Notably, [18]annulene is the first annulene after
benzene ([6]annulene) to be fully
aromatic: its π-system contains 4
n + 2 electrons (
n = 4), and it is large enough to comfortably accommodate six hydrogen atoms in its interior, allowing it to adopt a planar shape, thus satisfying
Hückel's rule. The discovery of aromatic stabilization for [18]annulene is historically significant for confirming earlier theoretical predictions based on
molecular orbital theory, since simple versions of valence bond theory did not readily explain the 4
n + 2 rule. The 1H
NMR of this compound exhibits hallmarks of a system with an
aromatic ring current, with the 12H signal of exterior hydrogens at 9.25 ppm, while the 6H signal of interior hydrogens resonates at a remarkable −2.9 ppm in THF-
d8 at −60 °C. On the other hand, a single signal at 5.45 ppm (the weighted average of the two individual signals) is observed at 120 °C. This is consistent with rapid exchange of exterior and interior hydrogens at that temperature. The
bond lengths in [18]annulene are in between those of single and double carbon–carbon bond, with two bond lengths observed
crystallographically: 138.9 pm (concave edges) and 140.7 pm (convex edges) which are indicative of significant
delocalization. The favorability of delocalization is, in turn, interpreted as evidence for aromaticity. For comparison, these values are close to bond length of benzene (140 pm). File:18annulene.png|thumb|The bond lengths of [18]annulene are around 139 to 141 pm, indicating delocalization and a bond order between 1 and 2. In general, carbon-carbon single bonds and double bonds are 154 pm and 134 pm, respectively, while benzene, with a bond order of 1.5, the bond-length is 140 pm. Based on the enthalpy of hydrogenation, the overall
resonance energy has been estimated to be 37 kcal/mol. This is about the same as that of benzene; however, this energy is spread out over 18 atoms instead of 6, so [18]annulene experiences a weaker stabilization than benzene. In terms of reactivity, it is somewhat more air- and light-stable than cyclotetradecaheptaene|[14]annulene and cyclodecapentaene|[10]annulene, which are, respectively, weakly aromatic and nonaromatic due to transannular interactions. Nevertheless, it rapidly undergoes
electrophilic additions, much like other
polyenes, and attempts to effect
Friedel–Crafts type reactions on [18]annulene failed. Despite the usual interpretation of [18]annulene as an 18-electron aromatic system, a 2014 theoretical study suggested that [18]annulene may be thought of as having only three completely delocalized
π bonds associated with its aromaticity, while the other six π bonds represent conjugated
three-center-two-electron (3c–2e) π bonds on the periphery of the molecule. ==Synthesis==