Guest Order and Disorder

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 

The origins of these effects were identified by a full X-ray crystal structure determination, which shows remarkable hydrogen bonding interactions between certain urea NH groups and the carbonyl 

such as 2-decanone, cooling to -85 °C also results in a similar commensurate structures. 

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The lattice gas approach is valid within certain limits for typical metallic hydrides, binaries as well as ternaries. Deviation from this idealized picture indicates that metallic hydrides are not pure host-guest systems, but real chemical compounds. An important difference between the model of hydrogen as a lattice gas, liquid, or solid and real metal hydrides lies in the nature of the phase transitions. Whereas the crystallization of a material is a first-order transition according to Landau s theory, an order-disorder transition in a hydride can be of first or second order. The structural relationships between ordered and disordered phases of metal hydrides have been proven in many cases by crystallographic group-subgroup relationships, which suggests the possibility of second-order (continuous) phase transitions. However, in many cases hints for a transition of first order were found due to a surface contamination of the sample that kinetically hinders the transition to proceed. 

FIGURE 13.6, Scattering curves for the host and guest molecules in a / -cyclodextrin (host) - 2iV-acetylphenylalanine methyl ester (guest) complex. Data for the host (J cyclodextrin) are indicated by solid lines, and for the guest (the methyl ester) by broken lines. The guest is somewhat disordered in the channel provided by the more ordered structure of the crystallized host (Ref. 24). 

Over the past four years there has been considerable interest in the NMR of Hofmann-type and related metal-cyanide inclusion compounds, with regard to local order or disorder and the dynamics of both guest and host molecules. 

Metallocenes (but only with unsubstituted cyclopentadienyl rings) also form thiourea inclusion compounds. X-ray diffraction and Mdssbauer data on the ferrocene inclusion compound show that at 295 K the guest molecules are orientationally disordered at the sites of 32 symmetry. A phase change at 162 K generates a more ordered structure78). 

In order to find a possible disordering of the guest, the isomorphous replacement method was used. 1-methylnaphthalene was replaced with 1-bromonaphthalene, and the resulted inclusion compound was isostructural with the methylnaphthalene one. 

Low and high are relative to the guest species, meta-bromonitrobenzene shows the transition at about 60°C, while meta-dinitrobenzene shows transition of this sort at about 95°C. Description is somewhat simplified here. The ordered guest molecules shown on the picture still have some 10-15 % of disordering over two different orientations. This, however, is qualitatively different from the symmetric disordering around the threefold axis. 

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