Water Cluster Architecture, Based on Gas Clathrates

As well as forming icosahedral water clusters, 14-water-molecule water tetrahedra can form large regular, and relatively undistorted,^a clusters around other clathrate cavities ('Bucky-ice' structures such as the tetrakaidecahedral (5¹²6²) and hexakaidecahedral (5¹²6⁴) cavities are found in crystalline gas clathrate structures CS-I and CS-II, respectively). Such structuring allows icosahedral-like network structuring around larger guest molecules, as the cavities have radii 4.33 Å and 4.68 Å (compare with pentagonal dodecahedral cavities (5¹²) at 3.91 Å). They are also capable of forming an infinite network, without further hydrogen-bond distortion (see below) in the same way gas hydrates form regular crystalline solids. Although both non-crystalline ES clusters and crystalline gas-containing clathrates have similar inner-shell water clustering, the topology of the outer clustering is very different. The lack of evidence for the partial formation of crystalline clathrates in liquid water containing clathrate-forming solutes cannot, therefore, be used to indicate the absence of formation of other clathrate-structuring as suggested by some authors.

The network of the 336-molecule cluster

network of the 336-molecule cluster

Twenty-four 14-molecule water tetrahedra arranged around the clathrate-I cavity, given below. Only the 336 oxygen atoms are shown.

Twenty-eight 14-molecule water tetrahedra arranged around the clathrate-II cavity, given below. Only the 392 oxygen atoms are shown.

The network of the 24-molecule cluster

5¹²6² cavity

5¹²6⁴ cavity

For interactive Figures, see Jmol.

Clathrate-I-like crystal structure containing 280-molecule water icosahedra — Clathrate-I-like crystal structure containing

280-molecule water icosahedra

The use of such clathrate cages in the formation of solid gas-hydrate ices is shown on another page.

280-molecule icosahedra plus 336-molecule tetrakaidecahedra (5¹²6²) can be arranged in a similar cubic network to give a fully tessellated structure. The oxygen atoms of water in the unit cell of such a structure are shown right. It is not known whether such a structure exists, as it does involve some distortion with standard deviations of 2.4% in the O···O nearest neighbor distances and 4.4% in the tetrahedral hydrogen-bonded angles.

For interactive Figures, see Jmol.

Footnotes

^a The hydrogen-bond angles and bond lengths are slightly more distorted (5¹²6⁴ > 5¹²6²) than in icosahedral clusters surrounding dodecahedral 5¹², cavities, but not prohibitively so. [Back]