Konstantin A. Udachin

Structure H Hydrate: A Single Crystal Diffraction Study of 2,2-dimethylpentane·5(Xe, H2S)·34H2O

Abstract Although Structure H hydrate has been studied intensively for a number of years, its detailed crystal structure has remained unknown. Single crystal X-ray diffraction was used to determine the structure of 2,2-dimethylpentane·5(Xe, H2S)·34H2O hydrate. The hexagonal unit cell, space group P6/mmm with z=1, has the following parameters: a = 12.212(2), c = 10.143(2) A (t = −100°C). Cage geometries and fractional coordinates for the atoms are presented and discussed. Structure H hydrate is only the third hydrate type for which a large number of molecules are suitable as guests.

Double clathrate hydrates with helium and hydrogen

Abstract Double cubic structure II clathrate hydrates of tetrahydrofuran (THF) with helium (space group Fd3m, a = 17.155(5) A, 120 K) or hydrogen and those of hexafluorophosphoric acid with hydrogen have been obtained. These hydrates are solid solutions due to the variable degree of the filling of the small cavities by helium and hydrogen molecules. The degree of the filling of the small cavities of the hydrate of THF by helium at 3.5 kbar is 24.5%; hydrogen can occupy 100% of the cavities, and at 7.0 kbar it is possible for two molecules to be accommodated in the small cavity.

The formation of solid solutions in the tetrahydrofuran-tetra(n-propyl)ammonium fluoride-water system

The phase diagram of the tetrahydrofuran (THF)-tetra(n-propyl)ammonium fluoride (Prp4 NF)-water system has been studied by differential thermal analysis (DTA). Solid solutions have been detected of the general formula THF·x Prp4NF·(17−2x) H2O, wherex changes from 0.06 to 0.5. The formation of solid solutions is due to the variable degree of the filling of the fragment consisting of four D-cavities in CS-II clathrate hydrates by the tetra(n-propyl)ammonium cation. The solid solution obtained is peculiar in that its vacancies are occupied by the blocks of four D-cavities each. Besides, this kind of displacement of two water molecules by an ionic pair Prp4N++F−, where only the charges of the substituting and substituted groups are equal, takes place here. It appears that this type of solid solution has been discovered for the first time.

Double Clathrate Hydrate in the (CH3)4NF-(C3H7)4NF-H2O System

Abstract Clathrate formation in the system (CH3)4NF-(C3H7)4NF-H2O was studied using the differential thermal analysis technique. The double clathrate hydrate (CH3)4NF. 0.5(C3H7)4NF·15H2O melting congruently at 21.7 °C was found in the system. The hydrate forms a hexagonal cell with a = 12.290(5), c = 59.35(1) A; D = 1.091 g·cm−3, and the packing coefficient is 0.603. Pressure increase causes the stabilization of the double hydrate.

Clathrate formation in (i-C5H11)4−k (C4H9) k NF−H2O (k=1,2,3) binary systems

Phase diagrams of some binary aqueous systems with tetraalkylammonium fluorides are examined. The size of the hydrophobic moiety of the guest is consecutively varied in the series (i-C5H11)4−k(C4H9)kNF (k=0, 1, 2, 3) by replacing bulky isoamyl radicals with n-butyl radicals. Changes in clathrate formation caused by variations of the sizes and forms of guests are analyzed in the series (i-C5H11)k−4(C4H9)kNF−H2O (k=0, 1, 2, 3, 4). All tetraisoamylammonium fluoride hydrates are more stable than other hydrates of this series. The stability of the compounds increases due to the fact that the isoamyl radicals use the host cavities more effectively than the butyl radicals. In all hydrates of the series, tetragonal structures-I (TS-I), which were earlier thought typical only for hydrates of tetrabutylammonium salts, are formed. Hydrates of the orthorhombic system are formed until three isoamyl radicals have been replaced by butyl radicals. Hydrates with 26–28 water molecules (mp 27.4–34.6°C) are the most stable hydrates of the series, except for i-AmBu3NF·25.9 H2O, melting 0.3°C lower than the tetragonal hydrate in the same system. All compounds are defined chemically, and for some of them crystal data are given.

Phase Diagram of the (C3H7)4NF-H2O System and Crystal Structure of the Layered (C3H7)4NF·11H2O Hydrate

Abstract The phase diagram of the binary (C3H7)4NF-H2O system has been studied. Three polyhydrates, (C3H7)4NF·12H2O, (C3H7)4NF·11.5H2O and (C3H7)4NF·11H2O, which melt congruently at -5.1; -6.7; -7.8°C, respectively, were found in the system. A single crystal X-ray structure of the novel hydrate Pr4NF·11H2O is reported. The space group for the layered hydrate is orthorhombic Fddd with a = 7.649(2); b = 14.920(6); c = 43.61(2)A, z = 8, DC = 1.077 g · cm−3 (t = -50°C). The layers of water and those of cations (C3H7)4N+ are held together by electrostatic and van der Waals interactions. Water forms a double water-fluoride layer which consists of interconnected fragments 54.

The Structure of Solvates in the Solid State — Systems Model for Heteromolecular Association?

It happens quite often during chemical preparation work that products of chemical reactions are obtained in the form of “solvates” (or “adducts”, etc.) in which solvent and/or other foreign molecules co-crystallize with the “main” component. The large amount of structural information which has been accumulated over the last decades provides interesting data on the role played by inclusion of molecular species in the crystalline state.