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 similar to 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.