Mass spectrometric characterization of small oxocarboxylic acids and gas phase ion fragmentation mechanisms studied by electrospray triple quadrupole-MS/MS-TOF system and DFT theory

Monocarboxylic acids with acyl functional group such as 5-oxohexanoic acid and 6-oxoheptanoic acid were characterized experimentally by electrospray ionization coupled to a triple quadrupole and TOF analyzer hybrid system. Collision-induced dissociation experiments at different activation energies were done to elucidate possible fragmentation pathways. These pathways were also studied on the theoretical level using DFT B3LYP/6-311++G(3df,3pd)//B3LYP/6-31+G(d) + ZPVE calculations. While all monocarboxylic acids fragment under loss of CO2 and H2O starting from their parent anion [M − H]−, an unusual fragmentation behaviour could be observed in case of 5-oxohexanoic acid. Synchronous CO2 and CH2CH2 elimination in a concerted mechanism was responsible for the lack of [M − H–CO2]− fragment ion. New anionic gas phase cyclization processes could be discerned in case of 5-oxohexanoic acid. Mechanistic differences in the fragmentation pathways of [M − H]− anions formed from 5-oxohexanoic acid and 6-oxoheptanoic acid after deprotonation were investigated both experimentally and through DFT calculations. Successive water, ketene CH2CO, and H2 eliminations were observed in the CID spectra of 6-oxoheptanoic acid parent anion. CO2 ejection from the parent ion of 6-oxoheptanoic acid was inhibited due to the lack of stabilization for the formed fragment ion.