Title : Features of cluster structures of supercooled water and methanol
Abstract:
In the gas phase at low pressures, water is a collection of isolated molecules. The vibrational spectrum of such water consists of one deformation and two stretching vibrations. In the liquid phase, the vibrational spectrum of water becomes much more complicated. This effect is due to the fact that the approach of molecules in a liquid leads to the formation of molecular clusters. The vibrational bands inherent in the gas phase are split and shifted to the low-frequency region. Instead of narrow lines in the gas phase, wide bands are fixed in liquids. The low-frequency shift of such vibrational bands increases with the number of molecules in the cluster.
At normal external pressure, as the temperature decreases, monomers, dimers, trimers, tetramers, and a small number of clusters with a large number of molecules in them are fixed in the structure of liquid water. At temperatures below 273 degrees Kelvin, pentamers begin to predominate in the structure of water and hexamers appear. The symmetry of pentamers suggests the presence of a 5th order symmetry axis. However, such structural elements cannot be part of crystalline structures. For the formation of a crystalline structure in water, it is necessary to further decrease its temperature and the predominance of clusters in its structure, consisting of six molecules - hexamers.
In the temperature range in which the predominance of pentamers in the structure of water changes to the predominance of hexamers, crystalline water cannot form. The water is in a supercooled state. It is interesting to note that the supercooled state of water is observed only when it is cooled from the liquid phase. When ice is heated, water passes from a crystalline state to a liquid state without intermediate stages. Similar processes are observed in methanol.
Audience Take Away:
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