Yuri A. Dyadin

The Structure of Tetrabutylammonium Bromide Hydrate (C4H9)4NBr·21/3H2O

Abstract The results of the investigation using single crystal X-Ray diffraction analysis of tetrabutylammonium bromide hydrate structure (C4H9)4NBr·21/3H2O, which was discovered in the tetrabutylammonium bromide–water binary system within the range of concentrated solutions, are presented in this communication. The (C4H9)4NBr·21/3H2O compound is crystallized in the trigonal space group R3c, a=16.609(1) A, c=38.853(2) A. The structure is a packing of tetrabutylammonium cations and clusters composed of hydrogen-bonded water molecules and bromide anions. The change of water functions is discussed within one binary system tetrabutylammonium bromide–water. In the clathrate formation region water plays the role of the host, forming the water–anion host framework in clathrate polyhydrates, while in the region of concentrated solutions, in the case of (C4H9)4NBr·21/3H2O compound, water functions as a guest, forming water-anion guest particle.

Thermodynamic Dissociation Constants for [MPy4(NO3)2]*2Py Clathrates (M=Mn, Co, Ni, Cu)

Abstract Stoichiometry and thermodynamic parameters of the title clathrates dissociation have been studied with thermoanalytical and strain method techniques. The [MPy4(NO3)2]*2Py (M = Mn, Co, Ni) clathrates dissociate with collapsing clathrate porous phase and destruction of the host complex to give the respective tripyridine complexes and gaseous pyridine. The [CuPy4(NO3)2]*2Py dissociates with collapsing clathrate phase but giving the host [CuPy4(NO3)2] complex as individual phase, with the tripyridine complex forming in further course of decomposition. The comparison of the thermodynamic dissociation parameters for the [MPy4(NO3)2]*2Py series with M = Mn, Co, Ni, Cu, Zn and Cd shows that the differences in the stability of the compounds do not correlate with structural parameters of the clathrates but depend on the nature of the metal cation in the host complex. Thermodynamic stability of these clathrate phases follows the general sequence of stabilty for complexes of the 3d transition metals known as Irwing-Williams sequence: MnZn. These results disclose the main issue of instability of the [MPy4(NO3)2]*2Py clathrates as instability of the respective host complexes.

On the stoichiometry of clathrates

Abstract There are two aspects to this problem: stoichiometry determined by the framework structure and changes in composition of clathrate phase due to the possibility of the partial filling of the cavities in it. In the simplest case structural stoichiometry is defined as the ratio of the number of the host molecules to the number of cavities in a unit cell but more complicated cases are not difficult to understand either.

Phase Diagrams of the Ternary Gas Hydrate Forming Systems at High Pressures. Part 1. Propane–Methane–Water System

Abstract Cross sections of the ternary system propane–methane–water at pressure up to 15 kbar have been investigated by means of differential thermal analysis. It is stated that a double gas hydrate of the cubic structure II is formed in the system. It is stable within the whole pressure range investigated. It is most likely that at pressures above 3.4 kbar one more double hydrate exists in the system. The possibility of the formation of solid solutions of methane in propane hydrates and of propane in methane hydrates existing under different P–T conditions is discussed on the basis of P–T curves corresponding to monovariant equilibria in the system.