NMR Studies of 4n π-Conjugated Polycyclic Anions

Abstract

The energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) de termines the line shape and chemical shift of 4n π polycyclic doubly charged systems. Narrower energy gaps correspond to more signif icant line broadening in the 1 H NMR bands. This line broadening is attributable to a singlet-triplet equilibrium that depends on the HOMO-LUMO gap. The topology of the system influences the par atropicity of these 4n π polycyclic dianions. Whereas the triphenylene dianion does not exhibit a 1 H NMR spectrum, a gradual change toward a highly resolved spectrum is obtained for the tri -benzo[a,c,i]phenazine dianion. A correlation between the HOMO -LUMO gap and the paratropic shift of these dianions is demonstrated. THE NOTI ON OF AROMATI CI TY is one of the most intriguing problems inorganic chemistry and has drawn the attention of chemists since Kekulé's ingenious proposal (1) in 1872 of the structure of benzene. The concept of antiaromaticity introduced by Breslow (2, 3) about a century later emphasized the perplexity of aromaticity. Nevertheless, numerous physical and chemical properties of many groups of compounds are attributed to their