Channel hosts urea clathrates

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). 

Crystalline inclusion complexes (IC s) have been also formed between polymers and another small-molecules, host clathrated provide a unique environment for observing the solid - state behavior of isolated polymer chains. In their IC s with small-molecule, host clathrates, such as urea (U) [1] and perhydrotriphenylene (PHTP) [57], the included polymer chains are confined to occupy narrow channels (ca. 5.4 A in diameter) where they are extended and separated from neighboring chains by the channel walls, which are composed exclusively of the host clathrate, crystalline matrix. Choi et al. [58] have been studied the behavior of isolated, extended polymer chains included in their IC s with U and PHTP by a combination of molecular modeling [59,60] and experimental observations in an effort to determine their conformations and mobilities in these well-defined, containing environments. 

Inclusion compounds are crystalline hosts that have channels in which a guest molecule may reside, e.g., templated urea. Non-stoichiometric ICs are typically formed. Clathrates, or cage compounds, are a special type of inclusion compound that possess fully enclosed voids. Integral stoichiometries for their ICs are expected. For example, hydroquinone (1) forms a hydrogen-bonded trimer that can confine a... 

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. 

A number of compounds with chemical compositions that are similar to that of the original Hofmann-type clathrates were reported. However, as usual for other inclusion compounds, similarity in composition does not necessarily co relate with similarity of structure. For example, in the [Cd(pn)Ni(CN)4] G series with aliphatic guests, the host structure is entirely different from the Hofmann-pn-type pyrrole clathrate [Cd(pn)Ni(CN)4]. (3/ 2)C4H5N. Two subseries of structures were observed for the channel cavities, one being a snake-like extension similar to chat observed for the urea-host inclusion... 

Basically, there are two main classes of urea/thiourea inclusion compounds the classical channel-type clathrates having a host lattice constructed from urea (Figure 8.1) or thiourea molecules (Figure 8.2) [1] and those whose host lattices contain anions as... 

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