Infrared multiphoton dissociation spectroscopy study of protonated p-aminobenzoic acid: Does electrospray ionization afford the amino- or carboxy-protonated ion?

Infrared multiphoton dissociation spectra of protonated p-aminobenzoic acid generated by electrospray ionization (ESI) from aqueous methanol and acetonitrile solutions were recorded in the gas phase from 2800-4000 cm ^-1. The O-protonated ion is more stable than the N-protonated structure in the gas phase, whereas the opposite is true in both solutions. When CH₃OH/H₂O was used as the ESI solvent, only the O-protonated ion was observed. In contrast, a 70:30 mixture of the O- and N-protonated species were produced from CH₃CN/H₂O. These structural assignments are based on an assortment of experimental data (action spectra, photofragments, photofragmentation kinetics, and H/D exchange) and are fully supported by extensive computations. This work shows that ESI can lead to isomerization and that the ionization site may be varied by changing the solvent from which the substrate is analyzed.