Urea inclusion channel

Eig. 12. Hydrogen-bonded network of an urea inclusion channel (--hydrogen bonds (90). 

Observation of the Norrish Type II reaction presents some difficulty in that generation of the biradical intermediate 12 requires a six-membered transition state and this is in conflict with the linear guest arrangement normally expected in the channel. However, as noted earlier, accommodation of planar six-membered rings in urea inclusion complexes has been observed 38. It appears that in this case the necessary six-membered transition state can be produced in the channel without destruction of the crystal structure. 

One of the main interests in urea inclusion compounds is their potential application in the separation of linear and branched hydrocarbons in the petroleum industry. Because the channel diameter is only slightly larger than the van der Waals diameter of a linear hydrocarbon, only a small amount of branching can be tolerated. Whether a particular urea inclusion compound will form or not may be assessed simply by comparison of the channel size with the diameter of the guest. For example, benzene is... 

Urea inclusion complexes. Normal alkanes having seven or more carbon atoms form inclusion complexes in which hydrogen-bonded urea molecules are oriented in a helical lattice in which a straight-chain hydrocarbon fits. The guest molecule is not bonded to the host but merely trapped in the channel. Discovered in 1940 by Bengen, the hydrocarbon-urea complexes have been studied particularly by Schlenk, Schiessler and Flitter, and Smith (X-ray analysis). The principal findings are as follows. 

Fig. 17. Schematic of the nonadecane/urea-inclusion compound. From left to right single-crystal experiment by rotation around the channel axis. Critical inhomogeneous linebroadening on approaching the transition temperature from above, with a lineshape invariant by rotation around c hole burning in the last spectrum proves the inhomogeneous nature of the broadening. Schematic of rotation patterns that gives the chain orientation in the low-temperature phase.

In the disordered crystal structure of the urea inclusion compounds of C 2n 2 (n = 12, 16), the orientation of the included paraffin chain was deduced and the hydrogen (deuterium) position determined for the hexadecane complex (n = 16) using neutron diffraction data [33]. A wide variety of guest molecules accommodated within urea channels were reported recently, including the synthesis and structural characterizations of a dialkylamine adduct [59] and a series of polymer-urea inclusion compounds [39e], Inteichannel ordering of n-alkane guest molecules have been experimentally and theoretically studied [57]. 

A considerable amount of recent work has been directed towards the study of detailed molecular orientations and motions of guest molecules in urea channel inclusion compounds, as well as the generation of crystalline modifications in different space groups, such as P2i2i2i [30a,b] R3c [60] and Pbcn [67], but the principal structural characteristics of the channel-type host lattices remain virtually unaltered. Structural properties of the 1,10-dibromodecane/urea and 1,12-dibro-modecane/urea inclusion compounds have been determined by single-crystal X-ray... 

Both experimental evidence [63] and Monte Carlo simulation [64] have shown that the removal of guest molecules from urea inclusion compounds leads to collapse of the channel host framework to produce the tetragonal crystal structure of pure urea. Thus there is always a dense packing of guest molecules within the channel structure of urea inclusion compounds [36a]. The migration of molecules into the channel structure of urea inclusion compounds has been probed by high-resolution solid-state NMR spectroscopy [34]. 

The channel-type host structure of 1.14 can be compared with those of the classical urea inclusion compounds, in which the separation between the centers of two adjacent channels is 8.2 A, leading to an effective cross-sectional diameter of... 

Figure 2.5.17 Urea inclusion compounds (e.g., with fatty acids) are only characterized in the crystalline state, but they are formed in aqueous solutions. They provide an industrial storage form for pure polyunsaturated fatty acids. Within urea channels they are not oxidized by oxygen.

The host architecture has some gross structural characteristics, which are repeated in a series of its inclusion compounds. Among the most extensively studied channel-type inclusion coinpounds are the conventional urea inclusion compounds (see Urea), in which the urea molecules form an extensive hydrogen-bonded honeycomb network containing parallel helical tunnels (Fig. la) with diameter between 5.5-5.S A and the tunnel centers separated by ca. 8.2 The walls of the tunnels are... 

Fig. 1 Perspectire view along the channel axis of the honeycomb host network in (a) an urea inclusion compound and (b) a thiourea inelusion compound View, this art m color at www dekker com )...

Thiourea forms channel-type inclusion compounds with globular molecules.This is partly because the space group E3c lacks a screw axis that the space group of the urea inclusion compounds P6i22 possesses. The larger diameter of the channel than in the urea compounds accommodates cyclohexane and monosubstituted cyclohexane as well as carbon tetrachloride and hexachloro-ethane in the channel. With these more or less spherical molecules as the guest species, thiourea inclusion compounds are rich in their polymorphism, most undergoing two or more phase transitions below room temperature. 

Shown m Fig 4a is the iOkl) section of the diffraction pattern from the DBD-Urea inclusion compoimd In urea inclusion compounds, the urea molecules form a hydrogen-bonded network containing hexagonal channels that run along the c-direction The channels can accommodate... 

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 . 

When urea crystallizes, channels with a diameter of 0.8-1.2nm are formed within its crystals and can accomodate long-chain hydrocarbons. The stability of such urea adducts of fatty acids parallels the geometry of the acid molecule. Any deviation from a straight-chain arrangement brings about weakening of the adduct. A tendency to form inclusion compounds decreases in the series 18 0 >18 1 (9) >18 2 (9, 12). 

Selenourea [630-10-4] ]5k.e urea and thiomea can form channel inclusion compounds (87) with a variety of hydrocarbons. Though the difference in channel diameter between thiourea and selenourea is small, selenourea seems to be much more selective for the inclusion of certain guest molecules (eg, cis/trans isomers). 

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