Temperature dependence of nuclear shielding and quadrupolar coupling of noble gases in liquid crystals

• Published in: The Journal of chemical physics 1992-12, Vol.97 (12), p.8977-8985
• Main Authors: LOUNILA, J, MUENSTER, O, JOKISAARI, J, DIEHL, P
• Format: Article
• Language: English
• Subjects: Atomic and molecular physics; Atomic properties and interactions with photons; Atomic spectra; Exact sciences and technology; Magnetic resonance spectra; Physics
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.463324

A theoretical model for nuclear magnetic resonance (NMR) observables of noble gas atoms in nematic and smectic A liquid crystal environments is developed. It is used to account for the behavior of the nuclear shielding of 129Xe (σ) and the quadrupolar coupling of 21Ne (B) measured in 1-butyl-c-4-(4′-octylbiphenyl-4-yl)-r-1-cyclo-hexan-carbonitrile (NCB 84). The analysis provides detailed information on the orientational and positional order of the liquid crystal solvent and of the solute atoms. In the nematic phase, the decrease of σ with decreasing temperature is attributed to the increase of the density and orientational order parameter S of the solvent. The former factor determines the variation of the isotropic part of σ, while the latter governs the change in the anisotropic part (the anisotropy of the shielding tensor Δσ is of the order of −10 ppm). The prominent increase of σ in the smectic A phase is ascribed to the tendency of the atoms to occupy the interspaces between the smectic layers rather than their interiors (the deviation from the uniform distribution of the atoms is of the order of 5%). The main result is that the isotropic part of σ increases as the density of the solvent in the immediate neighborhood of the solute decreases. In the case of the quadrupolar coupling, the effect of the redistribution of the atoms is small, as there is no isotropic contribution to B. The behavior of the quadrupolar coupling of 21Ne in the nematic phase indicates that there are two distinct contributions to B. One is due to the distortion of the electron cloud from the spherical symmetry and the other arises directly from the external electric field gradient produced by the neighboring molecules. The temperature dependence of the latter contribution is not determined by the variation of S alone.