Cyclic conjugation effects in cyclacenes

Cyclic conjugation is studied in Hückel and Möbius cyclacenes as well as in a few other related open-chain and cyclic belt-shaped hydrocarbons (e.g., zig-zag cyclarenes). The main conclusions are that (a) the energy effect of the conjugation along the perimeter of cyclacenes is relatively large, (b) conjugation along the 6 -, 10 - and 14-membered cycles has a much greater effect, in spite of the fact that these cycles embrace bonds that formally are not involved in cyclic conjugation. Möbius cyclacenes are non-alternant hydrocarbons and their odd-cycles have a significant contribution to conjugation. The contribution of conjugation in individual cycles to the total resonance energy is briefly discussed.