Forming Bonds While Breaking Old Ones: Isomer-Dependent Formation of H(3)O(+) from Aminobenzoic Acid During X-ray-Induced Fragmentation

[Image: see text] Intramolecular hydrogen transfer, a reaction where donor and acceptor sites of a hydrogen atom are part of the same molecule, is a ubiquitous reaction in biochemistry and organic synthesis. In this work, we report hydronium ion (H(3)O(+)) production from aminobenzoic acid (ABA) after core-level ionization with soft X-ray synchrotron radiation. The formation of H(3)O(+) during the fragmentation requires that at least two hydrogen atoms migrate to one of the oxygen atoms within the molecule. The comparison of two structural isomers, ortho- and meta-ABA, revealed that the production of H(3)O(+) depends strongly on the structure of the molecule, the ortho-isomer being much more prone to produce H(3)O(+). The isomer-dependency suggests that the amine group acts as a donor in the hydrogen transfer process. In the case of ortho-ABA, detailed H(3)O(+) production pathways were investigated using photoelectron-photoion-photoion coincidence (PEPIPICO) spectroscopy. It was found that H(3)O(+) can result from a direct two-body dissociation but also from sequential fragmentation processes.