Quantum confinement of hydrogen in ice based clathrates

In general clathrate hydrates are inclusion compounds, formed by a network of hydrogen-bonded water molecules that is stabilized by the presence of foreign (generally hydrophobic) molecules, hosted in cages of different forms present in the structure. We have studied the effects of the confinement dimensions on the dynamics of the molecular hydrogen, confined into water cages of 5 Å, and about 7 Å dimension. In both cages we observe translational modes and rotational transitions of the hydrogen molecules. In the small cage, however, a new additional feature sets in around 2.4 THz. The analysis of the data complemented by molecular dynamic simulations leads to the conclusion that the nature of this feature is a new type of hybrid motion caused by strong coupling of the quantum rotational and translational degrees of freedom of the hydrogen molecules.