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

Abstract

The nuclear shielding and self-diffusion coefficient of xenon-129 dissolved in the so-called critical mixture of two nematic liquid crystals with opposite diamagnetic anisotropy were determined as a function of temperature applying ¹²⁹Xe NMR spectroscopy. In the critical mixture, the liquid crystal director rotates 90° from the parallel direction to the perpendicular one with respect to the external magnetic field when approaching the critical point from high temperature side. The director reorientation offers a straightforward means to determine the self-diffusion coefficients of xenon, D_‖ and D_⊥, in the parallel and perpendicular directions, respectively, relative to the liquid crystal director simply by applying pulsed Z-gradient only. The self-diffusion tensor was found to be slightly anisotropic, the ratio D_⊥/D_‖ being ca. 0.78, over the whole temperature range investigated. The fit of a theoretical model function to the experimental shielding data gives the ¹²⁹Xe shielding constant, σ₀, and the anisotropy of the shielding tensor, Δσ₀, 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.