Tropylium cation

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).

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.

skeletal formula	ball-and-stick model	space-filling model

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

Mass spectrometry

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

Electron ionization mass spectrum of toluene [1].
Note parent peak corresponding to molecular mass M = 92 (C₇H₈⁺) and
highest peak at M-1 = 91 (C₇H₇⁺, quasi-stable tropylium cation).