Alternating linewidths. A novel relaxation process in the electron resonance of biradicals

The factors affecting the electron resonance linewidths of a biradical, showing strong spin exchange, tumbling isotropically in solution are considered. For a biradioal with two equivalent magnetic nuclei the hyperfine lines should alternate in width provided the dominant relaxation mechanism is a modulation of the exchange interaction between the unpaired electrons. Certain nitroxide biradicals do indeed exhibit an alternating linewidth effect which we attribute to this mechanism.