Inclusion host-guest orientation

Host-Guest Orientation and Geometry of CyD Inclusion Complexes... 

In the case of a-CD — m-nitrophenol(MNP) inclusion-conplex, there are two possible geometries as shown in Fig.2. Namely, the first is that the nitrophenyl group is included and the hydroxyl group is protruded from the a-CD cavity. This structure has been proposed based on the above-mentioned characteristic carbon-13 shifts of MNP observed in aqueous solution [4]. X-Ray crystallographic analysis of solid complex has also support this type of inclusion [8]. The second is the reverse of the first one, and has been proposed based on also carbon-13 chemical shifts in aqueous solution [9]. Here we tried to determine the host-guest orientation and position of the guest MNP in the a-CD inclusion-coirplex in aqueous solution by the quantum chemical analysis... 

Four out of the five associates in our studies display 1 1 host guest stoichiometry (those of 1, 20, 26, and 37). Only the corresponding inclusion of 41 has a 1 2 stoichiometry. This difference could be linked up with the altered orientation of the vicinal carboxyl groups. 

With simple crowns, complex formation may involve various degrees of inclusion of the guest into the cavity of the crown and a conformational rearrangement of the crown is almost always necessary for strong complexation to occur. This will normally involve a redirection of the donor electron pairs on complex formation so that their final orientations optimize a particular host-guest interaction. 

Solid inclusion complexes of photoreactive ketones with cyclodextrins as hosts (Figure 6) provide interesting examples of how a fairly stiff, somewhat heterogeneous (in terms of polarity, size, and guest orientation) reaction... 

Dianin s compound 8 is a famous example of a lattice inclusion host from the early days of inclusion chemistry [26,27], It is usually prepared as the race-mate and forms inclusion compounds with many guests in space group R3. Six molecules of 8 assemble by means of their phenolic groups into a near-planar hydrogen-bonded (-0-H)6 cycle, with alternate molecules being orientated up... 

The results discussed above highlight the fact that topochemical host-guest reactions in solid inclusion compounds are sensitive to the size and shape of the inclusion cavities and the relative orientations of the host and guest molecules within the crystal structure. Furthermore, these examples demonstrate that reactions in solid inclusion compounds can be associated with high selectivities that are difficult to achieve with conventional synthetic methods in the solution state. 

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