Aromaticity: The Alternating CC Bond Length Structures of [14]‐, [18]‐, and [22]Annulene

Abstract

First among unequals: Equal CC bond lengths, as found in benzene, are not necessary for aromaticity. [18]Annulene is the next Hückel monocycle where D6h symmetry is possible topologically (see picture). However, this widely accepted structure of [18]annulene is not correct. The 1H NMR chemical shifts and energy computations show that [18]annulene prefers CC bond-length alternation and C2 symmetry.