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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 336-molecule cluster

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.

network of the 24-molecule cluster

              51262 cavity

network of the 28-molecule cluster

               51264 cavity

 

For interactive Figures, see Jmol.

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 right. 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 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]

 

 

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This page was established in 2004 and last updated by Martin Chaplin on 20 July, 2015


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