Inclusion structures

Fig. 10. Stereoview of the inclusion structure formed by host 6 with acetonitrile, showing dipolar pairing of the host molecules. The latter are nearly planar, and contain an intramolecular hydrogen bond. The —NH site of the host is approached by the nitrogen end of acetonitrile, forming an N—H... N bond (crystal data a = 12.977, b = 8.021, c = 16.830 A, P = 106.95°, space group P2Jc) 36>...

Fig. 17. Stereoview of the inclusion structure formed by host II with diethyl ether (crystal data a = 9.092, b = 24.809, c = 9.135 A, p = 102.60°, space group P2,) (taken from Ref.4I))...

The formation of such materials may be monitored by several techniques. One of the most useful methods is 1H- and 13C-nmr spectroscopy where stable complexes in solution may give rise to characteristic shifts of signals relative to the uncomplexed species (43). Solution nmr spectroscopy has also been used to detect the presence of solid inclusion compound (after dissolution) and to determine composition (host guest ratio) of the material. Infrared spectroscopy (126) and combustion analysis are further methods to study inclusion formation. For general screening purposes of solid inclusion structures, the x-ray powder diffraction method is suitable (123). However, if detailed structures are required, the single crystal x-ray diffraction method (127) has to be used. 

The best-known example of an achiral molecule forming a chiral inclusion structure is that of the hexagonal urea tubulate [4] lattice (Figure 1). Helical assembly... 

Figure 1 Projection in the ab plane of the host lattice of a typical urea tubulate inclusion structure in space group P6j22 with the guest molecules omitted. Atom designators O stippling, N horizontal hatching, C and H unshaded.

The work reported here has shown that inclusion complexation of organic and organometallic chromophores by thiourea, TOT and cyclodextrins can induce second harmonic generation capability in the polar crystals which result, even when the original bulk materials are themselves incapable of SHG. Structural evidence has been presented to show tht the solid state inclusion structures are acentric, and a simple electronic picture t0 the polarization response of these materials within the two-state modeP ° has been discussed. In an earlier section we remarked that of the many complexes we have made, only one has NOT been acentric. This result was not anticipated. We postulate that it is a natural tendancy in such materials, rather that an exception. If we consider a dipolar molecule in isotropic solution, we can imagine that if it were to aggregate, it would do so in a head to tail fashion in order to minimize electrostatic repulsion. The situation is illustrated in Scheme 3. The arrangement that would result is centrosymmetric. 

Fig. 10a-f Examples of the structural motifs of p-ferf-butylcalix[4] arene derivatives a guest-free structure b 1 1 complex c 2 1 host-guest complex d hydrogen-bonded structure with an amine guest e self-inclusion structure f 1 1 host-guest clay mimic structure. (Adopted from [44] with permission)... 

Nine subcells with different volume fractions (corresponding to linear grading) are combined to represent the graded part (fig. 1, left and middle). Due to the fact that these models can only represent a pure matrix/inclusion structure, a switch from nickel-matrix to alumina-matrix is necessary. To investigate the particular influence of the effective matrix phase for the center sublayer two different crises are considered, viz. the 50% layer shows a nickel-matrix (marked as Ni) or an alumina-matrix (marked as A1203). 

At high pressure (9 MPa at room temperature) and low temperature (90 K) the oxygen inclusion structure is obtained [17, 18]. Oxygen molecules are coherently aligned in a linear fashion and the chain structure is most likely regulated by the channel structure. Since the observed temperature is higher than the melting point... 

A particularly obvious characteristic of the Ca + entry blockers is their diversity of structure. This diversity makes it quite unlikely that a single inclusive structure-activity relationship exists for this class of compounds. Consequently it is probable that there exist multiple modes and sites of action... 

Fig. 14.7. Inclusion structures of tranllast/y-CyD complexes with 2 1, 1 1, and 1 2 (guest host) stoichiometries.

Fig. 14.8. Proposed inclusion structures of tolbutamide/m-CyD (upper) and tolbutamide/yS-CyD (lower) complexes.

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