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We report the observation and assignment of high resolution rotation-vibration-tunnelling spectra of aniline (C₆H₅NH₂) with the Zürich GHz spectrometer from 75 GHz to 500 GHz and by high resolution Fourier Transform Infrared (FTIR) THz spectroscopy from 0.8 to 42 THz (26 to 1400 cm⁻¹, in part synchrotron based at our highest resolution spectrometer at the Swiss Light Source, SLS, achieving a best possible resolution Δ𝜈 = 0.00052 cm⁻¹ and effective resolutions generally better than 0.0008 cm⁻¹). Rotation-vibration tunnelling transitions have been assigned and analysed for the vibrational ground state, the associated inversion tunnelling level I₁ at 40.953 cm⁻¹ as well as the lowest frequency out of plane CN-bending fundamental 10b₁ at 216.466 cm⁻¹ and the torsional fundamental at 275.930 cm⁻¹ and the corresponding inversion tunnelling sublevels. High resolution results are also obtained for the excited levels I₂(423.046 cm⁻¹) and I3(699.369 cm⁻¹) and further vibrational modes interacting with the tunnelling motion. Accurate band centres and rotational parameters of the effective Hamiltonian are presented for numerous vibrational-tunnelling levels. The results are discussed in terms of the tunnelling quantum dynamics of inversion at the nitrogen atom with a pyramidal equilibrium structure for this prototypical aromatic amine. We discuss mode selective promotion or inhibition of tunnelling by excitation of different vibrational modes. We also discuss the accurate spectroscopic results as possible benchmarks for theoretical treatments of tunnelling in complex polyatomic molecules and the possibilities of analysing rotation vibration-tunnelling spectra in dense complex spectra of transiently chiral molecules and molecules potentially useful as candidates for the measurement of molecular parity violation.
Wichmann et al. (Thu,) studied this question.