Unimolecular and collision-induced fragmentation of protonated nitroarenes

The unimolecular fragmentation reactions of 28 protonated nitroarenes, occurring on the metastable ion time-scale, are reported. In addition, the collision-induced fragmentation of the same species have been studied at 10 eV and at 50 eV collision energy. When an OH, COOH or NH₂ substituent is ortho to the nitro function, the dominant fragmentation involves loss of H₂O, for both unimolecular and collision-induced reactions. When there is an electron-releasing substituent ortho or para to the litro group, loss of OH is the dominant fragmentation reaction both on the metastable ion time-scale and for ions activated by collision. When the electron-releasing substituent is meta to the nitro group, loss of NO₂ is the dominant low-energy unimolecular fragmentation reaction while loss of HNO₂ is the most important fragmentation for ions activated by 50 eV collisions. Elimination of NO from MH]⁺ occurs to a significant extent in the unimolecular fragmentation of protonated nitrobenzene and those protenated nitrobenzenes containing electron- attracting substituents. In the collision-induced dissociation of these species loss of HNO₂ occurs at the expense of loss of NO. The results are consistent with protonation predominantly at the nitro group. The results are discussed in terms of the use of neutral loss scans in tandem mass spectrometry to monitor complex mixtures for nitroarenes.