Guest structure

It is not easy to control the steric course of photoreactions in solution. Since molelcules are ordered regularly in a crystal, it is rather easy to control the reaction by carrying out the photoreaction in a crystal. However, molecules are not always arranged at an appropriate position for efficient and stereoselective reaction in their crystals. In these cases inclusion chemistry is a useful technique, as it can be employed to position molecules appropriately in the host-guest structure. Chiral host compounds are especially useful in placing prochiral and achiral molecules in suitable positions to yield the desired product upon photoirradiation. Some controls of the steric course of intramolecular and intermolelcular photoreactions in inclusion complexes with a host compound are described. 

Phthalocyanines have attracted particular attention as potential surface modifiers due to their stability and tendency to form ordered structures directed by dispersion forces. They are inherently host-guest structures with a readily interchangeable coordinating metal ion, which in the solid state results in a tunable bandgap. At a surface, in addition to possibly interesting electronic... 

Table 1. Comparison of structural parameters of GS inclusion compounds and pure guest structures.

Fig. 12 The composite incommensurate structure of Sc-II, as viewed down the c axis. The eight-atom host framework is shown in grey, and the ID guest chains are shown in black. The insets show perspective views of (a) the body-centred guest structure of Fujihisa et al. and (b) the C-centred guest stmcture of McMahon et al. The crystallographic axes are labelled...

Because of the very different cell dimensions and symmetries of the two guest structures, they could be distinguished trivially using single-crystal data, as their... 

Fig. 13 Rietveld refinement of Sc-II at 23 GPa using the C-centred guest structure. Reflections are indexed using their hklm indices. The upper tick marks show the calculated peak positions for hklG) host reflections and (hkOO) host/guest reflections, and lower ones mark (hkOm) guest-only reflections. The difference between the observed and calculated profiles is shown below the tick marks. The inset shows an enlarged view of the asymmetric peak at 17.5°, showing that it is accounted for by the (4400) and (0002) peaks. The vertical dashed line is a guide to the eye...

Fig. 15 2D diffraction pattern from a quasi-single-crystal of Sc-II at 48 GPa. The relative spacing of the layers of reflections from the host (hkl) and guest (hld)Q structures (compare to Fig. 14), and the presence of the (OOl) guest reflection confirms the guest structure to be C-centred... 

Fig. 17 The host-guest composite structure of the tll9 phase of Na shown in projection down the c-axis. The host atoms are shown in light grey and guest atoms in dark grey. The monoclinic guest component unit cell is outlined with dashed lines, and a separate perspective view of the guest structure is shown...

Local structuring the prehydrate structure consists of a locally ordered water-guest structure rather than individual hydrate cavities. 

Mechanisms of interactions between cyclodextrins and enantiomers were reported by Parker and Kataky [15,16]. The complexes have a 1 1 stoichiometry and they all have a host-guest structure. 

Early in the development of the field, compounds were often prepared to define the limits of both the synthetic methods and the stable products that could be formed. To date, many thousands of heteromacrocycles have been prepared. The dominant application of the vast family of host or receptor molecules has been to bind or complex a guest structure. The guests can be metal cations, organic cations, neutral substrates, anions, or complementary molecules. The complexation process can be understood from the simple example of 18-crown-6 complexing K+Cl- in solution. Ignoring structural and solvation/desolvation issues, the process can be described simply as... 

Ionic or molecular recognition by a host molecule will depend upon the degree of structural and electronic complementarity between host and guest. Structural... 

---