Computer Counting of the Number of Water Molecules in the First Layer of Crystalline Water Clathrate

Abstract

Abstract Semiempirical calculation of the number of water molecules in the first layer of water clathrates in crystalline state was developed by computer aided construction of hydrogen-bonded network of water molecules. In the generalized thermodynamic considerations on four possible types of “unit clathrate,” van der Waals stabilization energy, “strain” energy of the unit clathrate, and entropy term due to the motional freedom of the guest were taken into account for the calculation of free energy change for the formation of the water clathrate. The results of calculations on 37 hydrophobic organic molecules well represented the trend of the reported experimental data of X-ray studies. The present calculation also disclosed such a large unfavorable strain energy (8.72 kcal·mol−1) of pentakaidecahedron that it counteracts the van der Waals stabilization. The result is in a good agreement with the fact that pentakaidecahedral cavity is rarely seen in the gas hydrates. Most importantly, these semiempirical calculations of the energetics of water clathrates afforded valuable new insights into water assembly and may be extended to give better understandings of structured water in biological system and of water clusters that develop around apolar solutions in water.