trans-cyclononatetraenylium

Other names: cyclo­nona­tetra(1E,3Z,5Z,7Z)enyl­ium cation

C₉H₉⁺

35/50 uncommon

The trans-cyclononatetraenylium cation exemplifies an unusual phenomenon called Möbius aromaticity. Its saddle-like geometry physically resembles a Möbius strip with a single half-twist, creating a unique topology in its π-system. With 8π electrons (=4n), it would be antiaromatic in a conventional Hückel system, but the Möbius topology makes it aromatic instead – inverting the 4n/4n+2 rule. NMR studies confirm this unexpected aromaticity through characteristic downfield shifts. As of 2025, this compound remains the smallest synthesizable trans-cycloalkene, providing critical experimental validation of Möbius aromaticity theory and demonstrating how molecular topology can fundamentally alter electronic behavior.