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

Abstract

Methods are discussed for estimating the energy of formation of a clathrate network from ice Ih based on the structural data for clathrate and semiclathrate 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 semiclathrate hydrates, two versions of estimation are suggested-Estimations for 7 hydrates of amines and quaternary ammonium base salts showed that formation of semiclathrate hydrates requires much energy for network deformation. Two methods are described for estimating the network energies according 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 of polyhedron units forming the lattice; it is applicable to the most widespread class of networks constructed from Allen’s polyhedra. The energies of the tetragonal and orthorhombic networks belonging to this class are estimated. A relative stability series involving 9 structures of empty undistorted networks is formed.