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 (C ₆H₆) 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 C ₄H₄, C₆H₆, C₈H ₆, 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 C₄H ₄and C₈H₈) or derealization (in C ₆H₆ and C₁₀H₁₀) of the double bonds.