Collision-induced mass spectrometric fragmentation mechanisms of gaseous-ions derived from some organic carboxylic acids and related compounds

Mass spectrometric fragmentation mechanisms of some small organic molecules containing carboxylic acid group and their derivatives were investigated with special emphasis on gas-phase site specific double-hydrogen transfer mechanisms which were scrutinized with isotope labeling experiments and theoretical calculations.;Collision-induced dissociation (CID) of anions derived from carboxyanilides provided a novel way of producing gaseous ketenyl anions. Even though, the fragment-ion spectra of all ring isomers of carboxyanilides show a peak for the ketenyl anion, the intensities of the peak for ortho isomers were significantly higher than those for meta and para isomers. In addition the existence of a fragmentation pathway specific for meta isomers was disclosed. This pathway enables unequivocal identification of meta isomers from ortho and para isomers of carboxyanilides. The fragmentation follows a specific channel to eliminate a molecule of m-benzyne, from the anion produced after the initial decarboxylation of the precursor ion.;The CID mass spectra recorded from anions derived from O-alkyl ethers and O-acyl esters of ortho-hydroxybenzoic acid disclosed a dramatic "ortho effect." For example, the spectra recorded from O-alkyl ethers of ortho-hydroxybenzoic acid showed peaks for an initial CO2 loss followed by a loss of benzene to form an enolate anion which can be attributed to a double-hydrogen transfer from the positions 1 and 2 of the alkyl chain to the phenyl ring. Under the condition used, only the spectra from ortho isomer showed peaks at m/z 77 and m/z 93 for the phenyl phenoxide anions respectively, in addition to that for the ortho-specific enolate anion. The spectra recorded from O-acyl esters of ortho -hydroxybenzoate anions showed a prominent peak at m/z 137 for the elimination of a ketene molecule from the precursor. Deuterium labeling studies confirmed that an &agr;-hydrogen atom transfer is required for the elimination of the ketene molecule.;Collision-induced dissociation (CID) spectra of depotonated diesters of ortho-hydroxybenzoic acid (salicylic acid) made from glutaric, adipic, and pimelic acids showed a peak for a 120-Da "neutral loss" specific to the ortho isomers. In contrast, the spectra of anions derived from diesters of meta-, and para- hydroxybenzoic acids showed a peak for 138-Da loss for an elimination of elements of hydroxybenzoic acid by a charge-remote mechanism.