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

Abstract

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.