the compound

I was thinking that it might be non aromatic since the ring is very big and it might bend just like COT (cyclo-octatetraene) does to avoid destabilization due to antiaromaticity but I don't know if the given does that or not.

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    $\begingroup$ This question may be much more interesting then one could think, as I read about a claim, that this cation shows Möbius aromaticity! $\endgroup$ – Mithoron Apr 17 '18 at 13:48
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    $\begingroup$ If you call cyclooctatetraene "non-aromatic" (I wouldn't argue with that), then what sort of molecule would you actually call antiaromatic? $\endgroup$ – orthocresol Apr 17 '18 at 14:50
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    $\begingroup$ en.wikipedia.org/wiki/… $\endgroup$ – Mithoron Apr 17 '18 at 15:18
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    $\begingroup$ @Mithoron Indeed, it is the poster boy for Möbius aromaticity: Wikipedia. $\endgroup$ – Martin - マーチン Apr 17 '18 at 15:34
  • $\begingroup$ @orthocresol By definition, there cannot be stable (i.e. ground state) anti-aromatic compounds; these need either be hypothetical molecules or excited states. $\endgroup$ – Martin - マーチン Apr 17 '18 at 15:37

Cycloenneatetraenyl cation, using the Greek root for nine, is certainly not antiaromatic. A ring that large will in general be nonplanar unless strong aromatic coupling is there to drive a planar shape, and if the planar form is antiaromatic so much the better for nonplanarity. By classical rules such a nonplanar form would be nonaromatic.

Whether this nonplanar form is Mobius aromatic is open to interpretation, or at least it seems that way given the twisted nature of the ring. In any event Mobius aromaticity is unlikely to give the strong stabilization we see in the Huckel version because the p orbitals do not overlap perfectly. Most likely the structure and stability of the ion are more determined by sigma bond interactions (minimizing steric strain) than pi bond interactions.


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