13C chemical shift sensitivity to charge in delocalized carbocations and its relation to ring current anisotropy

Abstract

The total carbon shift changes that occur upon oxidation of a variety of delocalized hydrocarbons have been empirically separated into a charge term and a term related to ring current anisotropy. The largest low-field total shift is observed for the most paratropic di-cations, where the second term is the most significant one. The shielding contribution due to charge is considerably larger for di-cations than for the negatively charged species. Field and ion-pair effects are discussed in this context.