Fragmentation of Deprotonated Diacylhydrazine Derivatives in Electrospray Ionization Tandem Mass Spectrometry: Generation of Acid Anions via Intramolecular Rearrangement

The gas-phase fragmentation pathways of deprotonated diacylhydrazine derivatives (R1(C = O)-N(t-Bu)NH(C = O)R2, Compounds 1–6) were investigated by the combination of electrospray ionization tandem mass spectrometry (ESI-MS/MS) and theoretical calculations. Upon collisional activation, the deprotonated molecular ions [M – H]− dissociate in two reaction channels, both of which involve intramolecular rearrangement. The main product ion is confirmed to be an anionic acid species, [R1-CO2]−, generated through intramolecular rearrangement of [M – H]− initiated by the nucleophilic attack of the amide O6 on the carbonyl C2 (Path-1). The minor fragment channel (Path-2) involves methylpropene elimination of the precursor ion, followed by a similar nucleophilic displacement reaction to produce another acid anion [R2-CO2]−. Density functional theory calculations at the B3LYP/6-31+G(d,p) level indicate that Path-1 is more favorable than Path-2 for dissociation of the deprotonated halofenozide.