Host and guest

In our opinion the lower frequency band found by Korzeniowski et al. (1977 b, 1981) in 1 1 mixtures of host and guest at wavenumbers which are almost identical ( 5 cm-1) with that of the free diazonium ion may be a charge-transfer band. 

N. ..N axes parallel to each other) with the trityl groups in contact and the space between the diketopiperazine rings filled by methylene chloride. In the resulting structure every guest moiety is within pseudo-cage-type voids surrounded by four adjacent hosts, representing lattice inclusion which is not assisted by any specific coordination between host and guest. 

The possibility to resolve the two enantiomers of 27a (or 26) by crystalline complexa-tion with optically active 26 (or 27a) is mainly due to differences in topological complementarity between the H-bonded chains of host and guest molecules. In this respect, the spatial relationships which affect optical resolution in the above described coordination-assisted clathrates are similar to those characterizing some optically resolved molecular complexes S4). This should encourage additional applications of the lattice inclusion phenomena to problems of chiral recognition. 

Furthermore, all the examples shown above in Sects. 4.1-4.3 emphasize the significance of both steric and functional features in selective crystallizations. The latter are needed not only for binding between the individual host and guest constituents, but also for effecting a continuous and relatively unflexible pattern of intermolecular arrangement in the crystal lattice. This observation appears to be a very useful one in the systematic design of novel clathrate-based synthetic receptors. 

Interrelations Attributable to Special Host and Guest Features. 109... 

Beside a crystal cavity of suitable size, additional information is required, e.g. designed polarity gradients in the cavity or carefully located and specific binding sites, respectively. At best, the crystal inclusion will enjoy an ideal lock and key relation 23 between host and guest components from a chemical and spatial point of view. 

The structures of the acetic acidS0) and of the propionic acid71 inclusions of 26 (Fig. 20, type III) are isomorphous to each other. The increased guest volume with respect to formic acid yields 1 1 stoichiometry, with no H-bonds between host and guest molecules in either case. The tunnel where the dimers of guests are situated (see Fig. 32 a) is functionally the same as in the case of the self-dimerized pairs of the formic acid guests. 

The basic type I is represented by the 1 1 coordinatoclathrate of 20 DMSO 851 (Fig. 24). It is seen that an O—H. .. O interaction occurs between host and guest. Besides, one of the methyl C-atoms is proximal to the fixing —COOH thus forming a co-planar pseudo-ring arrangement of six non-H atoms (8-membered ring including... 

Inclusion Compounds Without Specific Binding Contacts Between Host and Guest Apolar Molecules as Guest Species... 

Fig. 27. Packing relations in the crystal structure of 47 benzene (1 1) 64>. Stereo drawing of complementary stick style and space filling representations of host and guest molecules, respectively (atomic radii of the corresponding guest atoms in the space filling style are set to about half of their common van der Waals values the H atoms of the host molecules are omitted)...

In accordance with this behavior, specific interactions between host and guest molecules are not indicated in the structures. The guest molecules take an inclination of approximately 45 degrees to both planes of the rectangular aromatic systems of... 

One of the aims of the crystallographic studies is to visualize the spatial conditions of non-H-bond type of interactions. Van der Waals forces (dispersion and exchange repulsion) and polarization are representatives of such interactive forces. They are governed by geometric features such as contact surfaces and volumes of the host and guest matrices. 

A further example of the steric fit and thus the conditions of the second rank interactions between host and guest is illustrated by the channel structure of the acid inclusions of 26 (see inclusion compound with acetic acid, Fig. 32a). The tunnel has a mostly hydrophobic character being made up mainly from the aromatic portions of the roof-shaped host molecule. We must note that this arrangement applies possibly for the acetic acid clathrate of 1 as well. 

Fig. 34. Stereo drawing of the packing in the 20 DMSO clathrate 851 (complementary space filling and stick style representations of host and guest molecules, respectively O atoms of the host are shaded). Space around guest molecules in the center of the drawing, related by the symmetry center operator, indicates the opportunity for disorder...

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