Clathrates crystallizations

Let us consider a clathrate crystal consisting of a cage-forming substance Q and a number of encaged compounds ( solutes ) A, B,. . ., M. The substance Q has two forms a stable modification, which under given conditions may be either crystalline (a) or liquid (L), and a metastable modification (ft) enclosing cavities of different types 1,. . ., n which acts as host lattice ( solvent ) in the clathrate. The number of cavities of type i per molecule of Q is denoted by vt. For hydroquinone v — for gas hydrates of Structure I 1/23 and v2 = 3/23, for those of Structure II vx = 2/17 and v2 = 1/17. 

As our system we choose a clathrate crystal containing NQ molecules of Q and occupying a volume V at the temperature T. We further suppose that it has been crystallized while in equilibrium with the solutes A,. . ., J,. . M, having absolute activities XA). . ., kM, i.e., chemical potentials... 

When studying heterogeneous equilibria involving clathrates, one is faced with peculiar difficulties owing to the hysteresis effects mentioned in the introduction the solute in a clathrate crystal of hydroquinone, for instance, will not come to thermodynamic equilibrium with the vapor in which it is placed. Consequently it is impossible, or at least very difficult, to measure the equilibrium vapor pressure of the solute in a clathrate by placing some crystals in a tensometer (cf. the experiments of Wynne-Jones and Anderson,58 and those of Leech and Richards reported by Powell33). 

Upon recrystallization from ethanol 1 afforded the clathrate crystal with a host-guest molar ratio of 1 2. Grinded l-2EtOH crystals were irradiated by means of a high-pressure mercury lamp at ambient temperature for 6 h. The photoproducts were chromatographed on a silica gel column to give monoether 2a (43%) and diether 2b (21%) along with unreacted 1 (36%). 

The experiment began by charging the equilibrium cell with about 30 cm3 of either phenoPp-cresol or phenol-water solution mixture. The cell was then pressurized with either methane or carbon dioxide until the phenol clathrate formed under sufficient pressure. The systems were cooled to about 5 K below the anticipated clathrate-forming temperature. Clathrate nucleation was then induced by agitating the magnetic spin bar. After the clathrates formed, the cell temperature was slowly increased until the clathrate phase coexisted with the liquid and vapor phases. The nucleation and dissociation steps were repeated at least twice in order to diminish hysteresis phenomenon. The clathrates, however, exhibited minimal hysteresis and the excellent reproducibility of dissociation pressures was attained for all the temperatures and found to be within 0.1 K and 1.0 bar at each time. When a minute amount of phenol or p-cresol clathrate crystals remains and the system temperature was kept constant for at least 8 hours after attaining pressure stabilization, the pressure was considered as an equilibrium dissociation pressure at that specified temperature. 

A crystal of this sort, containing molecules that are not bonded to the framework of the crystal, is called a clathrate crystal (from the Latin word clai/iri, lattice). 

This framework of water molecules is found in many crystals, such as xenon hydrate, Xe-5fH20 (or 8Xe-46H20, the contents of the unit cube outlined in the drawing), and methane hydrate, CH4 -5f H20. The drawing shows the xenon molecules (xenon atoms) in the chambers. Xenon hydrate can be classified with Prussian blue (plates 27 and 28) as a clathrate crystal. 

HERBSTEIN asked if the method would work if a benzene molecule, for example, were contained in a clathrate crystal. SNYDER replied that, if the molecule migrates through the clathrate relatively rapidly so that it sees an average magnetic environment, then one might indeed be able to obtain a spectrum with very large dipole interactions and quite a bit of structure. But if it tends to stay put in the clathrate site for 0.001 s or so, one would observe the typical broad-band spectrum characteristic of a solid. 

The stable and selective inclusion of highly strained halocyclohexanes conformers or of monomeric carboxylic acids reveals the outstanding mutual fit of the TOT molecules constituting clathrate crystal, thus leading to a remarkable preservation of the cohesive van der Waals forces in the host lattice. But for a few exceptions, once formed the TOT clathrates prove to be thermally stable. 

Clathrate crystals obtained by slow evaporation of a solution of 1 in suitable guest solvents conform to the general formula 2 G, where G is a guest species ran-... 

There are two major types of structural phase transitions order-disorder and displacive. An order-disorder phase transition is characterized by disorder of the atoms or molecules in the structure of one of the phases. Sometimes both phases are disordered in different degrees. The disordered (or more disordered) phase is more symmetric, because disorder makes the average distribution of atoms more even. Most of the phase transitions in clathrate crystals are of this type. 

The host tri-o-thymotide 5 was reported to form over 140 clathrates with guests of the most varied shapes. These clathrates crystallize in about a dozen isostructural series, the most populous belonging to space group P3i21. For several of these series, a linear relationship between unit cell size and guest molecular volume was observed. 

Kuruma, K. Nakagawa, H. Imakubo, T. Kobayashi. K. Guest-exchange and guest-release via gas-solid contact in clathrate crystals based on 2.5-bis(9-hydroxyfluoren-9-yl)thieno[3,2-b]thiophene as a host compound. Bull. Chem. Soc. Jpn. 1999. 72 (6), 1395-1401. 

On the other hand, the so-called solid-state complexes of multiarmed podands (e.g., hexahosts) with neutral molecules are more likely to be classed with clathrates (crystal void inclusion compounds). ... 

The most surprising anesthetic agents are the noble gases, such as xenon. Xenon is completely unreactive chemically. It has no ability whatever to form ordinary chemical compounds, involving covalent or ionic bonds. The only chemical property that it has is that of taking part in the formation of clathrate crystals. In these crystals the xenon atoms occupy chambers in a framework formed by molecules that interact with one another by the formation of hydrogen bonds. The crystal of this sort of greatest interest to us is xenon hydrate, Xe 5%HiO. The crys-... 

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