ROTATIONAL TUNNELING OF METHYL-GROUPS IN THE HYDROQUINONE ACETONITRILE CLATHRATE - A COMBINED DEUTERON NMR, INS, AND COMPUTATIONAL STUDY/

The methyl groups (CH3 and CD3) of the acetonitrile molecules in the c lathrate of hydroquinone are investigated by single-crystal deuteron N MR and inelastic neutron scattering (INS). The NMR spectra below 40 K exhibit the characteristics of rotational tunneling with tunnel freque ncies on the same scale as the quadrupole coupling constant of the deu terons. By comparing the spectra with simulations, the tunnel frequenc ies of the three crystallographically inequivalent CD3 groups are dete rmined with high precision. By INS, the energies of librational excita tions of the CH3 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 calculat ions based on a newly determined low-temperature neutron structure. In coherent reorientations dominate the NMR spectra above 40 K. Their cor relation times are determined in a wide temperature range and compared to theoretical predictions. (C) 1998 Elsevier Science B.V. All rights reserved.