Water cluster

In chemistry a water cluster is a discrete hydrogen bonded assembly or cluster of molecules of water. 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.

In silico structures
In silico, cyclic water clusters (H2O)n are found with n = 3 to 6. 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.

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 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

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.