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 found in crystalline gas clathrate structures sI and sII respectively). Such structuring would allow icosahedral-like network structuring around larger guest molecules as the cavities have radii 4.33 Å and 4.68 Å (compare pentagonal dodecahedral cavity (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 both 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 lack of formation of other clathrate structuring as suggested by some authors.

network of the 392-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.

5¹²6² cavity

5¹²6⁴ cavity

For interactive Figures, see Chime.

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 (only the oxygen atoms of water are shown.). A unit cell of such a structure is shown above. It is not known whether such a structure actually 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 Chime.

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]