A. Detken et al., ROTATIONAL TUNNELING OF METHYL-GROUPS IN THE HYDROQUINONE ACETONITRILE CLATHRATE - A COMBINED DEUTERON NMR, INS, AND COMPUTATIONAL STUDY/, Chemical physics, 238(2), 1998, pp. 301-314
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.