Methyl group rotational tunnelling in glasses: a direct comparison with the crystal

We present a neutron scattering study of the methyl group dynamics at low temperature in sodium acetate trihydrate, Na(CH3COO) · 3H2O, in crystalline and glassy state. As previously found in glassy polymers, the spectra of this low molecular weight system show an apparent quasielastic feature in the glassy state instead of the characteristic rotational tunnelling lines observed in crystals. This can be understood in terms of an asymmetric distribution of rotational tunnelling lines resulting from a Gaussian distribution of potential barriers for methyl rotation. The average barrier of this distribution is found to be notably higher than the unique barrier present in the crystal.We acknowledge the support from the projects: DGICYT, PB97-0638; GV, EX 1998-23 and PI-1998-20; UPV/EHU, 206.215-G20/98. A.J.M acknowledges a grant of the Basque Government.Peer reviewe