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 being the water dimer (H2O)2 . Shu et al. reported the images of water clusters of 100 micrometres. 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 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 (see: water models), cyclic water clusters (H2O)n are found with n = 3 to 60. Structures of water molecules with the highest resolution have been demonstrated in the studies of Richard Saykally of Berkeley College of Chemistry. 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 (H2O)28 is called the water buckyball 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. The 280 molecule icosahedral structure, which is 3 nm in diameter, consists of icosahedral shells with 280, 100 and 20 molecules (the 100 molecule structure is shown the figure above). There is increased stability with the addition of each shell. A look at the recent scientific literature may reveal good reviews on the studies of water clusters employing ab initio methods. These clusters are also important for studying hydration phenomena at molecular level since they form the basic building blocks of the hydrated clusters. There are theoretical models of water clusters of more than 700 water molecules by Martin Chaplin and Stanislav Zenin. They have not been proven experimentally.
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