ELECTRIC-FIELD GRADIENTS EXPERIENCED BY THE NOBLE-GAS ISOTOPES NE-21, KR-83 AND XE-131 IN THERMOTROPIC LIQUID-CRYSTALS

The Ne-21, Kr-83 and Xe-131 NMR spectra of these noble gases dissolved in various thermotropic liquid crystals were recorded in the nematic phases at varying temperatures. The spectra display splittings due to the interaction between the nuclear quadrupole moment and the electric field gradient (EFG) at the nuclear site and, consequently, allow one to investigate electric field gradients. The temperature dependence of the EFG is found to be such that the field gradient arising from the electric multipoles of the liquid crystal molecules is not sufficient alone to explain the observations, but one additional contribution has to be taken into account. The quadrupole couplings were analysed in terms of a model based on the contributions from the liquid crystal molecules and from the deformation of the electron cloud due to the collisions of the atoms with liquid crystal molecules. The model provides fairly detailed information on the EFG contributions as well as on the liquid crystal order.