Diffusion in Molecular Crystals

From the fact that the structure of hexamethylethane is close-packed, we may infer that the point defects responsible for the translational motion of the molecules are monovacancies. The data indicate an overexponential increase in diffusivity with temperature near Tm, analogous to some observations made on inorganic crystals. Various explanations could be given for this increase such as anharmonicity of the 

The picture is less clear for molecular crystals when the molecules deviate strongly from a globular form. NMR data and tracer diffusion data are then often in disagreement. Diffusion profiles (In c, vs. distance) are found to be curved, which is usually attributed to additional heterogeneous and fast diffusion pathways. For plastic crystals, this could indicate that many of them possess a highly defective structure. Even for the aromatic ring molecule benzene, which forms a non-plastic crystal, one finds a D (NMR)/D (tracer) ratio on the order of 103. This cannot be understood unless one invokes other than bulk lattice mechanisms of diffusion. 

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