Proton chemical shifts and electron densities in aromatic and heteroaromatic molecules. I. Procedure and chemical shift corrections; Applications to azines

Abstract

In this series, the observed proton chemical shifts of heteroaromatic compounds are used to obtain estimates of the electron distribution in these systems. These estimates are deduced from the measured shifts after applying corrections for the shielding contributions arising from ring currents, magnetic anisotropy of neighbouring atoms, and the electric field from the lone-pair dipoles on heteroatoms, in a manner closely related to that of Gil and Murrell. The corrected chemical shifts are then used to deduce an ?experimental? π-electron distribution via the electric field dependence of the proton shifts on the excess charge at all ring positions. However, in most cases, there are not enough experimental shifts to determine uniquely the excess charge at each ring atom, and it is necessary to assume that some inaccessible charges are close to values obtained from theoretical calculations. With these limitations, a comparison is made between the excess charges deduced in the above fashion from chemical shift data on mono- and poly-cyclic azines and diazines and those obtained by theoretical calculations using the BJ VESCF method. It is emphasized that this interpretation is tentative and that the extent of σ-electron polarization is not well understood at present.