Estimation of gas-phase acidities of a series of dicarboxylic acids by the kinetic method

Abstract

The gas-phase acidities (GA) of a series of dicarboxylic acids were estimated by applying the extended kinetic method. Proton-bound heterodimeric anions [A^-H⁺B^-] of a series of dicarboxylic acids (A) and reference compounds (B) were generated under electrospray ionization conditions, and the dissociation of these cluster ions was examined using a triple-quadrupole mass spectrometer. The mass-selected proton-bound heterodimeric anions were fragmented to yield individual monomer anions as the only product ions, and the abundance ratios of these anions were used to estimate the GA values of the dicarboxylic acids. The experiment was performed over a range of collision energies in order to more accurately determine GA values and to estimate the differences in the entropy of deprotonation of the dicarboxylic acids and the reference compounds. The trends in GA values obtained for the dicarboxylic acids could imply cyclization of the structures via intramolecular hydrogen bonding. The lower homologues show higher GA values than the higher homologues. The GA order for lower homologues is comparable with that of their solution-phase pKa1 values.