Rotational tunneling of methyl groups in the hydroquinone/acetonitrile clathrate: A combined deuteron NMR, INS, and computational study

The methyl groups (CH and CD ) of the acetonitrile molecules in the clathrate of hydroquinone are investigated by single-crystal deuteron NMR and inelastic neutron scattering (INS). The NMR spectra below 40 K exhibit the characteristics of rotational tunneling with tunnel frequencies on the same scale as the quadrupole coupling constant of the deuterons. By comparing the spectra with simulations, the tunnel frequencies of the three crystallographically inequivalent CD groups are determined with high precision. By INS, the energies of librational excitations of the CH groups are measured. These two pieces of information allow for an accurate characterization of the rotational potentials of the methyl groups. The results are contrasted to force-field calculations based on a newly determined low-temperature neutron structure. Incoherent reorientations dominate the NMR spectra above 40 K. Their correlation times are determined in a wide temperature range and compared to theoretical predictions