E. V. Grachev

Clathrate formation in water-noble gas (Hydrogen) systems at high pressures

Phase equilibria in helium-water, neon-water, and hxdrogen-water svstems were studied at pressures up to 15 kbar. The results are compared with the data for the previously investigated water systems with argon, crypton, and xenon. It is concluded that classical polyhedral clathrate hydrates are formed in all the systems, the stability of the hydrates diminishing from xenon to neon. In all the systems, except the xenon system, the hydrates are based on the crystalline framework of ice II. Their formation demands high pressures; the larger the guest molecule, the higher the pressure required. The xenon molecule seems to be too large to fit the cage of the ice II framework; therefore, the xenon hydrate CS-I remains stable up to at least 15 kbar.

Phase formation and structure of high-pressure gas hydrates and modeling of tetrahedral frameworks with uniform polyhedral cavities

During the last decade, a number of high-pressure gas hydrates have been prepared and structurally characterized. One of the most interesting results obtained is the formation of compounds having tetrahedral water frameworks with uniform polyhedral cavities (e.g., clathrates THF·6H2O and 2Ar·6H2O), unprecedented among ambient pressure hydrates. In order to predict, reveal and perform effective investigations of such gas hydrates at high pressures it is plausible to know all possible tetrahedral frameworks of this kind. The solution of this problem demands to elaborate methods of a topological design of the tetrahedral frameworks represented as space-filling packings of uniform polyhedra with trivalent vertices (simple polyhedra). There are two related approaches to solve this problem: tiling of 3D space into symmetrically equal polyhedra (stereohedra) and generation of periodic four-connected nets. At present, the complete set of tetrahedral frameworks built of uniform simple 14-hedra (23 packings) was obtained and more than 800 frameworks were constructed from larger simple stereohedra. This article gives a discussion of structural and energetic characteristics of the frameworks generated, as well as the possibilities of using these results for interpretation of experimental structural data and a deliberate synthesis of clathrate compounds possessing new structures. Experimental and theoretical data show a high probability of finding a whole series of novel high-pressure gas hydrates with the tetrahedral water frameworks built of simple stereohedra. Of these, the most probable structures are those with 14-hedral cavities, these structures having the stoichiometry of six water molecules per cavity. In our opinion, the formation of such hydrates could be expected for guest molecules with van der Waals diameters from 5.8 to 7.2 Å. However, these hydrates cannot be excluded for substantially smaller guests as well, provided that the water framework cavities include two guest molecules. Packing polymorphism (different space filling arrangements of the same stereohedron) revealed in the search for tilings of 3D space into stereohedra offers experimental discovery of this phenomenon, which can be promoted by such delicate effects as the guest-guest interaction. The set of the derived water frameworks as stereohedra space-fillings gives opportunities to select starting structure models in the course of a diffraction study of polycrystalline high-pressure gas hydrate samples.

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.

Crystal structure of the dinitratotetrakis(pyridine)nickel(II) clathrate with pyridine as guest, [NiPy4(NO3)2]·2Py

The structure of the [NiPy4(NO3)2]·2Py clathrate was determined by single crystal X-ray diffraction analysis (KM-4 diffractometer, λMoKa radiation, graphite monochromator, ω/2θ scan mode, θmax=30°, 640 reflections, R=0.065). Orthorthombic base-centered unit cell, space group Ccca, a=12.148(5), b=15.019(4), c=16.96(1) Å, Z=4C30H30N8NiO6, dcalc=1.411 g/cm3. The structure is of island type and consists of distorted octahedral trans-[NiPy4(NO3)2] (host) and pyridine (guest) molecules. The Ni(II) cation coordinated by four nitrogens of the pyridine ligands and two oxygens of the nitrato groups lies in the special position 222; the twofold axes coincide with the directions of the metal-ligand bonds. The guest pyridine molecules lie on the twofold axes that are parallel to the c axes and are oriented in such a way that the total dipole moment of the guest subsystem is zero.

Topology of subsolidus isothermal sections of the phase diagrams of ternary reciprocal systems without continuous solid solutions

The topological properties were studied for isobaric-isothermal subsolidus sections of the phase diagrams of ternary reciprocal systems with phases of constant compositions or limited solid solutions. Systems with the triangulation schemes of diagonal and adiagonal types were analyzed. Relations between the topological parameters of graphs corresponding to such schemes were found. Algorithms for solving the problems of generation and enumeration of these triangulation schemes were presented. A wide diversity of types of triangulations was demonstrated. Their classification was proposed.

Crystal structure of a 1∶2∶1 packing complex formed by bis(isothiocyanato) tetrakis(pyridine) nickel(II), triphenylmethane, and metanol

The structure of the [NiPy4(NCS)2]·2(C6H5)3CH·CH3OH (Py is pyridine) packing complex was determined by single crystal X-ray diffraction analysis (KM-4 diffractometer, λMoKα, graphite monochromator, ω/2ϑ scan mode, θmax, 2073 reflections, R=0.059). The unit cell is base-centered monoclinic, space group C2/c, a=20.22(1), b=14.256(7), c=18.836(6) Å, β=87.40(4)°, Z=4C61H56N6NiOS2, dcalc=1.239 g/cm3. The structure is of the island type and consists of molecules of three types. In the trans-[NiPy4(NCS)2] molecule, the central atom has a distorted octahedral environment and is coordinated by six nitrogen atoms of two isothiocyanate and four pyridine ligands. In contrast to the phase of the complex itself, this molecule acquires a four-lobe propeller conformation, in which the pyridine rings are rotated relative to the equatorial plane by angles of 52.8(3) and 50.1(2)°. The triphenylmethane molecule has a three-lobe propeller conformation, as in triphenylmethane clathrates with benzene, thiophene, pyrrole, and aniline, but strongly deviates from threefold axis symmetry. The phenyl rings form dihedral angles of 25(3), 40(4), and 19(4)° with the planes formed by the bonds of the ternary carbon atoms with hydrogen atoms and by the appropriate carbon atom of the phenyl ring. The methanol molecules are located inside the direct channels running along the c axis and are disordered around the inversion centers.

Clathrate forming ability of some [MgA4X2] complexes where A=4-methylpyridine or pyridine and X=halogen or NCS

Three new compounds have been synthesized: [Mg(4-MePy)4(NCS)2]·0.67(4-MePy)·xH2O (x=0.07–0.24 [Mg(4-MePy)4(NCS)2]·(4-MePy) and [MgPy4(NCS)2]·2Py·2H2O. Their structures have been investigated by Raman spectroscopy and the first two compounds have been shown to be isostructural with the clathrates [Cu(4-MePy)4(NCS)2]·0.67(4-MePy)·0.33H2O and [Ni(4-MePy)4(NCS)2]·(4-MePy). Thermal and other properties have been studied. It was concluded from an analysis of the literature that pyridine complexes of Mg and Ca are capable of forming clathrates with Py as guest molecule, although these compounds have not been examined before from the point of view of clathrate chemistry.

Heterogeneous Nucleation of Methane Hydratein a Water–Decane–Methane Emulsion

Heterogeneous nucleation in disperse systems with metastable disperse phases plays an important role in the mechanisms of environmental and technological processes. The effect the concentration and activity of particles that initiate the formation of a new phase have on nucleation processes in such systems is considered. An approach is proposed that allows construction of a spectrum of particle activity characterizing the features of nucleation in a sample, based on the fraction of crystallized droplets depending on the level of supercooling and the use of Weibull’s distribution. The proposed method is used to describe experimental data on the heterogeneous nucleation of methane hydrate in an emulsion in a water–decane-methane system.

Partition of the composition tetrahedron during construction of subsolidus isobaric-isothermal sections of the phase diagrams of quaternary systems

A problem of constructing subsolidus isobaric-isothermal sections of phase diagrams with constant-composition compounds and limited solid solutions was considered. Relations between the topological characteristics of such sections were derived. Their topological classification was proposed. An algorithm for constructing sections using the theory of finite graphs was developed. A problem of enumerating sections for diagrams with a given set of classification characteristics was solved.

Trialing of the topological types of phase diagrams for unary systems

The topological isomorphism of polyhedra with tervalent vertices and phase diagrams of unary systems allows us to reduce the problem of constructing the full set of topological types of diagrams with tailored parameters to the problem of generating labeled graphs. Each of them is the Schlegel projection of some polyhedron onto one of its faces. This work concerns the problem of listing possible topological types of phase diagrams of unary systems. A great variety of the types of these diagrams is demonstrated.