Xe-129 NMR shielding and self-diffusion in the mixture of two thermotropicnematogens with opposite diamagnetic anisotropy

The nuclear shielding and self-diffusion coefficient of xenon-129 dissolved in the so-called critical mixture of two nematic liquid crystals with oppo site diamagnetic anisotropy were determined as a function of temperature ap plying Xe-129 NMR spectroscopy. In the critical mixture, the liquid crystal director rotates 90 degrees from the parallel direction to the perpendicul ar one with respect to the external magnetic field when approaching the cri tical point from high temperature side. The director reorientation offers a straightforward means to determine the self-diffusion coefficients of xeno n, D-parallel to; and D-perpendicular to, in the parallel and perpendicular directions, respectively, relative to the liquid crystal director simply b y applying pulsed Z-gradient only. The self-diffusion tensor was found to b e slightly anisotropic, the ratio D-perpendicular to/D-parallel to being ca . 0.78, over the whole temperature range investigated. The fit of a theoret ical model function to the experimental shielding data gives the Xe-129 shi elding constant, sigma (0), and the anisotropy of the shielding tensor, Del ta sigma (0), as well as the density change at the isotropic-nematic phase transition. Furthermore, temperature dependence of shielding and shielding anisotropy can be determined with the help of the adjusted parameters.