Revisiting protomers of aniline by high-resolution ion mobility spectrometry, capillary electrophoresis-mass spectrometry, and abinitio calculations

Protomers are ions which displays tautomerism due to the position change of a proton. Recent papers show that two stable protomers of aniline were observed using ion mobility spectrometry. Protonation of the amine functional group and the carbon in “para” position were identified [Lalli, 2012 and Attygalle, 2017]. Tautomerization was also observed for other amine functional group containing in aromatic compounds. We present here the use of capillary electrophoresis and high-resolution ion mobility spectrometry to detect other stable or metastable protomers predicted by ab-initio calculations such as protonation of the carbon in “ortho” and “meta” position of aniline. These data should allow the refinement of the energy landscape during the protomer formation and tautomerization. For this reason, it is required to characterize all the ions that can be implicated during protomers formation. High resolution ion mobility(TIMS) allowed the detection of more ions related to aniline than those presented by the stateof-the-art. In addition, benzenium (C6H7+) and phenol (C6H7O+) cations can also be formed according to the electrospray ion source settings from the mass spectrometer. All of these ions must be taken in account to refine the energy landscape during the protomer formation and tautomerization. Ab-initio computation was used to support the identification of these ions in regard of the experimental data. Preliminary calculations by Density Functional Theory show that the protonation in the carbon in “ortho” position should be almost as stable as the amine protonation and should be observed during IMS experiments using the appropriate ion mobility resolving power. Calculations using polarizable continuum model also predict that the ratio of protomers is affected in presence of solvents and especially its dielectric constant. The protonation of the amine functional group is the most predominant species when aniline is solubilized in protic solvents. To the contrary both protonations of the amine functional group and the carbon in “para” should be significantly present in aprotic solvents. Aqueous and non-aqueous capillary zone electrophoresis (CZE) coupled to mass spectrometry are then consider to confirm these hypotheses.