Calculations of energies for self-diffusion in n-alkanes

Calculations for diffusion in orthorhombic, monoclinic and triclinic subcells are reported. They show that self-diffusion can occur and support a vacancy mechanism. Vacancy energies are much larger than the barriers to molecular motion. Diffusion parallel to the basal plane can be by a rigid rod mechanism while perpendicular requires a conformational jog to traverse the crystal boundary. The diffusion coefficients are in order of magnitude agreement with the limited experimental data available. For a given subcell, diffusion parallel to the basal plane is about an order of magnitude greater than perpendicular. Diffusion coefficients for the triclinic subcell are larger than those in the other two. The computed coefficients are especially sensitive to some of the parameters used.