19F shielding anisotropies of the fluoromethanes obtained from NMR studies in a smectic liquid crystal solution

摘要

19F magnetic shielding anisotropies of the fluoromethanes have been obtained from studies in a smectic liquid crystal solution using a pulsed FT NMR spectrometer. The value of Dgr;sgr; obtained for methyl fluoride in the smectic phase is observed to be comparable in magnitude but higher in accuracy than the previous measurements obtained from theoretical, clathrate, molecular beam, and nematic single phase techniques, with the earlier value obtained using the nematicndash;isotropic twohyphen;phase method being obviously in error. The shielding anisotropy measured for fluoroform is found to agree with the prior nematic twohyphen;phase value, with the nematic singlehyphen;phase and clathrate values being considerably lower. The anisotropy measured in this investigation for methylene fluoride was found to be in disagreement with both previous values measured via molecular beam and nematic twohyphen;phase techniques. It is shown that on transforming the experimental shielding anisotropy values obtained in this study along the Cndash;F bond direction, that sgr;11, the shielding component parallel to the Cndash;F bond ,is essentially constant in magnitude for all three systems. In addition, it is also observed that from a knowledge of the signs of the orientation parameters that the absolute signs of the indirect protonndash;fluorine nuclear spinndash;spin coupling constants for the fluoromethanes are found to be unambiguously positive.