π-Electron ‘ring currents’ and proton chemical shifts in planar, condensed, benzenoid hydrocarbons: an empirical reassessment of the semi-classical theory

Abstract

The semi-classical theory of Waugh and Fessenden, and Johnson and Bovey, predicting the effect of π-electron ‘ring currents’ on the secondary magnetic fields at protons in molecules containing benzenoid rings, is tested against a set of modern, accurate, experimental chemical shifts in a wide range of planar, alternant, polycyclic hydrocarbons. It is shown that this theory overestimates the ‘ring current’ deshielding of coplanar protons in such molecules, thus confirming Dailey's preliminary conclusions, drawn from the study of a small range of hydrocarbons. The empirical nature of the theory is discussed, and it is shown that when the calculated shifts are suitably scaled, as suggested previously, correlations with observed τ values are obtained which are as good as those found when the ‘ring current’ shifts are calculated from the McWeeny M.O. theory; these correlations are then used to make empirical estimates of the contributions from steric effects to the chemical shifts of the overcrowded protons in these molecules. It is concluded that whilst there is some theoretical justification for this scaling of the predictions from the semi-classical theory, the procedure must be considered as being essentially empirical.