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In
chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...
, a water cluster is a discrete
hydrogen bond In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...
ed assembly or
cluster may refer to: Science and technology Astronomy * Cluster (spacecraft), constellation of four European Space Agency spacecraft * Asteroid cluster, a small asteroid family * Cluster II (spacecraft), a European Space Agency mission to study th ...
of molecules of water. Many such clusters have been predicted by theoretical models (
in silico In biology and other experimental sciences, an ''in silico'' experiment is one performed on computer or via computer simulation. The phrase is pseudo-Latin for 'in silicon' (correct la, in silicio), referring to silicon in computer chips. It ...
), and some have been detected experimentally in various contexts such as ice, bulk liquid water, in the gas phase, in dilute mixtures with non-polar solvents, and as water of hydration in
crystal lattice In geometry and crystallography, a Bravais lattice, named after , is an infinite array of discrete points generated by a set of discrete translation operations described in three dimensional space by : \mathbf = n_1 \mathbf_1 + n_2 \mathbf_2 + n ...
s. The simplest example is the water dimer (H2O)2. Water clusters have been proposed as an explanation for some anomalous properties of liquid water, such as its unusual variation of density with temperature. Water clusters are also implicated in the stabilization of certain supramolecular structures. They are expected to play a role also in the hydration of molecules and ions dissolved in water.


Theoretical predictions

Detailed water models predict the occurrence of water clusters, as configurations of water molecules whose total energy is a local minimum. Of particular interest are the cyclic clusters (H2O)n; these have been predicted to exist for n = 3 to 60. Research shows experimental result of the size of water cluster with 20 water molecules of 0.822 nm Research uses of graph invariants for efficiently generating hydrogen bond topologies and predicting physical properties of water clusters and ice. The utility of graph invariants is confirmed by considering two water clusters, the (H2O)6 cage and (H2O)20 dodecahedron, which, associated with roughly the same oxygen atom arrangements in solid and liquid phases of water. At low temperatures, nearly 50% of water molecules are included in clusters. 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 (H2O)6: from ring, book, bag, cage, to prism shape with nearly identical energy. Two cage-like isomers exist for heptamers (H2O)7, and octamers (H2O)8 are found either cyclic or in the shape of a cube. Other theoretical studies predict clusters with more complex three-dimensional structures. Examples include the fullerene-like cluster (H2O)28, named the ''water buckyball'', and the 280-water-molecule monster icosahedral network (with each water molecule coordinate to 4 others). The latter, which is 3 nm in diameter, consists of nested icosahedral shells with 280 and 100 molecules. There is also an augmented version with another shell of 320 molecules. There is increased stability with the addition of each shell. There are theoretical models of water clusters of more than 700 water molecules, but they have not been observed experimentally.


Experimental observations

Experimental study of any supramolecular structures in bulk water is difficult because of their short lifetime: the hydrogen bonds are continually breaking and reforming at the timescales faster than 200 femtoseconds. Nevertheless, water clusters have been observed in the gas phase and in dilute mixtures of water and non-polar solvents like benzene and liquid helium. The experimental detection and characterization of the clusters has been achieved with the following methods: far-infrared spectroscopy, far-infrared (FIR), vibration-rotation-tunneling spectroscopy, vibration-rotation-tunneling (VRT), Н-NMR, and neutron diffraction. The hexamer is found to have planar geometry in liquid helium, a chair conformation in organic solvents, and a cage structure in the gas phase. Experiments combining IR spectroscopy with
mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...
reveal cubic configurations for clusters in the range n=(8-10). When the water is part of a crystal structure as in a
hydrate In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...
,
x-ray diffraction X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
can be used. Conformation of a water heptamer was determined (cyclic twisted nonplanar) using this method. Further, multi-layered water clusters with formulae (H2O)100 trapped inside cavities of several polyoxometalate clusters were also reported by Mueller et al.


Cluster models of bulk liquid water

Several models attempt to account for the bulk properties of water by assuming that they are dominated by cluster formation within the liquid. According to the ''quantum cluster equilibrium'' (QCE) theory of liquids, n=8 clusters dominate the liquid water bulk phase, followed by n=5 and n=6 clusters. Near the triple point, the presence of an n=24 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, no model yet has reproduced the experimentally-observed density maximum of water as a function of temperature.


See also

*
Hydrogen bond In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...
* Mpemba effect * Properties of water *
Richard J. Saykally Richard James Saykally (born September 10, 1947) is an American chemist. He is currently the Class of 1932 Endowed Professor of Chemistry at the University of California, Berkeley. He has received numerous awards for his research on the molecu ...


References


External links

* Water clusters at London South Bank University
Link


- Includes water clusters calculated with various water models and th
water clusters explored with ''ab initio'' methods
{{DEFAULTSORT:Water Cluster Cluster chemistry Water chemistry