Guest molecules, topological

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. 

J. A. Swift, M. D. Ward, Cooperative Polar Ordering of Acentric Guest Molecules in Topologically Controlled Host Frameworks , Chem. Mater. 2000,12,1501-1504. 

Figure 3.15 The structure of [Tb(btc)(DMF)2] nH20. (a) Each Tb center is connected to six other Tb centers through four ligands, (b) The 13.5 x 7.6 rectangle channels viewed down the c direction (Tb, black O, grey C, white H, omitted DMF and H2O guest molecules in (b) have been removed for clarity). (Redrawn from the CIF file of Z. Li et al., Synthesis, structure, and luminescent and magnetic properties of novel lanthanide metal-organic frameworks with zeolite-like topology, Inorganic Chemistry, 46 (13), 5174-5178, 2007 [74].)...

It is clear that in the case of MFI, the zeolite pore entrances should not be considered as rigid apertures. Instead, zeolite framework topologies can show flexibility. While the O-Si-0 angle in the tetrahedral unit is rigid (109 + 1 °), the Si-O-Si angle between the units can vary between 145 and 180°. Based on isomorphous substitution of Si by other T-atoms in the framework [18], framework defects [19], cation positions, changes in the water content [16], external forces on the crystalline material [20] and upon adsorption of guest molecules [21] phase transitions can occur that have a dramatic influence in particular cases on the framework atom positions. 

Coming back to the case of polyoxometalates, Rohmer et al. [29] found that electronically inverse host anions are formed in solution by means of a template mechanism which tends to maximize the electrostatic potential at the place of the guest anion. These authors also provided a correlation between the topology of the host and its molecular electrostatic potential, which explains on the basis of simple geometric considerations the difference between electronically normal and electronically inverse hosts. With the aid of this correlation it can be shown that the host cage tends to adapt not only to the shape of the guest molecule, but also to its electrostatic potential distribution [29]. 

Cyclodextrin complexation also depends on a suitable molecular topology that allows the guest molecule to ht within the hydrophobic host cavity. Cyclodextrins form inclusion compounds with hydrophobic guest molecules in... 

The tetranuclear complexes mentioned above are endowed with a substantial cavity, which should, in principle, be suitable for the uptake of guest molecules. A notable feature of these adamantanoid complexes is their topological equivalence to... 

The basic idea is that the channels and polyhedral cavities of a zeolite crystal can be placed in correspondence with the bonds and vertices of a d — 3 dimensional, finite Cartesian lattice. Hence, the problem of determining the trajectories of guest molecules through zeolite crystals of finite extent is modeled by studying the random walk problem on, here, finite lattices of two different topologies. 

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