The dipolar interactions between the protons and the central C-13 nucleus o
f 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 spect
ra are derived for arbitrary values of the tunnel frequency nu(t). Similari
ties to ESR and H-2 NMR are pointed out. The method is applied to three dif
ferent materials. In the hydroquinone/acetonitrile clathrate, the unique fe
atures in the C-13 NMR spectra which arise from tunnelling with a tunnel fr
equency that is much larger than the dipolar coupling between the methyl pr
otons 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 t
he width of the quasi-elastic line in neutron scattering. Selective magneti
zation transfer experiments for studying slow incoherent dynamics are propo
sed. For the strongly hindered methyl groups of L-alanine, an upper limit f
or nu(t) is derived from the C-13 NMR spectrum. In aspirin(TM) (acetylsalic
ylic acid), incoherent reorientations dominate the spectra down to the lowe
st temperatures studied; their rate apparently increases with decreasing te
mperature below 25 K.