Quantum rattling of molecular hydrogen in clathrate hydrate nanocavities

We have performed high-resolution inelastic neutron scattering studies on three samples of hydrogenated tetrahydrofuran-water clathrates, containing either H2 at different para and ortho concentrations, or HD. By a refined analysis of the data, we are able to assign the spectral bands to rotational and center-of-mass translational transitions of either para- or ortho-H2. The H2 molecule rotates almost freely, while performing a translational motion rattling in the nanometer-size cage, resulting a paradigmatic example of quantum dynamics in a nonharmonic potential well. Both the H2 rotational transition and the fundamental of the rattling transition split into triplets, having different separation. The splitting is a consequence of a substantial anisotropy of the environment with respect to the orientation of the molecule in the cage, in the first case, or with respect to the center-of-mass position inside the cage, in the second case. The values of the transition frequencies and band intensities have been quantitatively related to the details of the interaction potential between H2 and the water molecules, with very good agreement.