Which of the following is not an aromatic species?
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To be aromatic, a cyclic, conjugated molecule must be planar and have $(4n+2)$ $\pi$ electrons. If it is planar and has $4n$ $\pi$ electrons, it is anti-aromatic. If it is not planar, it is non-aromatic, regardless of the electron count. Cyclooctatetraene adopts a non-planar tub shape to avoid being anti-aromatic.
