New Data on Biradicals

Data on biradicals, including both the results of studies by pre-1947 methods (static magnetic susceptibility, ortho–para conversion of hydrogen, etc.) and the results of studies on biradicals by electron spin resonance are reviewed. Biradicals of various types are discussed – biradicals in the singlet ground state which pass to a triplet state on excitation and biradicals in the triplet ground state which arise in consequence of the degeneracy of higher occupied levels, the presence of an insulating group between the monoradical components, atropisomerism, etc. A complete compilation is also given of all the data on biradicals with heteroatoms. The application of the electron spin resonance method to biradicals permitted an approach to the problems of the interaction of unpaired electrons in these systems. The theory of such interaction and the form of the spectra which should be obtained in its presence and absence are discussed. However, many studies on this interaction have proved erroneous because the data obtained did not refer to the substances to which the observed spectra were attributed but to the products of their further reactions – the formation of di- and polymers. These problems are not usually discussed in sufficient detail in physical papers on biradicals, while in the present review special attention has been given to them.