Proton affinities of the anions of aromatic carboxylic acids measured by kinetic method

Gas-phase proton affinities (PA) of the carboxylate anions are very important parameters for gas-phase as well as condensed-phase reaction studies. Comparison of the PA values with pKa values of the respective carboxylic acids in solution gives information about the solvation effects on the acidity of these compounds. In the present work we have measured the PA values of about 65 aromatic acids using Cooks’ kinetic method. About 30 of these acids have been investigated previously by other researchers and served as the reference compounds in our work. The results of MS measurements were evaluated using three methods differing by the level of simplification. We found that for the series of the studied acids there were no significant differences between these methods, so even the simplest one, which does not take into account entropy effects, can be used. The case of the only exception: o-hydroxybenzoic (salicylic) acid is discussed. It was also found that the kinetic method can be used for acids whose anions undergo further fragmentation under experimental conditions.