Magnetic shielding by the azo-group. A nuclear magnetic resonance study of phenylazoalkanes

Abstract

Measured chemical shifts for protons near the azo-group in cis and trans isomers of azobenzene (I), azoalkanes (II)—(IV), and phenylazoalkanes (V)—(X) appear qualitatively to be consistent with conical regions of shielding and deshielding about the azo-group as predicted from the point dipole model for magnetic susceptibility anisotropy of the group. Various field contributions to the proton shielding constants have been calculated, and include contributions due to (1) electrostatic fields arising in polar bonds and lone pairs, (2) magnetic anisotropy of the azo-group, (3) ring currents of phenyl groups, and (4) the reaction field of the solvent. Values for the magnetic anisotropy of the azo-group are obtained directly from single crystal magnetic susceptibility data for cis- and trans-azobenzene and biphenyl. Computed chemical shift differences based on experimental values for the anisotropy disagree with measured shift differences both quantitatively and with regard to sign, suggesting that a mechanism other than magnetic anisotropy is responsible for the observed shifts.