Dynamics of hydrogen guests in ice XVII nanopores

The present high-resolution inelastic neutron scattering experiment on ice XVII, containing molecular hydrogen with a different ortho/para ratio, allows one to assign the ${\mathrm{H}}₂$ motion spectral bands to rotational and center-of-mass translational transitions of either para- or ortho-${\mathrm{H}}₂$. Due to its structure, ice XVII confines ${\mathrm{H}}₂$ molecules to move in spiral channels of molecular size. Reported data demonstrate that ${\mathrm{H}}₂$ molecules rotate almost freely in these nanometric channels, though showing larger perturbation than in clathrate hydrates, and perform a translational motion exhibiting two low-frequency excitations. The agreement between the experimental spectra and the corresponding molecular dynamics results clearly enables one to portray a picture of the confined motions of a hydrophobic guest within a metastable ice framework, i.e., ice XVII.