7 H ‐benz [ kl ]acridin‐7‐ylium closed‐shell ion produced from 1,8‐bis(phenylamino)naphthalenes under electron impact: Tandem mass spectrometric structure elucidation

Not only strongly basic aromatic amines such as ‘proton sponges’ show characteristic [M – Me 2 NH – H] + peaks corresponding to cyclization to stable heterocyclic ions under electron impact. The fragmentation of the title compounds, which are weak bases, leads to [M – RNH 2 – C 6 H 5 ] + heteroaromatic ions. These are the 7‐phenyl‐ 7H ‐benz[ kl ]acridine radical cation and the delocalized stable 7H ‐benz[ kl ]acridin‐7‐ylium ion as shown by unimolecular metastable ion spectra, collision‐induced decomposition (CID) tandem mass spectrometry (MS/MS) and accurate mass measurements. The high‐collision‐energy CID tandem mass spectra of these ions are perfectly superimposable on those of the ions produced by the reference substance, 7‐phenyl‐ 7H ‐benz[ kl ]acridine obtained as a by‐product in the Ullmann phenylation of 1,8‐diaminonaphthalene. This combination of MS/MS experiments also provides strong support for the gas‐phase reaction mechanism leading to the benz[ kl ]acridine ions and for the structures of these ions.