Infrared Spectra of Isomers of Protonated Aniline in Solid para-Hydrogen

The protonation sites of aniline molecule play important roles in its chemical reactions, but the preferred protonation site of gaseous aniline has yet to be determined. In this work, we recorded infrared (IR) absorption spectra of three isomers of protonated aniline, H⁺C₆H₅NH₂, produced on electron bombardment during matrix deposition at 3.2 K of a mixture of aniline and para-H₂. The intensities of IR lines of H⁺C₆H₅NH₂ decreased during maintenance of the electron-bombarded matrix in darkness because of neutralization with electrons that were slowly released from their trapping sites. The observed lines were classified into three groups according to their behavior upon secondary photolysis with light at 375 and 254 nm and assigned to para-, amino-, and ortho-H⁺C₆H₅NH₂, the three most stable isomers of protonated aniline, according to comparison of experimental spectra with quantum-chemically predicted spectra of five possible isomers of H⁺C₆H₅NH₂. The spectra of para- and ortho-H⁺C₆H₅NH₂ are newly distinguished. The approximate relative abundance of these isomers in solid p-H₂ was para:amino:ortho ≈ (1.0 ± 0.1):(1.0 ± 0.6):(0.6 ± 0.1). The possible mechanisms of formation are discussed.