Water cluster

In chemistry a water cluster is a discrete hydrogen bonded assembly or cluster of molecules of water [cite journal | title = Water: From Clusters to the Bulk | author = Ralf Ludwig | journal = Angew. Chem. Int. Ed. | year = 2001 | volume = 40 | pages = 1808–1827 | doi = 10.1002/1521-3773(20010518)40:10<1808::AID-ANIE1808>3.0.CO;2-1] . These clusters have been found experimentally or predicted in silico in various forms of water; in ice, in crystal lattices and in bulk liquid water, the simplest one the water dimer (W₂). Ongoing academic research is important because the realization that water manifests itself as clusters rather than a isotropic collection may help explain many anomalous water characteristics such as its highly unusual density temperature dependence. Water clusters are also implicated in the stabilization of certain supramolecular structures. So little is understood about water clusters in bulk water that it is considered one of the unsolved problems in chemistry.

Theoretical Studies (In silico structures)

In silico (see: water models), cyclic water clusters (H₂O)_n are found with n = 3 to 60. With increasing cluster size the oxygen to oxygen distance is found to decrease which is attributed to so-called cooperative many-body interactions: due to a change in charge distribution the H-acceptor molecule becomes a better H-donor molecule with each expansion of the water assembly. Many isomeric forms seem to exist for the hexamer: from ring, book, bag, cage, to prism shape with nearly identical energy. Two cage-like isomers exist for heptamers and octamers are found either cyclic or in the shape of a cube. Even larger clusters are predicted: the fullerene-like cluster W₂₈ is called bucky water and even for a 280 water molecule monster icosahedral network (with each water molecule coordinate to 4 others) there is found a local energy minimum. A look at the recent scientific literature may reveal good reviews on the studies of water clusters employing "ab initio" methods. [cite journal | title = Structure and Stability of Water Clusters (H₂O)_n, n = 8-20: An "Ab Initio" Investigation | author = S. Maheshwary, N. Patel, N Sathyamurthy, A. D. Kulkarni, S. R. Gadre | journal = J. Phys. Chem. a | volume = 105 | pages = 10525 | year = 2001 | doi = 10.1021/jp013141b] [cite journal | title = High-level ab initio calculations for the four low-lying families of minima of (H₂O)₂₀. II. Spectroscopic signatures of the dodecahedron, fused cubes, face-sharing pentagonal prisms, and edge-sharing pentagonal prisms hydrogen bonding networks | author = G. S. Fanourgakis, E. Aprà, W. A. de Jong, S. S. Xantheas | journal = J. Chem. Phys. | volume = 122 | pages = 134304 | year = 2005 | doi = 10.1063/1.1864892]

Experimental structures

The experimental observation [cite journal | author = C. J. Gruenloh, J. R. Carney, C. A. Arrington, T. S. Zwier, S. Y. Fredericks, K. D. Jordan | journal = Science | volume = 276 | pages = 1678 | year = 1997 | doi = 10.1126/science.276.5319.1678 | title = Infrared Spectrum of a Molecular Ice Cube: The S4 and D2d Water Octamers in Benzene-(Water)8] [cite journal | author = M. R. Viant, J. D. Cruzan, D. D. Lucas, M. G. Brown, K. Liu, R. J. Saykally | title = Pseudorotation in Water Trimer Isotopomers Using Terahertz Laser Spectroscopy | journal = J. Phys. Chem. a | volume = 101 | pages = 9032 | year = 1997 | doi = 10.1021/jp970783j] of water clusters requires sophisticated spectroscopic tools such as "Far-infrared (FIR) vibration-rotation-tunneling (VRT) spectroscopy" (a infrared spectroscopy technique). With water trapped in a liquid helium environment the hexamer is found to be a cyclic planar assembly but in the gas-phase the cage is found and in an organic host (water trapped in the crystal lattice of an organic compound) a conformation reminiscent of a cyclohexane chair conformation. Experiments combining IR spectroscopy with mass spectroscopy reveal cubic configurations for clusters in the range W₈-W₁₀.

When the water is part of a crystal structure as in a hydrate, x-ray diffraction can be used. In a recent study the conformation of a water heptamer was determined (cyclic twisted nonplanar) using this method [cite journal | title = Phase Transition Accompanied by Transformation of an Elusive Discrete Cyclic Water Heptamer to a Bicyclic (H2O)7 Cluster | author = M. H. Mir, J. J. Vittal | journal = Angew. Chem. Int. Ed. | year = 2007 | volume = 46 | pages = 5925 –5928 | doi = 10.1002/anie.200701779]

Bulk water models

According to the so-called in silico method "quantum cluster equilibrium" (QCE) theory of liquids W₈ clusters dominate the liquid water bulk phase followed by W₅ and W₆ clusters. In order to facilitate a water triple point the presence of a W₂₄ cluster is invoked. In another model bulk water is built up from a mixture of hexamer and pentamer rings containing cavities capable of enclosing small solutes. In yet another model an equilibrium exists between a cubic water octamer and two cyclic tetramers. However, in spite of much model making all models are unable to reproduce the experimentally observed density maximum.

References

External links

* Water clusters at London South Bank University [http://www.lsbu.ac.uk/water/abstrct.html Link]
* [http://www-wales.ch.cam.ac.uk/CCD.html The Cambridge Cluster Database] - Includes water clusters calculated with various water models and the [http://www-wales.ch.cam.ac.uk/~wales/CCD/anant-watcl.html water clusters explored with "ab initio" methods] .