Fragmentation of Protonated N-(3-Aminophenyl)Benzamide and Its Derivatives in Gas Phase

An ion of m/z 110.06036 (ion formula [C6H8NO](+); error: 0.32 mDa) was observed in the collision induced dissociation tandem mass spectrometry experiments of protonated N-(3-aminophenyl)benzamide, which is a rearrangement product ion purportedly through nitrogen-oxygen (N-O) exchange. The N-O exchange rearrangement was confirmed by the MS/MS spectrum of protonated N-(3-aminophenyl)-O (18) -benzamide, where the rearranged ion, [C6H8NO (18) ](+) of m/z 112 was available because of the presence of O (18) . Theoretical calculations using Density Functional Theory (DFT) at B3LYP/6-31 g(d) level suggest that an ion-neutral complex containing a water molecule and a nitrilium ion was formed via a transition state (TS-1), followed by the water molecule migrating to the anilide ring, eventually leading to the formation of the rearranged ion of m/z 110. The rearrangement can be generalized to other protonated amide compounds with electron-donating groups at the meta position, such as, -OH, -CH3, -OCH3, -NH(CH3)2, -NH-Ph, and -NHCOCH3, all of which show the corresponding rearranged ions in MS/MS spectra. However, the protonated amide compounds containing electron-withdrawing groups, including -Cl, -Br, -CN, -NO2, and -CF3, at the meta position did not display this type of rearrangement during dissociation. Additionally, effects of various acyl groups on the rearrangement were investigated. It was found that the rearrangement can be enhanced by substitution on the ring of the benzoyl with electron-withdrawing groups.