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 CO₂ 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 α 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 α methylene group – and OH^? release from the carboxylate group to abstract an additional α 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.