NMR Studies of the Pressure Dependence of Self-Diffusion in Four Organic Solids

NMR pulse techniques have been used to study the influence of hydrostatic pressure on self-diffusion in four organic solids composed of nearly spherical molecules: cyclohexane, 2,2-dichloropropane, t-butyl chloride, and t-butyl bromide. The self-diffusion was studied from the pressure and temperature dependence of T2, the spin—spin relaxation time, at pressures between 1 and 680 atm, and at temperatures within 10–50 deg of the melting point. The measured activation volumes were 8%—15% of the molar volumes. Comparable ratios have been previously observed for rotational motion in similar solids. The activation enthalpies for self-diffusion were 5–6 kcal/mole, somewhat exceeding the values for rotational motion. Comparisons are made with the latent volumes and enthalpies of fusion.