Probing adsorption sites in a cubic II water clathrate cage by methyl group rotation of CH3I guest molecules

Methyl iodide enclosed in the 5(12)6(4) cage of cubic II water clathrate is investigated by neutron spectroscopy in the temperature regime from 1.8 to 145 K. The three tunnelling bands observed at the lowest temperature at 497, 372 and 243 mueV of relative intensities 2:3:5 represent three distinct adsorption sites of CH3I. Sites of occurrence probabilities corresponding closely to the observed line intensities are related to the three types of hydrogen bonds of the cage. The band at 372 mueV shows fast spin conversion compared to the two others. The corresponding site is facing the shortest O-H..O hydrogen bond with the fastest proton dynamics. Spin conversion may thus reflect hydrogen jumps within the H-bond network of host molecules. Jump rates of the order of 10(-4)s(-1) can be estimated. Above a temperature T similar to 45 K the methyl dynamics become classical and can be described by rotational diffusion on a circle. A second quasielastic process becomes visible at a higher temperature and is identified as head-to-tail flips of the whole molecule with a characteristic jump distance of about 5 A. Above T similar to 90 K translational freedom of the CH3I leads to a rattling mode appearing at similar to0.95 meV.