On the interpretation and significance of nuclear spin relaxation by translational diffusion in organic plastic crystals

Nuclear spin relaxation measurements are reported for the protons in polycrystalline samples of the low entropy of fusion plastic solids dl-camphene, cyclohexane and norbornylene. The contributions from molecular diffusion are shown to be inconsistent with Torrey's random-walk model for lattice diffusion which has hitherto been widely used in the interpretation of such measurements in plastic crystals. In fact, they are found to be typical of an isotropic diffusion process in which the molecules undergo random displacements with an undeterminable distribution of jump lengths about a mean comparable to or greater than a molecular diameter. On the other hand, measurements in polycrystalline perfluorocyclohexane, a high entropy of fusion solid, are consistent with a lattice diffusion model, implying that a simple monovacancy mechanism might be involved in this and other high entropy of fusion solids. There are distinct differences between the present N.M.R. and radiotracer measurements in dl-camphene and cyclohexane which can only be reconciled by postulating that the structure of the point defect and its migration are more complex in the polycrystalline samples employed in the N.M.R. experiments than anticipated for a simple monovacancy mechanism, as is implied by the tracer measurements on corresponding well-annealed single crystals.