The isotope effect and orientational potentials of methane molecules in gas hydrates

We report an inelastic neutron scattering experiment at T=2 K of a fully de uterated methane hydrate CD4.5.75 D2O. In the experiment the J=0 -->J=1 rot ational transition of an almost free quantum CD4 rotor at an energy transfe r of 0.4 meV has been found. No line splitting due to the two different kin ds of cages, which are occupied by the methane molecules in the host lattic e, has been found. Using a simple electrostatic model of the orientational interaction between the methane molecules and the water molecules in the ho st lattice we are able to explain the measured spectra of both the deuterat ed and protonated samples. The intrinsic linewidth found for the rotational transition in deuterated and protonated samples can be attributed to the f rozen in disorder of the water dipole moments in the cages. The contributio n of the van der Waals interaction has also been calculated on the basis of empirical atom-atom interaction parameters and has been found to be one or der of magnitude smaller than the electrostatic interaction. In addition th e calculated orientational scattering length density of the CD4 molecules i s found to be in good agreement with experimental values. It is shown, that previous diffraction experiments lead to only limited information about th e orientational distribution function due to the proton disorder. (C) 2001 American Institute of Physics.