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

IR absorption spectra of aniline cation, anilino radical, and phenylnitrene isolated in solid argon

Electron bombardment of aniline (PhNH₂) in an Ar matrix mainly generated the aniline cation (PhNH₂⁺), anilino (PhNH) and phenyl (Ph) radicals, and phenylnitrene (PhN). Further irradiation of the electron-bombarded matrix sample at 365 nm depleted PhNH₂⁺ and PhN, and resulted in the formation of PhNH₂, PhNH, and Ph. In separate experiments, irradiation of the PhNH₂/Ar matrix samples at 265 or 160 nm mainly generated PhNH and Ph radicals, but without the formation of PhNH₂⁺ and PhN. According to the observed photochemical behaviors, quantum-chemically predicted harmonic vibrational wavenumbers of each species, and the information reported in previous photodissociation studies, we unambiguously characterized the IR features of the aromatic species. The information of the vibrational fundamentals of PhNH is new and the formation mechanism is discussed.

Infrared Spectra of Monohydrogenated Aniline, ortho- and para-HC6H5NH2, Generated in Solid para-Hydrogen

The isomers of monohydrogenated aniline (HC₆H₅NH₂) are regarded as important intermediates in reduction reactions of aniline, but their spectral identification has been limited to electron paramagnetic resonance in an adamantane matrix. We report here infrared (IR) spectra of two least-energy isomers of HC₆H₅NH₂, produced on electron bombardment during the deposition of a matrix of aniline and para-hydrogen at 3.2 K. The intensities of IR lines of HC₆H₅NH₂ increased during maintenance of the electron-bombarded matrix in darkness for a prolonged period because of the neutralization of protonated aniline, H⁺C₆H₅NH₂, by trapped electrons and further reactions between aniline and the unreacted hydrogen atoms that were produced during electron bombardment. The observed lines were grouped according to their behaviors on secondary photolysis with light at 520, 465, and 375 nm. On comparison of experimental spectra with quantum chemically predicted spectra for four possible isomers of HC₆H₅NH₂, lines in one group were assigned to the most stable ortho-HC₆H₅NH₂ and those in the other group were assigned to the secondmost stable para-HC₆H₅NH₂. Their photolytic behaviors at varied wavelengths are consistent with predicted ultraviolet absorption bands. The mechanisms of formation of these isomers are discussed according to semiquantitative analysis.

Vibrational spectroscopy of protonated amine–water clusters: tuning Fermi resonance and lighting up dark states

The H-bonded NH stretching fundamentals of protonated amine–water clusters pass through the “Fermi resonance window” formed by bending overtones, generating split bands due to anharmonic couplings.