Author:
Black PJ,Brown RD,Heffernan ML
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.
Cited by
110 articles.
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