Clathrate forms

De Rosa, C. Guerra, G. Petraccone, V. Pirozzi, B., Crystal structure of emptied clathrate form (8e form) of syndiotactic polystyrene, Macromolecules 1997, 30, 4147 4152... 

Constructing a Solution Host from Clathrate-Forming Building Blocks The Cryptophanes... 

The inclusion ability of 7.19 is sufficiently robust that it can tolerate a variety of substitutions and still retain (modified) clathrate-forming ability. A wide variety of analogues have been prepared, which exhibit different size and shape cavities by the following modifications ... 

Considering the process of liquid clathrate formation, solid-state liquid clathrate forming materials such as 13.21 are subject to a dissolution equilibrium or liquefying phase change upon exposure to aromatic solvents (Equation 13.1). 

A great deal of more recent research has concerned liquid clathrates formed from strong acids such as HC1 in aromatic solvents in which oxonium ions such as H30+ are stabilised by crown ethers and play the role of the cation. This has allowed the isolation of a range of interesting oxonium salts, as discussed in Section 3.11.1. 

The Sill and Wilkening proposal that clathrates formed in the cold outer parts of the solar system and then transported to the inner solar system (e.g., in comets) might help account for the atmospheres of the terrestrial planets. They contend that infall of 1 ppm of ice-methane clathrate with noble gases dissolved as just described could account for the present inventories of Ar, Kr, and Xe in the terrestrial atmosphere. 

A potentially unique form of specific solute-solvent interaction has been proposed for ionic liquids. Blanchard and Brennecke [234] note that the solubilities of aromatic species are anomalously high in an imidazolium-based IL when compared to solutes of comparable molecular weight and dipole moment. This cannot be explained purely by ji-ji interactions, because while ji-ji interaction energies can be significant [235], the solubilization of a pure aromatic liquid must disrupt at least as many ji-ji contacts as it creates. However, work by Holbrey and co-workers [171] characterizes a cocrystalline clathrate form of an imidazolium-based IL with benzene, which shows distinctive ji-ji stacking. 

The reactions of N3P3C16 with ethylenediamine and ethanolamine give spirocyclic derivatives (25) and not ansa-compounds. Aliphatic and alicyclic diols also yield spirocyclic derivatives (26, 31). Inclusion clathrates formed by spirocyclic phosphazenes with aryldioxy substituents (2) have been studied by mass spectrometry, broadline NMR, and X-ray techniques (4). Some conclusions on molecular motion in these compounds have been deduced. 

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. 

The stable form of hydroquinone is a-quinol, Qa. The unstable form which gives the clathrate is / -quinol, QP. The difference, Ap — (ilqP — /aq ), between the chemical potentials of the two forms must therefore be positive. The clathrate formed from a gas, A, will just reach stability with the a form when the pressure, P, and the fraction of the holes filled, y, are such that... 

The transfer cycle for methane ([135], Ihble 2.10) shows that transfer from the inert solvent to the gas phase is favorable due to the large increase in entropy (JS = +14 kcal/mol-1 K-1) which compensates for the loss in van der Waals contacts indicated by a positive enthalpy (JH= +0.5 kcal mol-1). The transfer from the inert solvent and from the gas phase to water is unfavorable due to the large decrease in entropy (the water molecules arrange around methane in clathrate form), which outweighs the favorable change in enthalpy AH--2.7 and -3.2 kcal mol-1) arising from the increase in van der Waals contacts with water. 

The structure of another type of inclusion compound, 73-MeCN (1/1), suggests that the interaction between the ketone host and MeCN is purely of steric character " (Figure 18). The packing is highly sensitive, and clathrate forming will disappear with any changes in the size of either the host or the guest molecules. ... 

In order to investigate Lee et al. s [20] claims about the hydrogen content of clathrates formed from THF concentrations below the eutectic composition (approximately 1.0mol% at 1 atm), Anderson et al. [23] experimentally prepared... 

An X-ray analysis of the clathrate formed by 4-( )-hydroxy-phenyl)-2,2,4-trimethylthiochroman has been reported. 

Finally, ammonia and methane have both been proposed as minor constituents of the Galilean satellites in hydrated or clathrated form, possibly lowering the melting point in their interiors and promoting ice (or cryo-) volcanism (Hogenboom etal., 1997 Kargel, 1992 Lewis, 1973). However, as of the early 2000s, there has been no identification of these materials on the satellite surfaces. 

Ibragimov, B. T., Beketov, K. M., Talipov, S. A., and Mardanov, R. G. (1995). X-ray structural investigation of gossypol and its derivatives. XXVIII. Separation of the dilactol tautomeric form of gossypol hexamethyl ether into individual stereoisomers and evaluation of their clathrate-forming capacity. Chem. Nat. Compd. 31, 575-578. 

When ternary clathrates form, the structural problem concerning the arrangement of guests in the channel remains undefined. From this point of view useful information can be derived just from copolymerization, which acts as an unconventional probe for structural analysis. If we admit that interchange between included monomers is slow, the sequence of monomer units in the copolymer corresponds to that of the guests in the channel before polymerization. The polymer chain behaves as a recording tape or a permanent copy of an otherwise elusive intermolecular arrangement (23). 

---