Gerald P. Ceasar

Studies of Chemical‐Shift Anisotropy in Liquid‐Crystal Solvents. I. Experimental Results for the Methyl Halides

Analysis of the nuclear magnetic resonances of the methyl halides dissolved in nematic‐liquid‐crystal solutions has given measurements of the anisotropy of the proton chemical shift. The accurate determination of these anisotropies is made difficult by the effect of the liquid‐crystal solvent on the chemical‐shift measurements. The method was found to be unsuitable for the measurement of the methyl fluoride anisotropy, but the values for the other methyl halides have been determined to within ± 1.5 ppm. Methyl chloride and methyl bromide are found to have small proton chemical‐shift anisotropies indistinguishable from zero, and methyl iodide appears to have a small positive anisotropy.

Studies of Chemical Shift Anisotropy in Liquid Crystal Solvents. IV. Results for Some Fluorine Compounds

Measurement and analysis of the nuclear magnetic resonance spectra of 19F nuclei in nematic liquid crystal solutions have been used to obtain the fluorine chemical shift anisotropy in sym‐C6F3Br3, sym‐C2F3Cl3, and for CF3I. Values of the chemical shift anisotropy relative to the molecular symmetry axis were 112 ppm, 76 ppm, and − 8 ppm, respectively. An attempt to account for these results and other similar ones for 19F using the Karplus–Das equation was unsuccessful. A probable source of difficulty was the necessity of assuming axial bond symmetry for the C–F bond in order to transform the chemical shift anisotropies from the molecular symmetry axis to the C–F bond axis.

Studies of Chemical Shift Anisotropy in Liquid‐Crystal Solvents. II. Theoretical Calculations for the Methyl Halides

Experimentally determined values of the proton chemical‐shift anisotropy for the methyl halides have been used to test simple phenomenological theories which have been proposed to account for proton magnetic shielding. It is shown that the frequently used magnetic dipole model predicts that the long‐range contribution to the proton shielding anisotropy should largely depend on the average neighbor magnetic susceptibility. Anisotropies calculated from this model are much larger than those measured. Expressions have also been derived relating the proton shielding anisotropy to the electric dipole moment and charge densities of polar substituents. Results of these calculations are not as conclusive as in the magnetic dipole case.