Forming Bonds While Breaking Old Ones: Isomer-Dependent Formation of HO from Aminobenzoic Acid During X-ray-Induced Fragmentation.

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 (HO) production from aminobenzoic acid (ABA) after core-level ionization with soft X-ray synchrotron radiation. The formation of HO 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 HO depends strongly on the structure of the molecule, the ortho-isomer being much more prone to produce HO. The isomer-dependency suggests that the amine group acts as a donor in the hydrogen transfer process. In the case of ortho-ABA, detailed HO production pathways were investigated using photoelectron-photoion-photoion coincidence (PEPIPICO) spectroscopy. It was found that HO can result from a direct two-body dissociation but also from sequential fragmentation processes.