Aromaticity and Antiaromaticity in 4-, 6-, 8-, and 10-Membered Conjugated Hydrocarbon Rings

Recently, we presented a molecular orbital (MO) model of aromaticity that explains, in terms of simple orbital-overlap arguments, why benzene (C6H6) has a regular structure with delocalized double bonds whereas the geometry of 1,3-cyclobutadiene (C4H4) is distorted with localized double bonds. Here, we show that the same model and the same type of orbital-overlap arguments also account for the irregular and regular structures of 1,3,5,7-cyclooctatetraene (C8H8) and 1,3,5,7,9-cyclodecapentaene (C10H10), respectively. Our MO model is based on accurate Kohn−Sham DFT analyses of the bonding in C4H4, C6H6, C8H8, and C10H10 and how the bonding mechanism is affected if these molecules undergo geometrical deformations between regular, delocalized ring structures and distorted ones with localized double bonds. The propensity of the π electrons is always to localize the double bonds, against the delocalizing force of the σ electrons. Importantly, we show that the π electrons nevertheless determine the localization (in C4H4 and C8H8) or delocalization (in C6H6 and C10H10) of the double bonds.