V. Yu. Komarov

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.

Single crystal X-ray diffraction study of the cubic ionic clathrate hydrate of tetrabutyl ammonium propionate (C4H9)4NC2H5COO·27.0H2O

Single crystal X-ray diffraction is used for the first time to study the structure of the ionic clathrate hydrate of tetrabutyl ammonium propionate (C4H9)4NC2H5COO·27.0H2O with the host framework based on cubic arrangement I. The structural model is characterized by a high degree of disordering in both guest and host subsystems. The structural features indicate a colocation of cations and anions in multicell 4T·nD cavities with n = 0–4; the simplest option is a pairwise location of cations and anions in combined five-cell 4T·D cavities. The positional ordering of cations leads to the formation of a 2×2×2 supercell (space group

New red-luminescent cadmium coordination polymers with 4-amino-2,1,3-benzothiadiazole

I\bar 43d

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

, a = 24.312(6)Å). The resulting model confirms the characteristic features of ionic clathrate hydrates of tetrabutyl and tetraisoamyl ammonium carboxylates. Non-standard refinement techniques are described, which may be of interest for other structures with a high degree of disordering. The proposed structural interpretation can serve as a basis for a further investigation of this class of compounds by other methods.

Trialing of the topological types of phase diagrams for unary systems

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.

Influence of water on the structure and magnetic properties of a copper bromide coordination compound with 3-amino-4-ethoxycarbonylpyrazole

Abstract New polymeric cadmium complexes, α-[CdLCl2]n (1), [CdL2Cl2]n (2) and β-[CdLCl2]n (3) (L = 4-amino-2,1,3-benzothiadiazole), were obtained as products of the reaction of CdCl2 with L. The synthetic procedures allowing isolation of pure 1–3 were optimized. The structures of 1–3 were established by single-crystal X-ray diffraction and the compounds were characterized by UV–Vis and IR spectroscopy. In these compounds, L is either μ-bridging (1) or terminal (2 and 3). The UV–Vis spectra of the complexes in the solid state resemble that of free L. However, coordination of L leads to a significant shift of emission in photoluminescence spectra from yellow (free L) to red (1–3).

[ZnEn 3 ]CrO 4 as a precursor of zinc chromite spinel

Coordination compounds based on copper(II) halides with 4H-1,2,4-triazolo[1,5-a]benzimidazole (L1), 3-methyl-1,2,4-triazolo[1,5-a]benzimidazole (L2), and 4-methyl-1,2,4-triazolo[1,5-a]benzimidazole (L3) with the compositions [Cu(L1)2(Hal)2]·nH2O (Hal = Cl−, n = 0.5 (I) Br−, n = 1 (II)); [Cu(L2)2Cl2] (III), [Cu(L3)2Cl2] (IV) and [Cu(L3)2Br2(H2O)]·2H2O (V) were synthesized. The compounds were characterized using X-ray diffraction, UV-Vis spectroscopy (diffuse reflectance spectra), infrared spectroscopy, and static magnetic susceptibility measurements. By means of X-ray diffraction structural analysis, crystal structures were determined for the [Cu(L3)2(Hal)2(H2O)]·2H2O complexes, where Hal = Cl− (IVa) or Br− (V). Cytotoxic effects of the complexes and ligands on the cell line Hep-2 were then studied, and we found that the copper(II) complex with ligands L1–L3 leads to a considerable enhancement of cytotoxicity.