Exchange and dipolar interactions in solid organic radicals

Investigations of the E.S.R. linewidths of exchange narrowed lines from solid organic free radicals allow one to obtain information about both the exchange interaction and the electron-electron dipolar interaction in some instances. In cases in which the dipolar interaction is incompletely averaged by spin exchange one can use the frequency dependence of the E.S.R. linewidth to determine the magnitudes of the exchange frequency and the dipolar interaction. This paper reports results from a study of the frequency and temperature dependence of the linewidth of four solid organic radicals. The dipole-dipole interaction of the two nitroxide radicals included in this study was found to be temperature dependent, indicating changes in intermolecular spin separation with temperature. The exchange energy of these two radicals appeared to vary exponentially with the separation of the spins through most of the temperature interval which was investigated.