Ice-seventeen (Ice XVII)

ice-XVII, from [2796] — Ice-XVII crystal structure,

from [2796]

Molecular hydrogen (H₂) forms several different clathrates as the H₂ molecules can fit in most cages with multiple H₂ in the larger cages (see elsewhere). At high pressures (≈ 400 MPa, 280 K), a different structure is formed (intercalated filled ice rather than a clathrate) with the composition (H₂O)₂H₂ and consisting of interpenetrating spiral chains of water molecules showing topological similarity to the mineral quartz [2773]. All the hydrogen can be released by warming under vacuum for about one hour at a temperature of 110-120 K to give a new metastable hexagonal ice structure with space group P6₁22 with cell dimensions 6.3305 Å, 6.3305 Å, 6.0580 (a, b, c; 90°, 90°, 120°, 6 molecules, 75 K). The hydrogen bonding is random, following the 'ice rules'.

Above 130 K, the sample transforms into ice Ih. It is microporous due to its hexagonal helical pores (with 6.10 Å. channel diameter from the center of the oxygen atoms leaving a space of ≈ 3 Å minimum diameter) and will re-adsorb molecular H₂(≈ 1.8 Å diameter) if exposed to it. The structure of the heavy water version of this ice has been determined by neutron diffraction [2796].

Ice XVII consists of sheets of pentamers of hydrogen-bonded water molecules made up from (H₂O)₁₂ partial dodecahedra (see left, the xz-plane down the y-axis) that form hexagonal helices down the z-axis (see above right). This xz sheet-like structure is also formed in the [CoII(cyclam)Cl₂].3H₂O (cyclam =1,4,8,11-tetraazacyclotetradecane) clathrate [1871 ].

Confinement of Ne atoms or O₂ molecules inside the spiral hexagonal channels of the deuterated ice XVII is possible at mild pressure (~100 kPa) and low temperature (~40 K). These are filled ices rather than clathrates. The molar ratios are H₂O:Ne = 6 mol:1.27 mol (unit cell) and H₂O:O₂ = 6 mol:1.36 mol (unit cell) [3632]. Molecular dynamics simulations have found CO₂-filled ice XVII to be more stable than the CO₂-filled sI clathrate at pressures ranging from 0.6 GPa to 1.8 GPa and up to about 308 K [3749]. Ice XVII may be used as a cheap, useful, and environmentally friendly, microporous material to store CO₂ as (H₂O)_3.5CO₂. [3059]. Ice XVII can also be filled with N₂ forming an sX clathrate hydrate, (H₂O)_2.6N₂ [3750].

H₂ molecules rotate almost freely in these narrow channels [3172]. Confinement of different mixtures of ortho-H₂ and para-H₂ molecules within the channels allows the assignment of the H₂ motion spectral bands to rotational and center-of-mass translational transitions of either para-H₂ or ortho-H₂[2796].

D₂O ice XVII may be converted to pure D₂O cubic ice without stacking defectsunder dynamic vacuum to remove the contained H₂ [3857 ].

For an interactive Figure, see Jmol.

[Back to Top ]