7
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9H-fluoren-9-ylium cation

In this cation, there are two individual benzene rings which are aromatic. But in the entire π system, there are 12 π-electrons which should make it antiromatic. Which is correct?

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    $\begingroup$ Could be called aromatic, but much less than the corresponding anion. (see chemistry.stackexchange.com/questions/88615/…) $\endgroup$ – mykhal Nov 18 '18 at 18:31
  • $\begingroup$ And this is why the 4n+2 rule is for single cycles only. For anything more complex we don't really know much outside of experiments. $\endgroup$ – Oscar Lanzi Nov 18 '18 at 23:26
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    $\begingroup$ According to [literature][1] it is actually anti-aromatic. I guess it depends on when you call something like that aromatic or not. Is the system fully delocalized, are all bonds equal or close in bond lenghts, is there considerable out of plane twisting from the planarity, does the aromatic ring current extend through the whole molecule, ...? Here it seems to be the case and anti-aromatic since both ring-systems are still connected. [1]: pubs.acs.org/doi/10.1021/ja00533a030 $\endgroup$ – Justanotherchemist Nov 19 '18 at 7:23
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    $\begingroup$ @Justanotherchemist The way you have tried to include links doesn't work in comment. You can use [text](link) instead. Anyway, it is understandable this way, too. I think your comment is already halfway to an answer, care to share it as one? $\endgroup$ – Martin - マーチン Nov 21 '18 at 14:52
  • $\begingroup$ @Justanotherchemist To be precise, it's in quotes: Diphenylmethyl and the related but "antiaromatic" fluorenyl cations have been compared previously…, and it's talking about antiaromatic destabilization. $\endgroup$ – mykhal Nov 23 '18 at 18:38

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