Nmr Of Quadrupole Noble Gases In Liquid Crystals

NMR spectroscopy of quadrupolar noble gases, Ne-21 (spin 3/2), Kr-83 (9/2), Xe-131 (3/2), dissolved in thermotropic liquid crystals enables the derivation of versatile information about the physical properties of the materials. The spectra display fine structures, triplets and nonets, which reveal quadrupole couplings. These, in turn, can be used in the determination of electric field gradients (EFG), orientational order parameters, and tilt angles. Comparison of Xe-129 and Xe-131 chemical shifts reveals second-order quadrupole shifts (SOQS) in circumstances where the ratio of quadrupole coupling and magnetic flux density is suitable. The Xe-131 SOQS is shown to distinguish between uniaxial and biaxial nematic phases. Both Kr-83 and Xe-131 NMR spectra may display asymmetry around the central transition because of the SOQS, which can be used to identify biaxiality in nematic phases. As a curiosity, the effect of possible hexadecapole coupling on the Kr-83 NMR transitions of krypton in liquid crystals is discussed.[GRAPHICS].