Combined Trapped Ion Mobility and Infrared Ion Spectroscopy Study of Protonation Sites in Aromatic Amines

The protonation site of aromatic amines in the gas phase has been under substantial debate, as it involves a subtle competition between the higher electronegativity of the amine nitrogen and the better charge delocalization ability of the fused aromatic rings. Previous studies have unambiguously shown, especially by ion mobility measurements, that higher-energy tautomers are easily observed depending on the experimental conditions in the ion source, including voltage settings and the type of solvent used in spray sources. Here, we use a combination of ion mobility and ion spectroscopy and focus on the tautomeric structure after ion mobility separation, in particular for protonated 1-aminonaphthalene and 1-aminoanthracene. We employ an atmospheric pressure chemical ionization (APCI) source, with a direct insertion probe to avoid any solvent influence, mounted on an FTICR mass spectrometer with a trapped ion mobility (TIMS) unit and optical access to the ions to perform infrared (IR) multiple-photon dissociation spectroscopy using the Free-Electron Laser for Infrared eXperiments (FELIX). TIMS analysis indeed reveals the presence of both N- and C-protonated species, but the IR spectra recorded in the ICR cell also suggest that mobilization and scrambling of the proton occur after TIMS separation. We computationally investigate the energetics of tautomerization and experimentally explore ion activation after TIMS separation.