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

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Abstract
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 Δσ 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 nematic–isotropic two‐phase method being obviously in error. The shielding anisotropy measured for fluoroform is found to agree with the prior nematic two‐phase value, with the nematic single‐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 two‐phase techniques. It is shown that on transforming the experimental shielding anisotropy values obtained in this study along the C–F bond direction, that σ11, the shielding component parallel to the C–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 proton–fluorine nuclear spin–spin coupling constants for the fluoromethanes are found to be unambiguously positive.