Loss of atomic nitrogen from even-electron ions? A study on benzodiazepines

The fragment spectra of protonated nitro-substituted benzodiazepines show an unusual fragment [M + H - 14]⁺ , which is shown by accurate mass measurement to be due to the loss of a nitrogen atom. Our investigations show that this apparent loss of atomic nitrogen is rather an attachment of molecular oxygen to the [M + H - NO₂ ]^+• ion, which is the main fragment ion in these spectra. The oxygen attachment is exothermic, and rate constants have been derived. MSⁿ spectra show that it is not easily reversible upon fragmentation of the adduct ion and that it is also observed with some secondary and tertiary fragments, which allows to limit the attachment site to the aromatic ring annulated to the diazepine moiety. Fragments of the oxygen adduct ion indicate that the O₂ molecule dissociates in the adduct formation process, and the two oxygen atoms are bound to different sites of the ion. Comparison with radical cations generated by fragmentation of non-nitro-substituted benzodiazepines, none of which showed an oxygen attachment, and the fragmentation mechanisms involved in their formation indicates that the [M + H - NO₂ ]^+• ion is a distonic ion with the charge and radical site neighbored on the aromatic ring. From these results, we derive a proposal for the formation and structure of the [M + H - NO₂ + O₂ ]^+• ion, which explains the experimental observations. Copyright © 2015 John Wiley & Sons, Ltd.