Influence of paramagnetic impurities on molecular self-diffusion as seen by nuclear spin-lattice relaxation and dynamic polarization

Abstract

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.