Hydroquinone-clathrate compounds

The transition temperatures 7-15 K of hydroquinone-clathrate compounds containing very small polar molecules (HCl and H2S) are comparable with the separation... 

MOLECULAR MOTION AND PHASE TRANSITION IN HYDROQUINONE CLATHRATE COMPOUNDS... 

In the hydroquinone clathrate compounds the guest molecules are separated by the rigid hydroquinone lattice. The interaction among them is primarily electrostatic, perhaps with an additional effect of the van der Waals force. 

Previously we showed that the dipole-dipole interaction between the guest molecules has an appropriate magnitude to explain the phase transitions in the hydroquinone clathrate compounds [14,17]. We have now two more substances to compare with the model the phase transition in the hydrogen sulphide compound and the ordered structure of the acetonitrile compound. A Monte Carlo calculation showed that no phase tran-... 

Figure 4 shows the transition temperature of the hydroquinone clathrate compound plotted against the square of the dipole moment of the guest molecule. The data on the hydrogen sulphide, methanol and hydrogen cyanide compounds are those of the heat capacity anomalies. The transition temperature of the acetonitrile compound, if it exists, lies above the room temperature. One finds a good proportionality between the transition temperature and the square of the dipole moment. 

Examples of the hydroquinone inclusion compounds (91,93) are those formed with HCl, H2S, SO2, CH OH, HCOOH, CH CN (but not with C2H 0H, CH COOH or any other nitrile), benzene, thiophene, CH, noble gases, and other substances that can fit and remain inside the 0.4 nm cavities of the host crystals. That is, clathration of hydroquinone is essentially physical in nature, not chemical. A less than stoichiometric ratio of the guest may result, indicating that not all void spaces are occupied during formation of the framework. Hydroquinone clathrates are very stable at atmospheric pressure and room temperature. Thermodynamic studies suggest them to be entropic in nature (88). 

Not only hydroquinone, but also phenol and a number of related substances have been reported20,21 33,44 to form clathrate compounds of a similar type. But this class of substances proves to be... 

In the next section we shall give a brief account of the crystal structure of the hydroquinone clathrates and of the gas hydrates, as far as is needed for a proper understanding of the subsequent parts. The reader who is interested in the phenomenology of other clathrate compounds should consult one of the many review articles7,8 39 on inclusion compounds. 

Kroll process, 13 84-85 15 337 17 140 in titanium manufacture, 24 851-853 Kroll zirconium reduction process, 26 631 KRW gasifier, 6 797-798, 828 Krypton (Kr), 17 344 commercial, 17 368t complex salts of, 17 333-334 doubly ionized, 14 685 hydroquinone clathrate of, 14 183 in light sources, 17 371-372 from nuclear power plants, 17 362 physical properties of, 17 350 Krypton-85, 17 375, 376 Krypton compounds, 17 333-334 Krypton derivatives, 17 334 Krypton difluoride, 17 333, 336 uses for, 17 336... 

Hydroquinone, p-C6H4(OH)2, also known as quinol in the older literature) exists in three polymorphic forms (or crystalline modifications). a-Hydroquinone is the stable form at room temperature, whereas the metastable monoclinic y-form can be prepared by sublimation or rapid evaporation of a solution in ether. Crystallization of hydroquinone from a common solvent such as methanol generally yields a clathrate with guest solvent inolecules trapped inside cavities of the (3-hydroquinone host lattice.". The existence of the (3 polymorph of hydroquinone (empty P-hydroquinone clathrate), which can be obtained by crystallization from n-octane, was reported in 1981. Described in this article are the structural features of these compounds, and some recent developments are also covered. 

Qathrate compounds are of this type molecules of one substance are trapped in the open structure of molecules of another. Hydroquinone forms clathrate compounds with SO2 and methanol, for example. Urea and thiourea also have the property of forming complexes, known as adducts, with certain types of hydrocarbons. In these cases molecules of the hydrocarbons fit into holes or channels in the crystals of urea or thiourea the shape and size of the molecules determine whether they will be adducted or not. 

Many further cases of inclusion materials were discovered by happy accident during the next two centuries, for example, additional clathrate hydrates, the Hofmann clathrates, phenol inclusion compounds, Dianin s compound, urea tubulates, choleic acids, cyclodextrins, aud interpenetrated hydroquinone inclusion compounds. These substances remained problematic despite being the object of much painstaking scientific study. Mauy were unstable under ambient conditions and therefore it proved difficult to determine accurate ratios of their two components A and B. Furthermore, the substances did not follow the usual rules of covalent bonding, leading to their representation in the fonn (A)x (B)y. It was surmised that one component somehow trapped the other, but no experimental methods were available to analyze this phenomenon. 

The interstitial solutions are typical of clathrate compounds and they have been considered [l]. Studying water [2,3], urea [4,5], thiourea [6] and hydroquinone [7-9] clathrates we have found that only the latter (in the presence of the limited guest set [10-12]) forms the solutions of this type. 

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