Nuclear Magnetic Resonance Spectroscopy. The Effect of Structure on Magnetic Nonequivalence Due to Molecular Asymmetry

The difference in chemical shift between the methylene protons of a number of compounds related structurally to 1-phenylethyl benzyl ether has been measured. Correlations between structural features of these compounds and the magnitude of the magnetic nonequivalence induced by the center of molecular asymmetry provide a basis for qualitative identification of the most important factors contributing to the chemical-shift difference. Although it is clear that no single factor Completely determines the magnitude of the methylene proton nonequivalence in these ethers, it is suggested that the proximity of the asymmetric center to the benzyl group results in a preferred conformation of the phenyl ring with respect to the methylene protons, and that the principal contribution to the nonequivalence originates in the magnetic anisotropy of the phenyl group.