Spin Exchange in Nitroxide Biradicals

The dependence of the electron spin-exchange energy upon temperature and solvent has been studied for the carbonate, oxalate, and succinate diesters of tetramethyl-piperidinol-oxyl. An analysis is given of exchange effects on ESR line positions, intensities, and widths. For the carbonate diester the exchange energy is constant at low temperatures. As the temperature is raised this energy gradually decreases, passes through a minimum, and then increases exponentially. Interpretation suggests that at low temperatures exchange occurs through an indirect process involving the core of sigma electrons, whereas at high temperatures a direct exchange process is more important.