Clathrates hydrogen bonding

A variety of chemical interactions are possible between two molecules of two different species. Those that result in chemical reactions are considered in Chapter 5 for the purpose of separation of mixtures. There are a number of other, weaker, noncovalent interactions where the bond energy is less than 50 Id/mol. These include chelation, clathration, hydrogen bonding, hydrophobic interactions, ionic binding, pi bonding, van der Waals interactions, etc. These result in weak chemical complexes, which are... 

Fig. 18. Crystal structures of recent clathrate design (a) coordinatoclathrate between host (39) (Fig. 17) and / -butanol (host—guest hydrogen bonding in the shaded area) (b) perspective view of the hehcal inclusion channel formed by diol host (43) (Fig. 17 all except one host molecule are represented...

Probably the most familiar of all clathrates are those formed by Ar, Kr and Xe with quinol, l,4-C6H4(OH)2, and with water. The former are obtained by crystallizing quinol from aqueous or other convenient solution in the presence of the noble gas at a pressure of 10-40 atm. The quinol crystallizes in the less-common -form, the lattice of which is held together by hydrogen bonds in such a way as to produce cavities in the ratio 1 cavity 3 molecules of quinol. Molecules of gas (G) are physically trapped in these cavities, there being only weak van der Waals interactions between... 

According to these authors all gas hydrates crystallize in either of two cubic structures (I and II) in which the hydrated molecules are situated in cavities formed by a framework of water molecules linked together by hydrogen bonds. The numbers and sizes of the cavities differ for the two structures, but in both the water molecules are tetrahedrally coordinated as in ordinary ice. Apparently gas hydrates are clathrate compounds. 

The most important host for clathrates is hydroquinone. Three molecules, held together by hydrogen bonding, make a cage in which one molecule of the guest fits. Typical guests are methanol (but not ethanol), SO2, CO2, and argon (but not neon). 

On the basis of previous observations it was anticipated that the clathrate formation would be more selective in controlling guest selectivities if a functionality which could form hydrogen-bonds with guest species and add this strengthening feature to that... 

Additional observations reflect on the high stability of the DTU host lattice and illustrate its versatile capability to form clathrates with different guests. The crystalline 1 1 complexes of DTU with diethylether (Fig. 29) and diethylamine are isomorphous with the structure of the propanamide adduct (Fig. 24). However, in these structures the hydrogen bonds do not form a continuous pattern but are, rather, confined to... 

Fig. 31. Stereoview of the 24 acetone clathrate down the channel axis. Each acetone guest is within a hydrogen bonding distance from one of the hydroxyl sites of the host481...

Mass spectra for clusters formed by the adiabatic expansion of liquid droplets of different mole fraction (Xdio) 1,4-dioxane-water mixtures have been studied. For Xdio = 0.01, the hydrogen-bonded networks of water are predominant in the water-rich region with 1,4-dioxan molecules probably being captured in the network to form clathrates, but decrease exponentially with increasing Xdio <1999JML163>. 

Another type of hydrogen bonding of oxetane is clathrate formation with water. Clathrate compositions of 1 6.5 and 1 17 moles of water have been observed <72CR(C)(274)1108). Oxetane has been found to be more effective than other cyclic ethers in solvating uranyl hexafluoroacetonylacetate (81IC1415). 

Clathrate hydrates discussed in Section 8.3.3 also provide exciting examples of dynamic complexes. The cages formed by hydrogen bonded water molecules in these systems are constantly decomposed and reformed, but they are stabilized by appropriate guests [58]. If the latter are too small to fill the cage they, in turn, move inside it. 

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