13C NMR of tunnelling methyl groups

The dipolar interactions between the protons and the central C-13 nucleus of a (CH3)-C-13 group are used to study rotational tunnelling and incoherent dynamics of such groups in molecular solids. Single-crystal C-13 NMR spectra are derived for arbitrary values of the tunnel frequency nu(t). Similarities to ESR and H-2 NMR are pointed out. The method is applied to three different materials. In the hydroquinone/acetonitrile clathrate, the unique features in the C-13 NMR spectra which arise from tunnelling with a tunnel frequency that is much larger than the dipolar coupling between the methyl protons and the C-13 nucleus are demonstrated, and the effects of incoherent dynamics are studied. The broadening of the C-13 resonances is related to the width of the quasi-elastic line in neutron scattering. Selective magnetization transfer experiments for studying slow incoherent dynamics are proposed. For the strongly hindered methyl groups of L-alanine, an upper limit for nu(t) is derived from the C-13 NMR spectrum. In aspirin(TM) (acetylsalicylic acid), incoherent reorientations dominate the spectra down to the lowest temperatures studied; their rate apparently increases with decreasing temperature below 25 K.