Tropylium ion

  In organic chemistry, the tropylium ion is an aromatic species with a formula of [C₇H₇]⁺. Its name derives from the molecule tropane (itself named for the molecule atropine).

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It is a heptagonal, planar, cyclic ion; as well, it has 6 π-electrons (4n+ 2, where n=1), which fulfills Hückel's rule of aromaticity. It can coordinate as a ligand to metal atoms.

The structure shown is a composite of seven resonance contributors in which each carbon carries part of the positive charge.

The tropylium ion is frequently encountered in mass spectrometry in the form of a signal at m/z = 91. This fragment is often found for aromatic compounds containing a benzyl unit. On ionization, the benzyl fragment is cleaved off. It (PhCH₂⁺) rearranges to the highly stable tropylium cation (C₇H₇⁺).