Semi-clathrates

In contrast to the examples described above, there are partially disordered structures which may mimic ordered ones but with false or deformed geometry of molecular moieties. As a convenient example, the semi-clathrate hydrate1 of tetra(iso-amyl)... 

The term semi-clathrate refers to solid inclusion compounds where, in addition to non-bonded, clathrate interactions, there is a contribution of chemical bonding. In the example used above, this chemical contribution is of ionic nature fluoride anion from the ammonium salt is incorporated into the water host framework linked via hydrogen bonds. Thus, the host has anionic character and guest-host interactions have ionic component. 

An interesting example where infrared O-H frequencies were used to correlate structures is for choline chloride dihydrate, which is postulated to have a semi-clathrate hydrate structure by analogy with the known crystal structure of tetraethyl ammonium fluoride pentahydrate [162]. 

Unfortunately, none of the hydrate inclusion compounds which have less regular structures, such as the tetrabutyl and isoamylammonium salt hydrates [432] and the amine semi-clathrates [433, 434], has been studied by neutron dif-... 

In the alkylamine hydrates, with one exception, the functional group of the amine is hydrogen-bonded to or included with the water host lattice. These compounds are not true clathrates i.e., they, are sometimes referred to as semi-clathrates [433]. 

Here the layers contain only water molecules which form antidromic pentagons, quadrilaterals and homodromic hexagons (Fig. 21.10). Clathrate or semi-clathrate structures have been postulated for choline chloride hydrate, (H3Q3N+CH2CH2OH 2H20 CP, on the basis of similarities in the solid-state infrared spectra [162], but this has not been confirmed by crystal structure analysis. 

The majority of alkylamine hydrates are semi-clathrates . As Ihble 21.4 indicates, in all alkylamine hydrates except tert-butylamine, the amine molecule is hydrogen-bonded to the water framework, which, nevertheless, retains the cagelike structure. For this reason, these compounds are described as semi-clathrates . 

Figure 6 Simulated powder X-ray diffraction pattern by Cu Ka radiation following previonsly reported structural data for (a) tetragonal Ar hydrate at 0.7 GPa, (b) filled ice structure of CH4 hydrate at 2.0GPa, ° and (c) tetra-w-butyl anunonimn bromide (TBAB) semi-clathrate hydrate. Insets show each crystal structure used for the simulation.

Gholinezhad J, Chapoy A, Tohidi B (2011) Separation and capture of carbon dioxide from CO2/H2 syngas mixture using semi-clathrate hydrates. Chem Eng Res Des 89(9) 1747-1751... 

The strong affinity of ionic liquids for aromatics has been attributed to the formation of liquid clathrates (90-92). Liquid clathrates (93) are semi-ordered liquids containing complex salt hosts. They are formed by associative interactions between aromatic molecules and salt ions, which separate cation-anion-packing interactions to a sufficient degree that localized cage structures are formed. Although the aromatic compounds are highly soluble in the ionic liquid phase, the... 

Fig. 21.6 a -c. Other polyhedral voids observed in the clathrate hydrate structures, a tetrakaideca-hedron, 51262 b pentakaidecahedron, 51263 c hexakaidecahedron, 5t264. Unlike the dodecahedron (Fig. 21.3) which is an regular solid, these are semi-regular solids. If the edges and angles in a face are equal, the faces cannot be exactly planar... 

Systematic X-ray crystallographic studies have provided a structural rationale for the inclusion behaviour of tetraphenylene, leading to a semi-quantitative assessment of the effective molecular sizes and shapes of a wide range of encaged guest species. However, much work remains to be done in the spectroscopic, thermodynamic and theoretical investigation of this class of clathrates. 

In the past, this has been studied semi-quantitatively by means of NMR second moments and NMR quadrupole echoes (5,6,7). Here we show that at high temperatures ( -iyOK) and NMR lineshapes can be obtained with sharp, well-defined features, so that quadrupole coupling or nuclear shielding (chemical shift) tensor components can be measured directly to yield information on guest molecule orientation in the clathrate hydrate cages. 

---