The status of knowledge of several oJ the smaller radicals and atoms is considered with particular reference to those which take part in combustion processes.
WITH1N the last ten years, several techniques have been developed, or modified, to investigate the properties of free radicals. The achievements of each of these methods have been reviewed in some detail, in a journaP not readily available in the United Kingdom, and at two unreported conferences on the properties of free radicals (Quebec, 1956), and of trapped radicals (Washington, 1957). The aim of this review is therefore to consider the status of knowledge of several of the smaller radicals and atoms (particularly those which take part in combustion processes), both in the light of these, and other, sources of information. However, no attempt will be made to cover the subject of free radicals more widely, since this has recently been done by E. W. R. STEACIE'-'. The extensive photochemical investigations of free radical reactions and their kinetics will also not be considered here, since they would more properly form the subject of a separate review.
EXPERIMENTAL TECHNIQUES
Ultra-violet spectroscopy
Information is provided by the transition from one electronic level to a less energetic one (often, but not always, the ground state). As a result of the fine structure accompanying the main bands, and of the distribution of the intensity of individual lines, deductions can be made about the structure, shape and dimensions of the emitter (or absorber). The interpretations, however, are not clear-cut and, very often, the selection of one of several possible assignments is accompanied by much controversy. In general, monatomic and diatomic species can be analysed fairly readily, and recently, considerable progress has been made with triatomic species, in spite of considerable theoretical difficulties (see also A. G. GAYDON3).
Infra-red spectroscopy
Here, information is obtained as a result of changes in the vibrational quantum numbers of particular modes. Many of the observed bands are related, sometimes quite simply, to the vibration frequency of particular bonds within a molecule (e.g. ~C~O, ~N~H). Examination of rotational fine structure and intensity distributions can again be helpful in determining structure. Additional information is provided by the number of bands