Inclusion compounds urea clathrates

Pure tra i-l,4-polyisoprenes as well as 1,4-polybutadienes can be synthesized by polymerization in inclusion compounds [266-269]. As typical hosts for this dienes, the inclusion compounds or clathrates of urea, thiourea, or perhydrotriphenylene [PHTP Eq. (36)] are used [270,271]. The host forms the frame of the crystal and the guest is placed in the cavities existing in the lattice. Polymerization is generally started by subjecting the inclusion compound to irradiation with a-, y-, or x-rays and proceeds by a radical mechanism [272,273]. Also, free radical initiators such as di-/cr/-butylperoxide could be used [274]. Inclusion in urea yields crystalline trans-, A polymers, whereas trans-lA-polyisoprene obtained in PHTP is amorphous. There is no trace of, A-cis units or of 1,2, 3,4, and cyclic units. The reason for the amorphous product is the presence of a substantial number of head-to-head and tail-to-tail junctions in addition to head-to-jail junctions [275, 276]. 

The history of inclusion compounds (1,2) dates back to 1823 when Michael Faraday reported the preparation of the clathrate hydrate of chlorine. Other early observations include the preparation of graphite intercalates in 1841, the 3-hydroquinone H2S clathrate in 1849, the choleic acids in 1885, the cyclodextrin inclusion compounds in 1891, and the Hofmann s clathrate in 1897. Later milestones of the development of inclusion compounds refer to the tri-o-thymotide benzene inclusion compound in 1914, phenol clathrates in 1935, and urea adducts in 1940. 

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. 

A laser flash study of the photoreactions of hexan-2-one and 5-methylhexan-2-one has provided evidence for the existence of the triplet 1,4-biradicals produced by the y-hydrogen abstraction typical of Norrish Type II reactivity. The photochemical behaviour of the alkanone, nonan-5-one, in urea inclusion compounds has been studied. In solution, irradiation of nonan-5-one yields hexan-2-one, propylene, and two cyclobutanols. In the clathrate, the fragmentation products were essentially the same but only one cyclobutanol was observed. The cyclization fragmentation ratio was established as 0.67, compared with 0.32 in methanol. The authors suggest that the CIS-cyclobutanol has less stringent rotational requirements and that it is this isomer (43) which is formed in the clathrate. 

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... 

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... 

The chemistry of inclusion compounds also looks back on a lively history There are many events of significance in the area of inclusion chemistry till the middle of the twentieth century including the discovery of new inclusion compounds and hosts (Fig. 1), among them the graphite intercalates, the P-quinol and cyclodextrin inclusion compounds, the Hofmann-type clathrates as well as the inclusion compounds of tri-o-thymotide, Dianin s compound, the choleic acids, of phenols, of urea and others specified in comprehensive monographs... 

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... 

Hofmann-type clathrates Werner clathrates clathrate hydrates inclusion compounds of urea, thiourea, and selenourea cyclodextrins calixarenes gossypol hexa-hosts hydroquinone phenol and Dianin s compounds graphite intercalates natural and pillared clays and others. Such studies contributed to the birth of supramolecular chemistry, with relevance to a new understanding of the world of materials that was emphasized with a Nobel Prize. 

This article provides a brief overview of the theory of IR spectroscopy and Raman spectroscopy (for more in-depth descriptions of these methods see Refs. [1-3]). This is followed by a review of vibrational spectroscopic studies performed on clathrate hydrates (a class of inclusion compound) and macrocyclic suprainolecular compounds. Clathrate hydrates were highlighted in this article because of all the clathrate compounds, they are particularly amenable to vibrational spectroscopy and are of great industrial significance. Similar IR/Raman methods can be applied to other well-known clathrate compounds. including quinol and urea " clathrates. Finally, future directions on the use of vibrational spectroscopy in supramolecular compounds will be given. 

Since their discovery in 1949 , n-alkane urea inclusion compounds (UIC) have fascinated chemists and spectroscopists alike . However, there is disagreement about the interpretation of the deuterium NMR spectra of UICs. Originally, it was believed that in these channel clathrates the alkanes exist in an a -trans conformation. Recently, it was shown that significant amounts ca 30%) near the end of the chains are present in a gauche conformation. An illustration of the possible alkane conformations based upon MM2 calculations is shown in Figure 4 . 

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... 

The examples mi t have illistrated that functional grou (e.g. OH, COOH, NH,), as they are a component of classical crystal inclusion compounds are usually used for construction, cross-linking, and stabilization of the host lattice (Fig. 6a), and are not used, as could have been, for direct binding of guest molecules, e.g. via coordination or H-bonding (Fig. 6b). To speak with a newly developed classification system on inclusion compounds (see Chapter 1 of Vol. 140), tho are true clathrates and i t coordinatoclathrates (cf. Fig. 6, for a more detailed sj fication see Fig. 15 in Chapter 1 of Vol. 140). As in the case of urea and thiourea, a rather stable, but nearly invariable host lattice with rigidly... 

Many further cases of inclusion materials were discovered by happy accident during the next two centuries, for example, additional clathrate hydrates, the Hofmann clathrates, phenol inclusion compounds, Dianin s compound, urea tubulates, choleic acids, cyclodextrins, aud interpenetrated hydroquinone inclusion compounds. These substances remained problematic despite being the object of much painstaking scientific study. Mauy were unstable under ambient conditions and therefore it proved difficult to determine accurate ratios of their two components A and B. Furthermore, the substances did not follow the usual rules of covalent bonding, leading to their representation in the fonn (A)x (B)y. It was surmised that one component somehow trapped the other, but no experimental methods were available to analyze this phenomenon. 

The conditions of the formation of the solution of such kind for channel inclusion compounds are simplified substantially and seem to be reduced to those of the formation of the individual clathrates, i.e. to the conformity of the guest molecules dimensions to the cross-section of the clathrate framework channel (for urea for thiourea 6,9-7j4A). 

An important advantage of the inclusion complexes of the cyclodextrins over those of other host compounds, particularly in regard to their use as models of enzyme-substrate complexes, is their ability to be formed in aqueous solution. In the case of clathrates, gas hydrates, and the inclusion complexes of such hosts as urea and deoxycholic acid, the cavity in which the guest molecule is situated is formed by the crystal lattice of the host. Thus, these inclusion complexes disintegrate when the crystal is dissolved. The cavity of the cyclodextrins, however, is a property of the size and shape of the molecule and hence it persists in solution. In fact, there is evidence that suggests that the ability of the cyclodextrins to form inclusion complexes is dependent on the presence of water. Once an inclusion complex has formed in solution, it can be crystallized however, in the solid state, additional cavities appear in the lattice, as in the case of the hosts previously mentioned, which enable the inclusion of further guest molecules. ... 

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