Urea clathrates structure

With larger guest molecules that do not fit the cavity, all three principal cyclodextrins are capable of forming channel structures in which the cyclodextrin cavities line up in order to produce an extended hydrophobic channel into which guests can be threaded in a similar way to urea clathrates... 

While the resonance of nitrogen in a urea clathrate containing heptane appeared to be unobservable80), due to a fluctuation in the vicinity of urea molecules, the nitrogen resonance was recently found to be visible in thiourea-cyclohexanc clathrate81) in which a well-defined crystalline structure leads to a unique environment for each thiourea molecule. This observation needs further confirmation. 

Clathrates provide cavities of a specific size and shape and therefore they can be used very effectively for separating gases with different sizes of molecules. For example, urea clathrates have been used to separate linear from branched hydrocarbons. The hydroquinone clathrate can be used to store and deliver radioactive krypton. In addition, if the host is chiral, there can be chiral discrimination so that one enantiomer of a guest is enclosed in the clathrate structure in preference to the other enantiomer. The trapped guest molecules can be liberated, for example, by solution in an apolar solvent, at the convenience of the user,... 

Host guest ratios are ill-defined and depend on guest length, and the structures are incommensurate, rather like urea clathrates (Section 7.3). The host channels arise from the arrangement of 12 TOT molecules that form the channel repeat unit. Two sequences of six molecules form a doublehelix structure (Figure 7.26). 

From the examples in Figure 9.8, it can be seen that these urea clathrates (also known as inclusion compounds, adducts, or channel or cage compounds the process itself has been dubbed extractive crystallization17) are not confined to n-alkanes, but are also formed with straight-chain olefins, alcohols, esters, ketones, halides, etc. An X-ray structure determination18 first demonstrated the spiral hexagonal structure formed by the urea and the approximately 0.7 urea molecules per carbon of chain length.15... 

Urea is well-known to form clathrate structures (Section 4.3.1) however, it is also able to form layered structures in a tape motif. The urea functionalities are able to align in such a way that each carbonyl receives two NH - O hydrogen bonds from the molecule next to it (Figure 4.18(d)). This results in the formation of R (6) rings running throughout the structure. 

The attention was mainly focused on the C-Cl stretching range of the supposedly mostly syndiotactic chain. Polymerization in urea clathrates provides the purest sample of planar syndiotactic PVC [130], Such a chain shows a large energy gap in the CCl stretching range ( 650 to 820 cm ). It is very likely that defect modes from other structures may generate localized gap modes which may be used as useful structural probes. 

The inclusion properties of urea were discovered by Ben-gen in 1940 and this tubulate host has since become one of the most studied. Thiourea and selenourea form related, but slightly different, clathrate structures. Figure 1 illustrates the structure of the (thiourea)3 -(carbon tetrachloride) compound. Many 1,3-diarylurea derivatives also include guest molecules, but these produce hydrogen-bonded complexes with acceptor guest species, rather than clathrates. " ... 

Systems that react in this manner fall into two classes. In the first of these the framework that dominates the crystal structure scarcely participates in the reaction. This is the case, for example, in the reaction of an organic molecule intercalated in graphite or a clay, or of a guest molecule held in a clathrate of urea or thiourea. Some cases of this sort will be treated in the next section. 

Common hosts such as urea or p-f-butylcalix[4]arene can exist as various crystal phases, some if which do not contain cavities. The crystal form of the pure host without cavities is called the a-phase. The 30 (apohost) phase contains unfilled cavities while the p -phases have the same host structure but contain different guests. Such structures are sometimes referred to as pseudopoly morphs. Further pure phases (y-phase) or clathrates (y1. phases) may also exist in some cases, as in tri-o-thymotide. Apohosts are usually relatively unstable but allow the inclusion of interesting guests such as gases. 

Host — A - molecular entity that forms an -> inclusion complex with organic or inorganic -> guests, or a - chemical species that can accommodate guests within cavities of its crystal structure. Examples include cryptands and crowns (where there are -> ion-dipole interactions between heteroatoms and positive ions), hydrogen-bonded molecules that form clathrates (e.g., hydroquinone and water), and host molecules of inclusion compounds (e.g., urea or thiourea). The - van der Waals forces and hydrophobic interactions (- hydrophobic effect) bind the guest to the host molecule in clathrates and inclusion compounds. 

The most common hosts for inclusion polymerization are urea, thiourea, perhydrotriphenylene (PHTP), deoxycholic acid (DCA), apocholic acid (ACA) and tris(o-phenylenedioxy)cyclotriphosphazene (TPP)(Fig. 2). They have the common feature of forming channel-like clathrates, but differ in many specific properties. For instance, urea and thiourea have a rigid structure in which the host molecules are connected by hydrogen bonds and possess a high selectivity towards the guests. In urea channels are rather narrow whereas in thiourea they are wider as a consequence, linear molecules include only in urea and branched or cyclic molecules in thiourea. On the contrary, chainnels existing in PHTP clathrates are very flexible and can accomodate linear, branched and cyclic molecules. 

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