Nuclear magnetic shielding of noble gases in liquid crystals

Abstract

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.

Determination of sample temperature and temperature stability with 129Xe NMR

“…Assuming pairwise additivity for xenon shielding perturbations, it appears that 129 Xe shielding depends upon the solvent density in isotropic solutions according to [10] r gas Xe À r sol…”

129Xenon NMR: Review of recent insights into porous materials

“…Important theoretical developments have been performed for a better understanding of the NMR chemical shift and line shapes of 129 Xe [28][29][30][31][32][33][34]. influenced by the size of the cavities [51,52] and the nature of metal ions into the zeolite framework [53].…”

“…During the last 35 years, 129 Xe NMR spectroscopy has been applied on various systems, such as porous materials, systems with supported metals [6][7][8][9][10][11], polymers [12][13][14][15][16][17][18], biomolecules [19][20][21][22][23], liquid crystals [24][25][26][27][28] etc. Important theoretical developments have been performed for a better understanding of the NMR chemical shift and line shapes of 129 Xe [28][29][30][31][32][33][34].…”

Structure and dynamics elucidation of ionic liquids using multidimensional Laplace NMR

“…S6, ESI †). 20 This indicates that there are no phase transitions within that temperature interval. Furthermore, the observation supports the abovementioned interpretation that the disappearance of the fast diffusion component at higher temperatures cannot be associated with such a change.…”

“…19 Typically, the phase change of liquid or liquid crystals is manifested by an abrupt change in the chemical shift of xenon dissolved in the liquid. 20 The linear decrease of the 129 Xe chemical shift of xenon dissolved in the [P 6,6,6,14 ][BMB] IL with temperature indicates that there are no abrupt changes in the density and orientational order parameter of the IL in the 294-373 K temperature range (see Fig. S6, ESI †).…”