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.