Intramolecular screening effects on nuclear magnetic resonance chemical shifts. Part II. The anisotropy in the magnetic susceptibility of C–F and C–Cl bonds

Abstract

Values, apparently representing the anisotropies in the magnetic susceptibilities of the C–F and C–Cl bonds, are determined from ¹H n.m.r. shifts. It is suggested that apparent Δ_χ values for some bonds may fortuitously accommodate electronegativity and electric-field effects characteristic of the bonds. Because of this it is concluded that the values for Δ_χ^CF and Δ_χ^CCI deduced herein and for Δ_χ^CH and Δ_χ^CC reported previously, need not represent actual bond anisotropies. Even so, it is shown that the values deduced to represent Δ_χ^CF and Δ_χ^CCI can be used to account for differences in proton shifts in diverse molecular situations by using a bond-induced point magnetic dipole approach. The points of action of the effective dipoles induced in the bonds are determined.