An NMR study on the shielding of the ¹²⁹Xe isotope of xenon dissolved in thermotropic liquid crystals

Abstract

The variation of the nuclear shielding of the ¹²⁹Xe isotope in natural xenon dissolved in various liquid crystals and liquid crystal mixtures has been studied over the temperature range from 300 to 360 K. The temperature dependence is linear in the isotropic phase of the liquid crystals. An abrupt change in the shielding is observed when passing through the nematic-isotropic and smectic A-nematic phase transitions as well as when the liquid crystal director rotates by 90° in the so-called critical mixture of ZLI 1167 and EBBA. This is interpreted as being mainly the consequence of the shielding anisotropy of xenon arising from the deformation of its electronic distribution. The shift changes observed for 4,4′-di-n-heptylazoxybenzene at the nematic-isotropic phase transition on the one hand and at the smectic A-nematic phase transition on the other are found to be opposite in sign, reflecting the change in the liquid crystal structure.