Theory of carbon NMR chemical shifts in conjugated molecules

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 σpAA is expected to dominate. By a detailed LCAO analysis of σpAA, 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.