Inelastic neutron scattering study of methyl groups rotation in some methylxanthines

The three isomeric dimethylxanthines and trimethylxanthine are studied by neutron spectroscopy up to energy transfers of 100 meV at energy resolutions ranging from 0.7 microeV to some meV. The loss of elastic intensity with increasing temperature can be modeled by quasielastic methyl rotation. The number of inequivalent methyl groups is in agreement with those of the room temperature crystal structures. Activation energies are obtained. In the case of theophylline, a doublet tunneling band is observed at 15.1 and 17.5 microeV. In theobromine, a single tunneling band at 0.3 microeV is found. Orientational disorder in caffeine leads to a 2.7 microeV broad distribution of tunneling bands around the elastic line. At the same time, broad low energy phonon spectra characterize an orientational glassy state with weak methyl rotational potentials. Librational energies of the dimethylxanthines are clearly seen in the phonon densities of states. Rotational potentials can be derived which explain consistently all observables. While their symmetry in general is threefold, theophylline shows a close to sixfold potential reflecting a mirror symmetry.