Exploring rearrangements along the fragmentation of glutaric acid negative ion: a combined experimental and theoretical study.

Glutaric acid, a common short-chain aliphatic dicarboxylic acid, was investigated in the negative ion mode by subjecting its [M-H](-) ion to collision-induced dissociation (CID) experiments in an infinity ion cyclotron resonance (ICR) cell coupled to a hexapole-quadrupole-hexapole ion guide. A 12 Tesla magnet was used for high-resolution measurements. Two distinctive main pathways were observed in the MS/MS spectrum. The fragmentation pathways were also thoroughly investigated in a density functional theory (DFT) study involving a B3LYP/6-311+G(2d,p)//B3LYP/6-311+G(d,p) level of theory. Elimination of CO(2) from the [M-H](-) ion of the dicarboxylic acid takes place in a concerted mechanism, by which a 1,5 proton shift occurs from the intact carboxyl group to the methylene moiety located in the alpha position relative to the deprotonated carboxyl group. This concerted mechanism stabilizes the terminal negative charge and deprotonates the second carboxylic acid group. Water elimination from the [M-H](-) ion does not take place by means of a simple proton removal from the alpha methylene group - and OH(-) release from the carboxylate group to abstract an additional alpha proton thus leading to the formation of a deprotonated ketene anion. In the case of this dicarboxylic acid, a new mechanism was found for water elimination, which differs from that known for aliphatic monocarboxylic acids. An intramolecular interaction between the deprotonated and the intact carboxyl groups plays a key role in making a new energetically favourable mechanism. The DFT study also reveals that a combined loss of CO2 and H2O in the form of H2CO3 is possible. Copyright 2010 John Wiley & Sons, Ltd.