Gas-Phase Collision-Induced Dissociation of Protonated N -Phenylphthalimide and Its Derivatives

The fragmentation of protonated N-phenylphthalimide (C6H4(CO)2NC6H5) in electrospray ionization generates an unexpected cation (C6H4COCN+) at m/z 130.029 as the most abundant ion by the loss of a neutral phenol molecule. One of the oxygen atoms in the two carbonyl groups is transferred to the phenyl ring by replacing the nitrogen atom. To confirm the oxygen migration to the benzene ring, mono-18O–N-phenylphthalimide was synthesized and subjected to collisional activation, resulting in two similarly abundant ions at m/z 130.029 and m/z 132.033 that corresponded to the 18O cation (C6H4C18OCN+). To confirm that one hydrogen atom on the phenyl ring may mediate the reaction to eliminate a water molecule, deuterated N-phenylphthalimide (C6H4(CO)2NC6D5) was prepared, and its MS/MS spectrum shows a peak at m/z 210.0848, corresponding to a neutral loss of HOD. Moreover, when the phenyl group is replaced by a pentafluorobenzene group (i.e., C6H4(CO)2NC6F5), HF loss is observed but without water loss. A nitrogen–oxygen rearrangement mechanism was proposed, in which the carbonyl oxygen atom mediates nucleophilic substitution of the nitrogen atom of the N-phenyl ring, leading to the loss of neutral phenol. The nitrogen–oxygen rearrangement can be extended to various substituted N-phenylphthalimide compounds, including heterocyclic rings such as triazole, most of which yield nitrogen–oxygen rearrangement ions as major peaks in MS/MS spectra.