High resolution GHz and THz (FTIR) spectroscopy and quantum dynamics of tunnelling in aniline

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 1400cm−1, in part synchrotron based at our highest resolution spectrometer at the Swiss Light Source, SLS, achieving a best possible resolution Δν~=0.00052cm−1 and effective resolutions generally better than 0.0008cm−1). Rotation-vibration tunnelling transitions have been assigned and analysed for the vibrational ground state, the associated inversion tunnelling level I1 at 40.953cm−1 as well as the lowest frequency out of plane CN-bending fundamental 10b1 at 216.466cm−1 and the torsional fundamental at 275.930cm−1 and the corresponding inversion tunnelling sublevels. High resolution results are also obtained for the excited levels I2(423.046cm−1) and I3(699.369cm−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.