Studies of Chemical-Shift Anisotropy in Liquid Crystal Solvents. III. The NMR Spectra of Nematic Solutions of Ethane and 1,1,1-Trifluoroethane

Abstract

The proton and fluorine NMR spectra of ethane and 1,1,1-trifluoroethane have been observed in the nematic liquid crystal solvent 4,4′-di-n-hexyloxyazoxybenzene. High pressure NMR techniques were used to achieve the necessary concentration of the gases in the nematic phase. Relative internuclear distances in these compounds have been determined from ratios of the dipolar coupling constants. A positive value for the absolute sign of the scalar coupling J(CH3,CH3) has been determined from the spectrum of oriented ethane. A positive J(CF3,CH3) in CH3CF3 gave internuclear distances in best agreement with microwave data. The proton chemical-shift anisotropies are found to be small; that of ethane is essentially indistinguishable from zero. Calculations of the fluorine shielding anisotropies of five compounds in terms of primary and secondary shielding indicates that double bond character in the C–F bond is of some importance in determining the anisotropy of the fluorine chemical shift.