NMR of quadrupole noble gases in liquid crystals

ABSTRACT

NMR spectroscopy of quadrupolar noble gases, ²¹Ne (spin 3/2), ⁸³Kr (9/2), ¹³¹Xe (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 ¹²⁹Xe and ¹³¹Xe chemical shifts reveals second-order quadrupole shifts (SOQS) in circumstances where the ratio of quadrupole coupling and magnetic flux density is suitable. The ¹³¹Xe SOQS is shown to distinguish between uniaxial and biaxial nematic phases. Both ⁸³Kr and ¹³¹Xe 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 ⁸³Kr NMR transitions of krypton in liquid crystals is discussed.

Graphical Abstract

KEYWORDS:

• Quadrupolar noble gases
• electric field gradient
• orientational order
• second order quadrupole shift
• biaxiality
• tilt angle
• hexadecapole coupling

Acknowledgments

The author thanks his colleagues in the former NMR research group, now NMR Research Unit in the University of Oulu for long-lasted co-operation. Dr. L.P. Ingman is tanked for the ²¹Ne, ⁸³Kr and ¹³¹Xe NMR experiments in ZLI 1167 and Dr. A.M. Kantola for the ⁸³Kr NMR spectrum displayed in Figure 11.

Disclosure statement

No potential conflict of interest was reported by the author.