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Water cluster

Water cluster

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In chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

 a water cluster is a discrete hydrogen bond
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...

ed assembly or cluster
Cluster chemistry
In chemistry, a cluster is an ensemble of bound atoms intermediate in size between a molecule and a bulk solid. Clusters exist of diverse stoichiometries and nuclearities. For example, carbon and boron atoms form fullerene and borane clusters, respectively. Transition metals and main group...

 of molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s of water. These clusters have been found experimentally or predicted in silico
In silico
In silico is an expression used to mean "performed on computer or via computer simulation." The phrase was coined in 1989 as an analogy to the Latin phrases in vivo and in vitro which are commonly used in biology and refer to experiments done in living organisms and outside of living organisms,...

 in various forms of water; in ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...

, in crystal lattices and in bulk liquid water, the simplest one being the water dimer
Water dimer
The water dimer consists of two water molecules loosely bound by a hydrogen bond. It is the smallest water cluster. Because it is the simplest model system for studying hydrogen bonding in water, it has been the target of so many theoretical studies that it has been called "a theoretical Guinea...

 (H2O)2 . Ongoing academic research is important because the realization that water manifests itself as clusters rather than an isotropic collection may help explain many anomalous water characteristics such as its highly unusual density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 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
Unsolved problems in chemistry
Unsolved problems in chemistry tend to be questions of the kind "Can we make X chemical compound?", "Can we analyse it?", "Can we purify it?" and are commonly solved rather quickly, but may just as well require considerable efforts to be solved. However, there are also some questions with deeper...

.

Theoretical Studies (In silico structures)


In silico (see: water model
Water model
In computational chemistry, classical water models are used for the simulation of water clusters, liquid water, and aqueous solutions with explicit solvent. These models use the approximations of molecular mechanics...

s), cyclic water clusters (H2O)n are found with n = 3 to 60. With increasing cluster size the oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 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
Fullerene
A fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and they resemble the balls used in association football. Cylindrical ones are called carbon nanotubes or buckytubes...

-like cluster W28 is called bucky water and even for a 280 water molecule monster icosahedral
Icosahedron
In geometry, an icosahedron is a regular polyhedron with 20 identical equilateral triangular faces, 30 edges and 12 vertices. It is one of the five Platonic solids....

 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.

Experimental structures


The experimental observation of water clusters requires sophisticated spectroscopic tools such as Far-infrared (FIR) vibration-rotation-tunneling (VRT) spectroscopy (a infrared spectroscopy
Infrared spectroscopy
Infrared spectroscopy is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic...

 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 spectrometry
Mass spectrometry
Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...

 reveal cubic configurations for clusters in the range W8-W10.

When the water is part of a crystal structure as in a hydrate
Hydrate
Hydrate is a term used in inorganic chemistry and organic chemistry to indicate that a substance contains water. The chemical state of the water varies widely between hydrates, some of which were so labeled before their chemical structure was understood....

, x-ray diffraction can be used. In a recent study the conformation of a water heptamer was determined (cyclic twisted nonplanar) using this method

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.

Bulk water models


According to the so-called in silico
In silico
In silico is an expression used to mean "performed on computer or via computer simulation." The phrase was coined in 1989 as an analogy to the Latin phrases in vivo and in vitro which are commonly used in biology and refer to experiments done in living organisms and outside of living organisms,...

 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
Triple point
In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases of that substance coexist in thermodynamic equilibrium...

 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
Chemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...

 exists between a cubic water octamer and two cyclic tetramers. However, in spite of much model-making, no model yet has reproduced the experimentally-observed density maximum.

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