The collisional activation of protonated N-propyl-2-nitroaniline obtained by electrospray ionization shows two major competitive dissociation pathways: the elimination of the elements of propionic acid, [M + H - C3H6O2]+ to give an m/z 107 ion, and of the elements of ethanol, [M + H - C2H6O]+ to give an m/z 135 ion. The mechanistic study reported here addresses these unusual fragmentations to reveal that both occur via a common intermediate formed by the transfer of an oxygen atom from the nitro group to the first carbon atom of the propyl group, allowing elimination of propionic acid and (H2O + ethene), respectively. The corresponding loss of CH4O does not occur when the propyl group is replaced by an ethyl group, but elimination of the elements of propanol does occur when propyl is replaced by a butyl group. Further, the product ions of m/z 107 and 135 are also formed when the propyl chain is replaced with a hexyl group.
Keywords: collisional activation; density functional theory; intramolecular oxidation; mass spectrometry; nitro group; oxygen atom rearrangement; protonated N-alkyl-2-nitroaniline.