An N.M.R. study of the pressure dependence of molecular self-diffusion and reorientation in organic plastic crystals

Abstract

The proton N.M.R. relaxation times, T ₁ and T _1ρ, of the organic plastic crystals, hexamethylethane, hexamethyldisilane, norbornane, norbornylene, norbornadiene and cyclohexane, have been measured as functions of hydrostatic pressure up to 2·8 kbar ‡ Bar = 10⁵ Pa. View all notes , and in most cases at two different temperatures. These materials provide examples of b.c.c., f.c.c. and h.c.p. crystal lattice structures. The T ₁ measurements are analysed in terms of the reorientational motion of the molecules. This molecular motion is found to have a weak pressure dependence associated with low values for the activation volume. The T _1ρ measurements are analysed in terms of the translational self-diffusion of the molecules. This motion has a strong pressure dependence and activation volumes, ΔV*, obtained for this motion are found to fall in the range 0·57 to 1·2 times their respective molar volumes, V _m. The nature of the pressure dependence of the self-diffusion shows a correlation with the entropy of fusion, ΔS _f, of the plastic crystal. The plastic crystals with higher values of ΔS _f show larger values of the ratio ΔV*/V _m and have activation volumes which appear to be independent of temperature and pressure, whereas those with lower values of ΔS _f are generally found to have lower values of the ratio ΔV*/V _m and also to have pressure and temperature dependent activation volumes. These observations are discussed in terms of the defect responsible for the self-diffusion, which is thought to be a relaxed vacancy.