Nuclear magnetic shielding of noble gases in liquid crystals

• Published in: The Journal of chemical physics 1999-04, Vol.110 (13), p.6381-6388
• Main Authors: Ylihautala, Mika, Lounila, Juhani, Jokisaari, Jukka
• Format: Article
• Language: English
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.478541

A theoretical model for nuclear magnetic shielding of noble gas atoms soluted in liquid crystal solvents is developed. It is found that the solvent effect on the shielding can be represented as a linear combination of products of the liquid crystal orientational order parameters of varying rank. In a special case of pairwise additive shielding perturbations, most of the coefficients vanish and the shielding reduces to a sum of two terms, the isotropic and anisotropic parts. Both contributions are directly proportional to the density of the liquid crystal, and the anisotropic part is also directly proportional to the second rank orientational order parameter of the liquid crystal. The developed model is used to account for the behavior of the 129Xe shielding in the nematic liquid crystal 4-ethoxybenzylidene-2,6-dideutero-4′-n-butylaniline (d2-EBBA). The pairwise additivity approximation of the shielding perturbations is found to explain the observed temperature dependence of the 129Xe shielding satisfactorily. In particular, the temperature dependence of the isotropic part is mostly due to the change in the liquid crystal density, whereas the anisotropic part is mainly controlled by the temperature dependence of the Xe–liquid crystal molecule pair correlation function and the second rank orientational order parameter of the liquid crystal. This result differs from the results of the phenomenological theory of Lounila et al. [J. Chem. Phys. 97, 8977 (1992)], where only the density and the orientational order parameter were assumed to be significantly temperature dependent.