Neutron scattering measurements and computation of the quantum dynamics of hydrogen molecules trapped in the small and large cages of clathrate hydrates

We report inelastic neutron scattering (INS) measurements on molecular hydrogen trapped in simple (D₂O) and binary (D₂O plus perdeuterated tetrahydrofuran) clathrate hydrates, performed at a low temperature using two different neutron spectrometers to probe both energy and momentum transfer. The INS spectra of binary clathrate samples exhibit a rich structure containing sharp bands arising from both the rotational transitions and the rattling modes of the guest H₂ molecule. They agree well with the rigorous fully quantum simulations, which account for the subtle effects of the anisotropy, angular and radial, of the host cage on the H₂ microscopic dynamics and the resulting spectra. The simple clathrate samples present a much greater challenge, due to the multiple H₂ occupancy of the large cages, which makes the quantum calculations an extremely difficult task. In addition, we discuss in detail various physical aspects of the experimental and simulated INS spectra, such as their temperature dependence, the effects of the cage geometry, and the different features associated with the ortho-hydrogen and para-hydrogen species.