Vibrational spectroscopy of aniline cations and their H-loss cations in helium droplets

We produced aniline cations ($c$–C₆H₅ NH₂⁺) and their dehydro- (H-loss) cations inside helium droplets by electron ionization and observed their mass-selected vibrational spectra in the NH stretching region. We observed vibrational bands at m/q = 93 which were identified as aniline cations. These bands showed only a few cm⁻¹ shifts from the symmetric and anti-symmetric NH stretching bands for the amino (NH₂) group in the gas phase. For the H-loss cation at mass m/q = 92, the agreement of the observed NH band frequency with the result of DFT calculations suggests several candidate species, including a seven-membered ring structure, 4-monodehydro azepinylium ($c$–C₆NH₆⁺). A new reaction pathway to this cation was discussed by considering large excess energy in the ionization process.