The properties of some forms of water ice reserve still intriguing surprises. Besides the several stable or metastable phases of pure ice, solid mixtures of water with gases are precursors of other ices, as in some cases they may be emptied, leaving a metastable hydrogen-bound water structure. We present here the first characterization of a new form of ice, obtained from the crystalline solid compound of water and molecular hydrogen called C0-structure filled ice. By means of Raman spectroscopy, we measure the hydrogen release at different temperatures and succeed in rapidly removing all the hydrogen molecules, obtaining a new form of ice (ice XVII). Its structure is determined by means of neutron diffraction measurements. Of paramount interest is that the emptied crystal can adsorb again hydrogen and release it repeatedly, showing a temperature-dependent hysteresis.
We have performed high-resolution inelastic neutron scattering studies on
three samples of hydrogenated tetrahydrofuran-water clathrates, containing
either H2 at different para/ortho concentrtion, 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 nanometric-size cage, resulting a paradigmatic example of
quantum dynamics in a non-harmonic 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 a very good
agreement
The Raman spectrum of hydrogen clathrate hydrates has been measured, as a function of temperature, down to 20 K. Rotational bands of H(2) and HD, trapped into the small cages of simple (H(2)O-H(2)) and binary (H(2)O-THF-H(2)) hydrates, have been analyzed and the fivefold degeneracy of the molecular J=2 rotational level has been discussed in the light of the available theoretical calculations. The vibrational frequencies of H(2) molecules encapsulated in the large cages of simple hydrates turn out to be well separated from those pertaining to the small cages. Comparison with the equivalent D(2) spectra allowed us to assign the large cavity vibrational frequencies to three couples of Q(1)(1)-Q(1)(0) H(2) vibrational modes. Populations of ortho and para species have been measured as a function of time from rotational spectra and the rate of ortho-para conversion has been estimated for both simple and binary hydrates. We suggest, using the H(2) vibrational spectra, a model to analyze the cage population in simple hydrates.
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