Proximity effects in mass spectrometry. Electron impact ionization-induced cyclization of 1,2-bis(1,4-dithiafulven-6-yl)benzenes

The electron impact ionization mass spectra of o-, m- and p-bis(1,4-dithiafulven-6-yl)benzenes were studied by means of accurate mass measurements, metastable analysis and collision-induced dissociation. Differences observed in the spectra of the ortho isomers are due to a cyclization reaction leading to molecular ions with the same structure as those generated from certain cyclic compounds, as confirmed by comparison of linked scans at constant B/E of metastable and collisionally activated molecular ions. Parallels of this cyclization of molecular ions with their electrochemical or acid-induced isomerization are also discussed.