[18]annulene

Other names: cyclooctadecanonaene, (1Z,3E,5E,7Z,9E,11E,13Z,15E,17E)-cyclooctadeca-1,3,5,7,9,11,13,15,17-nonaene

C₁₈H₁₈

26/50 common

[18]annulene is a fully conjugated cyclic hydrocarbon containing 18 carbon atoms and 18 π-electrons, making it aromatic by the Hückel criterion. First synthesized by Sondheimer and colleagues in 1962, this molecule provides compelling evidence for the concept of aromaticity in large rings. It displays a noteworthy property in its ¹H NMR spectrum, where the ring's aromatic current creates a remarkable magnetic anisotropy. While the outward-facing protons show an expected downfield shift (δ ~9 ppm) due to ring current deshielding, the inward-facing protons instead experience shielding (recall Lenz's Law) and appear dramatically upfield (δ ~-2.0 ppm) – a powerful demonstration of aromatic ring current effects. This unusually large chemical shift difference serves as textbook evidence for extended aromaticity beyond conventional benzene systems and highlights how molecular geometry interacts with electronic structure to produce surprising spectroscopic properties.