Vladislav Yu. Komarov

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.

Calorimetric and structural studies of tetrabutylammonium chloride ionic clathrate hydrates.

In the present work, characteristic properties of tetrabutylammonium bromide (TBAB) ionic clathrate hydrates structures were studied by single-crystal X-ray structure analysis. The structures of three different tetragonal TBAB ionic clathrate hydrates that were formed in our experiments were based on the same water lattice of tetragonal structure I (TS-I) differing in the ways of including bromide anions and arranging tetrabutylammonium cations. We demonstrated that (1) Br(-) can be included into the water lattice, replacing two water molecules, (2) the butyl group of the cation can be inserted not only in large T and P cavities but also in small D cavities of the water lattice TS-I, and (3) one of the reasons for polytypism of ionic clathrate hydrates on the basis of TS-I is the occurrence of alternative modes of arrangements of four-compartment cavities in adjacent layers of the water framework. The compositions of three TBAB ionic clathrate hydrates TBAB·38.1H2O, TBAB·32.5H2O, and TBAB·26.4H2O were determined by chemical analysis, and their enthalpies of fusion were measured by differential scanning calorimetry (DSC). From the obtained results, the enthalpies of the TBAB hydrate formation from TBAB and water were calculated thermodynamically.

The structure of the ionic clathrate hydrate of tetrabutylammonium valerate (C4H9)4NC4H9CO2·39.8H2O

For the first time the detailed crystal structure of the ionic clathrate hydrate of tetrabutylammonium valerate (C4H9)4NC4H9CO2·39.8H2O has been determined by single crystal X-ray analysis at 150 K. The idealized water–anion host framework consists of five-compartment polyhedral cavities 4T·P and small D cavities. Butyl chains of the cation and the anion of (C4H9)4NC4H9CO2 are included in the five-compartment cavities with displacement of two “inner” host water molecules. The inclusion of the guest entities leads to the distortion of D-cavities which are partially filled by additional “guest” water molecules. The other D-cages are empty. The structure exhibits significant disordering. The adequate model of disordering and inclusion mode of the guest moieties have been obtained. Special features of the crystal structures of ionic clathrate hydrates of tetraalkylammonium carboxylates are discussed.

Semiclathrate Hydrates in Tri-n-butylphosphine Oxide (TBPO)–Water and TBPO–Water–Methane Systems

In the present work, we studied semiclathrate hydrates in the TBPO-H2O and TBPO-H2O-CH4 systems. The stoichiometry, temperature, and enthalpy of dissociation of TBPO semiclathrate hydrate crystals formed in the TBPO-H2O binary system were found to be TBPO·33.6 ± 0.9H2O, 280.0 K, and 253.1 ± 4.7 J g-1, respectively. The crystal structure determined by single crystal XRD analysis (150 K) was the orthorhombic with space group Pbam and unit cell parameters a = 19.9313(8) Å, b = 23.4660(7) Å, and c = 12.1127(5) Å. The structural stoichiometry is TBPO·34.5H2O. The TBPO guest molecules arrangement within the host water framework has been refined for the first time. The discrepancy between the analytically measured and structural stoichiometry is likely to be attributed to the structure defects, which cannot be revealed by the routine single-crystal XRD analysis. The methane capacity of TBPO + CH4 double hydrate was measured by the thermovolumetric method in the range 14.9-55.8 wt % TBPO aqueous solution at a methane pressure of 8.5 ± 0.5 MPa and temperature of 274 ± 1 and 286 ± 1 K. The maximum included methane volumes of 61.6-74.6 mL/g were observed for the TBPO + CH4 double hydrates synthesized from ∼26-30 wt % TBPO aqueous solutions. Powder X-ray diffraction measurements of the hydrate samples used in the thermovolumetric experiments revealed that the TBPO + CH4 double hydrate has the same structural characteristics as the simple TBPO hydrate. The study of the Raman spectra of the TBPO + CH4 double hydrate and TBPO simple hydrate showed that in the TBPO + CH4 double hydrate CH4 molecules selectively occupy the small D cages. The results of the present study did not confirm the earlier suggestion of the formation of several structural types of hydrates in the TBPO-H2O system. The obtained results indicate that the TBPO-H2O binary system has a potential for application in gas separation and as cold storage and transportation media.