Theory of Carbon NMR Chemical Shifts in Conjugated Molecules

Abstract

A molecular orbital theory for carbon‐13 chemical shifts in conjugated molecules is formulated. Consideration of the various possible contributions to the shift shows that the local paramagnetic term σ_p^AA is expected to dominate. By a detailed LCAO analysis of σ_p^AA, its variation with the salient features of the electron environment is determined. It is demonstrated that there is a significant local‐charge dependence which agrees in sign and order of magnitude with an experimentally established correlation. In addition, the shielding is shown to be a function of the free valence of the atom under consideration and of the polarity of its sigma bonds. A linearized equation for the carbon‐13 chemical shift (with respect to benzene) is presented and compared with the available experimental data.