The 1H NMR spectrum of an aromatic molecule such as benzene is iconic; one learns that the unusual chemical shift of the protons (~δ 7-8 ppm) is due to their deshielding by a diatropic ring current resulting from the circulation of six aromatic π-electrons following the Hückel 4n+2 rule. But rather less well-known is the spectacular inversion of these effects as induced by the paratropic circulation of 4n electrons. A 4n+2 rule can be converted to a 4n one by the addition of two electrons, and chemically this can be done by reduction with lithium metal to form a dianion. Fortunately, this experiment has been done for a molecule known as methanoannulene. This is a 4n+2 aromatic molecule 1 with ten π-electrons (n=2) that can be reduced with lithium metal to form an ion-pair 2 comprising lithium cations and the twelve π-electron (4n, n=3) methanoannulene dianion.
- D. Schmalz, and H. Günther, "1,6-Methanoannulene Dianion, a Paratropic 12π-Electron Dianion with a C10Perimeter", Angewandte Chemie International Edition in English, vol. 27, pp. 1692-1693, 1988. http://dx.doi.org/10.1002/anie.198816921