Diffusivity and nuclear spin relaxation measurements at high pressure in methanol

Diffusivity D and nuclear spin relaxation times T ₁ and T ₂ have been measured by NMR to 4.0 GPa in methanol, using a diamond anvil cell probe. In pure MeOH, D ^-1 and T ₂ show essentially identical activation volumes. However, these are approximately 18% larger than the activation volume of viscosity. By relating these observations to an average molecular correlation time a pressure-dependent infinite-frequency shear modulus G _∞ can be inferred, using two independent approaches. The relation between diffusivity and viscosity shows increasing departure from Stokes-Einstein behavior with increasing pressure, if a constant hydrodynamic radius is assumed. This departure is attributed to the pressure dependence of G _∞, and can be described empirically by a simple modification of the Stokes-Einstein relation.