Nitroxide radical pairs in the plastic crystal camphene J. Chem. Phys. 62, 956 (1975); 10.1063/1.430511Effect of a nitroxide free radical on the nuclear magnetic relaxation rates of thallium(I)205 in aqueous solutionIn order to check the theory of spin diffusion with a new kind of sample, we present measurements of dynamic polarization and spin-lattice relaxation, at 3300 G, on the plastic phase of cyclohexane doped with a nitroxide free radical. The EPR lines are sufficiently narrow to allow an ideal "solid effect." The enhancement of the proton resonance was measured as a function of microwave power for various temperatures. The decay rate of these enhanced signals both as a function of temperature and radical concentration was studied, in order to determine the proton spin-lattice relaxation time. The electronic spin-lattice relaxation time was measured by the saturation method. The data were found to be fairly consistent with a rapid diffusion regime between -10 and -60°C. The critical radius (i.e., the radius of a sphere, centered on the electronic spin, inside which the diffusion coefficient is zero) was found to be ~1.7 A. This value is much smaller than the distance between the free radical and the nearest cyclohexane molecules. The meaning of this fact is not yet clear.

Measurements, near the melting point, of nuclear spin-lattice relaxation and dynamic polarization in plastic cyclohexane doped with a nitroxyde free radical show deviations from the expected spin-diffusion theory. These deviations are explained by the quenching of the molecular self-diffusion around the radical and interpreted semiquantitatively in terms of a two zone model. It is shown that the quenching extends up to more than four intermolecular distances and the spin-diffusion coefficient in this region is shown to be around 0.3× 10−12 cm2 sec−1. It seems that similar measurements would be of interest in any crystal which exhibits NMR motional narrowing due to fast molecular self-diffusion.