Clathrate inclusion compounds, phase

Channel Inclusion Compounds, p. 223 Clathrate Inclusion Compounds, Phase Transitions irz. p.289... 

OTHER CLATHRATE INCLUSION COMPOUNDS UNDERGOING PHASE TRANSITIONS... 

Shielding and Stabilization. Inclusion compounds may be used as sources and reservoirs of unstable species. The inner phases of inclusion compounds uniquely constrain guest movements, provide a medium for reactions, and shelter molecules that self-destmct in the bulk phase or transform and react under atmospheric conditions. Clathrate hosts have been shown to stabiLhe molecules in unusual conformations that can only be obtained in the host lattice (138) and to stabiLhe free radicals (139) and other reactive species (1) similar to the use of matrix isolation techniques. Inclusion compounds do, however, have the great advantage that they can be used over a relatively wide temperature range. Cyclobutadiene, pursued for over a century has been generated photochemicaHy inside a carcerand container (see (17) Fig. 5) where it is protected from dimerization and from reactants by its surrounding shell (140). 

In a thorough review of calorimetric studies of clathrates and inclusion compounds, Parsonage and Staveley (1984) presented no direct calorimetric methods used for natural gas hydrate measurements. Instead, the heat of dissociation has been indirectly determined via the Clapeyron equation by differentiation of three-phase equilibrium pressure-temperature data. This technique is presented in detail in Section 4.6.1. 

The process of guest release for the two types of inclusion compound of 9.4 can be followed by thermo-gravimetric analysis (TGA, Box 9.1). This reveals the clathrate-like behaviour of the y phase (trace 2) and contrasts significantly with the zeolite-like behaviour of the microporous /1-phase (trace 1), Figure 9.19b. The mass loss stages followed by both compounds after initial wetting in benzene are as follows ... 

Liquid clathrates offer a great advantage over solid-state separations (e.g. by formation of Hoffman-type inclusion compounds, Section 9.4) because of the extremely fast mixing kinetics, the avoidance of the need to wait for crystallisation to occur and the easy separation of the two liquid phases. It should also prove possible to run liquid clathrate separations in a continuous extraction manner. The avalues of a number of liquid clathrate-based separations have been reported and are summarised in Table 13.1. 

A different problem arises when simple monomers like ethylene and propylene are subjected to polymerization. All efforts made so far have been unsuccessful, mainly because the correct conditions for inclusion were not known and for the widely diffused erroneous opinion that these molecules cannot act as guests for structural reasons. We directed our work to a knowledge of the decomposition curve of their PHTP clathrates by assuming that the inclusion compound exists in the temperature range in which the equilibrium pressure of the solid phase is lower them that of the pure monomer. 

It is clear that pure 1,3-dialkyimidazolium ILs in the solid, liquid and gas phases can be described as well-organized hydrogen-bonded polymeric supramolecu-les. Moreover, there is now much evidence indicating that this polymeric nature is maintained to a great extent when they are mixed with other substances. 1,3-Dial-kylimidazolium ILs-aromatic mixtures form liquid clathrates and in the case of a [CjCjIm]PFg-benzene mixture the inclusion compound [([CjCjIm]PFg)2(benze-ne)] could be trapped and its X-ray structure determined [77]. The conductivity of... 

Clathrates. or inclusion compounds, are the typical representatives of supennolecular species. We may define them as the compounds formed by inclusion of one kind of molecules, called guest molecules, into the cavities of a crystalline framework composed of the molecules of another kind (or into a cavity of one large molecule), called host molecules, without forming any specific chemical bond between guest and host. Unlike the case of traditional chemical compounds, favorable spatial complementarity of the guest and host subsystems, not chemical reactivity, plays the important role in formation of these compounds from the components. This principle of formation allows molecules that are coordination saturated and do not interact chemically with each other to be brought together, so that they form supermolecules or supennolecular crystalline phases that are thermodynamically more stable than a mixture of initial components. 

Isostructurality occurs frequently in many classes of inclusion compounds. Here, selected examples are drawn from several classes displaying this tendency, starting with clathrate hydrates and metal-containing inclusion compounds. The inherent stabilities of many host frameworks are conducive to the formation of isostructural series. At the other extreme, however, isostructurality may also arise in situations where the host is unstable, originating only within the clathrate phase, which self-assembles in the presence of guest molecules. 

---