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 evaluatio n 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 particip ate 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 coope rative functional group interactions. Collisionally activated dissocia tion studies of deuterium-labeled ions provide evidence of proton migr ation from the amino group to the carboxylic acid group prior to fragm entation. Results of semi-empirical molecular orbital calculations pro vided structures for the various protonated aminobenzoic acids. For o- aminobenzoic acid, the proton bridges between the acid and amine group s.