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

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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.

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