Comparison of gas‐phase basicities and ion–molecule reactions of aminobenzoic acids

Functional group interactions and substituent effects of o ‐, m ‐ and p ‐aminobenzoic acids were examined in a quadrupole ion trap by evaluation of proton‐transfer and methylene substitution reactions. An electron‐withdrawing carboxylic acid group can enhance or reduce the gas‐phase basicity of aniline depending on its location and ability to participate directly in proton bridging. In fact, the gas‐phase basicity of o ‐aminobenzoic acid is enhanced by ∼ 7 kcal mol −1 relative to the meta and para isomers in which the substituents do not have cooperative functional group interactions. Collisionally activated dissociation studies of deuterium‐labeled ions provide evidence of proton migration from the amino group to the carboxylic acid group prior to fragmentation. Results of semi‐empirical molecular orbital calculations provided structures for the various protonated aminobenzoic acids. For o ‐aminobenzoic acid, the proton bridges between the acid and amine groups.