  The present high-resolution inelastic neutron scattering experiment on ice
XVII, containing molecular hydrogen with different ortho/para ratio, allows to
assign the H$_2$ motion spectral bands to rotational and center-of-mass
translational transitions of either {\it para}- or {\it ortho}-H$_2$. Due to
its structure, ice XVII confines H$_2$ molecules to move in spiral channels of
molecular size. Reported data demonstrate that 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 good agreement between experimental spectra and
corresponding molecular dynamics results clearly enables to portray a picture
of the confined motions of a hydrophobic guest within a metastable ice
framework, i.e. ice XVII.
