Urea inclusion compounds guest molecule

Linear guest molecules are included along these canals in an extended planar zig-zag conformation. Branched molecules are generally excluded unless branching occurs near the end of a long linear chain, but aromatic derivatives can be included if they have a long alkyl chain 38). The review article by Takemoto and Sonoda 21) contains an excellent survey of the types of molecules known to form urea inclusion compounds and of the means used to study their detailed conformations and thermal motion. 

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]. 

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. 

George. A.R. Harris, K.D.M. Properties of the guest molecules in the 1.10-dibromodecane/urea inclusion compound. J. Mater. Chem. 1994. 4. 1731-1735. 

The empty urea tunnel stmeture is unstable, and it has been shown by experiment and computer simulation that the tunnel collapses if the guest molecules are removed from the inclusion compound, leading to the pure crystalline phase of urea, which does not contain empty tunnels. The instability of the "empty" urea tuimel stmeture clearly limits the potential for developing certain applications of urea inclusion compounds. 

In this article, an overview of stmctural aspects of urea inclusion compounds is provided, and some of the interesting properties of these materials, such as the dynamic and conformational properties of the guest molecules, chemical reactions, and applied aspects, are elaborated upon. Also highlighted are the types of techniques used to investigate these properties. Several review articles have been published on urea inclusion compounds,and they contain comprehensive lists of references to the original literature in this field. The reader is encouraged to consult these review articles, and the original papers cited therein, in order to obtain more information. 

Although the focus in this article is on the conventional urea inclusion compounds, we note that the inclusion compounds formed between urea and certain specific guest molecules arc commensurate tunnel stme-tures. For such commensurate systems, the host stmeture is usually distorted from the hexagonal tunnel stmeture shown in Fig. I. Examples are the inclusion compounds formed between urea and the guest molecules 1,6-dibromohexane, - " sebaconitrile and (a + l),(to - 1)-... 

In both cases, the host structure in the low-temperature phase is orthorhombic and is based on a small distortion from the orthohexagonal description of the high-temperature structure. The structural relationship between the host and guest substructures along the tuimel remains incommensurate. Other urea inclusion compounds, such as 1,10-decanedicarboxylic acid/urea exhibit more complicated structural behavior in the low-temperature phase, with the formation of large superstructures in directions perpendicular to the tuimel axis. Thus, the exact nature of the distortion of the host structure in the low-temperature phase depends critically on the type of guest molecule. 

There have been various attempts to rationalize the phase transitions observed in urea inclusion compounds. One of these approachesdraws an analogy between the phase transition in alkane/urea inclusion compounds and the order-disorder phase transitions in alkali cyanide crystals and proposes that coupling between transverse acoustic phonons of the host structure and the orientational order of the guest molecules provides an indirect mechanism for orientational ordering of the guest molecules in the low-temperature phase. 

At sufficiently low temperatu es, most conventional urea inclusion compounds undergo phase transitions associated with changes in the symmetry of the host structure and changes in the dynamic properties of the guest molecules. For alkanelurea and oc.co-dibromoalkane/urea inclusion compounds, structural and dynamic " aspects of these phase transitions have been investigated extensively using a variety of techniques. Qualitatively, the alkane/ urea and a,co-dibromoalkane/urea inclusion compounds behave in a siinilar way with respect to these transitions. 

Guest molecules in solid host structures are often constrained to exhibit uncharacteristic conformational properties, which can be exploited as a means of carrying out spectroscopic characterization of such conformations. For example, guest molecules in the 1,6-dibromohexane/urea inclusion compound (a commensurate system) exist exclusively with both bromine end groups in the gauche conformation allowing a definitive characterization of the vibrational properties of this conformation. 

Several techniques have been used to investigate dynamic properties of urea inclusion compounds, including solid-state NMR, incoherent quasielastic neutron scattering ESR, molecular dynamics simulation, Raman, infrared, dielectric loss, and x-ray diffraction. In addition to investigations of the dynamics of the guest molecules, the dynamic properties of the urea molecules have also been studied. 

Early studies of guest motion in urea inclusion compounds focused on solid-state H-NMR, with measurements of line widths, second moments, and spin lattice relaxation times. Subsequent NMR work has mainly involved H-NMR of urea inclusion compounds containing fully deuterated or selectively deuterated guest molecules. These experiments probe the quad-... 

Although the alkanelurea and a,co-dibromoalkane/urea inclusion compounds exhibit similar dynamic behavior, the dynamic properties of other urea inclusion compounds can differ substantially. The dynamic properties of dioctanoyl peroxide guest molecules have been investi-gated by a combined approach employing IQNS and H-NMR. Both techniques indicate that the reorientation of the guest molecules about the tunnel axis can be described by a model of uniaxial rotational diffusion in a... 

Much early work relating to chemical reactions in urea inclusion compounds focused on studies,primarily using ESR spectroscopy, of radicals generated from the guest molecules by x-ray irradiation, and provided fundamental information about the spin density and its anisotropy in a variety of radical species. Subsequently, a number of other workers have used ESR spectroscopy to identify radicals formed by x-ray or y-ray irradiation and to study the dynamic, chemical, and electronic properties of these radicals.[ Details of these studies were reviewed elsewhere. 

The photolysis processes of alkanone guest molecules in urea inclusion compounds have also been studied and have been shown to involve Norrish Type 11 reactions. For 5-nonanonelurea and 6-imdecanone/urea, the reactions show an almost complete diastereoselective preference for the Z-cyclobutanol product. 

Papers concerning the physical properties of polymers as the guest components in urea inclusion compounds and polymerization reactions of guest monomer molecules within the urea tunnel structure have been reviewed elsewhere. The polymers studied included poly (ethylene), poly (acrylonitrile), poly (1,3-butadiene), poly(eth-ylene oxide), poly(tetrahydrofiiran), poly(acrolein), poly(vinyl chloride), poly(ethyl acrylate), poly(lactide), poIy(lactic acid), poly(ethylene adipate). poly(ethylene succinate), acrylonitrile-ethyl acrylate copolymer, and poly(hexanediol di acrylate). 

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