Using structural data for estimating the stability of water networks in clathrate and semiclathrate hydrates

Methods are discussed for estimating the energy of formation of a clathrate network from ice Ih based on the structural data for clathrate and semicla thrate hydrates. The energies of real networks in clathrate hydrates and of ideal undistorted networks (8 structures) are calculated from the crystal structure data using the Zimmerman-Pimentel quasiharmonic potential. For se miclathrate hydrates, two versions of estimation are suggested. Estimations for 7 hydrates of amines and quaternary ammonium base salts showed that fo rmation of semiclathrate hydrates requires much energy for network deformat ion. Two methods are described for estimating the network energies accordin g to topological information. The first technique involves calculating the number of cycles with different numbers of sides per unit cell; this method is applicable to networks involving strained cycles with 4, 7 and 8 sides. The second method represents the lattice energy via the partial energies o f polyhedron units forming the lattice; it is applicable to the most widesp read class of network constructed from Alien's polyhedra. The energies of t he tetragonal and orthorhombic networks belonging to this class are estimat ed. A relative stability series involving 9 structures of empty undistorted networks is formed.