On Measuring Nuclear Magnetic Shielding Anisotropies in Liquid Crystal Solvents

Up to the present time nuclear magnetic shielding anisotropies (Δσ) have been determined in nematic liquid crystal solutions by comparing the chemical shifts in the isotropic and nematic phases. Unknown changes in solvent-induced shifts arising from the isotropic → nematic phase transition constitute a serious problem in the interpretation of the results. To avoid the ambiguities inherent in these shifts, a method is described for measuring Δσ in the nematic phase alone. This technique has been used to determine Δσ in a variety of
¹
H- and
¹⁹
F-containing molecules and the results are compared with those obtained by the two-phase method.