods. These were OH, SH, SeH, TeH (Radford 1960, 1964) and SO (Daniels & Dorain 1964, 1966); in addition a number of atoms had been detected, notably H, O, N, P, F, Cl, Br and I. Kinetic studies in which electron resonance is used to monitor the concentrations of H and O atoms have become quite numerous. However, from the viewpoint of a spectroscopist, a potentially very large field has remained essentially unexplored. In §2 of this paper the relation between gas phase electron resonance spectroscopy and other forms of microwave spectroscopy, including the electron spin resonance of free radicals in liquids and solids, is demonstrated. Our experimental methods are described (§3) and the results we have obtained for a number of different free radicals reviewed. The exact analysis of the spectra is often compli­cated, but the description in §§4, 5 and 6 is non-mathematical and indicates qualitatively the structural information which can be obtained from the spectra. Finally possible future areas of development are considered (§7).