Protonation Sites of Aromatic Compounds in (+) Atmospheric Pressure Photoionization

Abstract

Reaction enthalpy of hydrogen transfer reactions of aromatic compounds has been observed to be greatly affected by the exact location of the protonation site. Therefore, to clearly identify the protonation location, each candidate protonation site for 43 aromatic compounds were theoretically determined and their location was compared with that determined based on experimental MS data. Only the basic nitrogen atom is favorable as a protonation site for pyridine‐type aromatic compounds, whereas carbon atoms are preferable for the protonation of pyrrole‐type compounds. The most favorable protonation sites for aniline or methylated aniline‐type aromatic compounds are either the nitrogen atom in the amine group or the carbon atom at the para‐position to the amine group. Like pyrrole‐type compounds, aromatic compounds with amine groups also favor protonation at the carbon atom instead of at the nitrogen atom. In addition, hydrocarbons having an anthracene structural motif without heteroatoms produced higher or equal percentages of protonated ions compared to that achieved with molecular ions. The results of this study can be used to improve the analyses of aromatic compounds.