Two-dimensional hosts

8—r Boudon, S., Decian, A., Fischer, J., Hosseini, M. W., Lehn, J.-M. and Wipff, G., Structural and anion coordination 

In order to avoid the problem of increasing difficulty in fully protonating macrocycles like hexacy-clen and higher analogues, which have two carbon bridges between the nitrogen atoms, a number of 

anion binding behaviour is similar in type to that observed for compounds of type 4.14, however, with binding constants increasing with anion charge. For anions of the same charge, structural effects were observed in which anions that matched the symmetry of the macrocycles were bound most tightly (e.g. three-fold axis of SO/ with 4.13-6H+ four-fold symmetry of squarate, C4O/, with 

Another interesting class of anion receptors based upon protonated nitrogen atoms are the expanded porphyrin macrocycles such as 4.17 (diprotonated sapphyrin) and compound 4.18. The tetrapyrrole porphyrin macrocycles are excellent hosts for metal cations such as Fe and Mg + (e.g. haemoglobin and chlorophylls. Sections 2.3-2.5) however, their cavity dimensions are too small to accommodate anions. Conversely, expanded porphyrins such as 4.17 comprising five or more pyrrole residues present a rigid macrocyclic cavity about 5.5k in diameter, in which (particularly when protonated) the NH 

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Fig. 7.2 Classification of intercalation compounds (a) host of chains weakly bonded together (b) three-dimensional host with one-dimensional lattice of sites for guest ions (c) layered host (two-dimensional host and two-dimensional lattice of sites) (d) three-dimensional host with three-dimensional lattice of sites.

The insertion of polymers, with electrical and ionic conductivity, into two-dimensional host structures continues to be a growing field of research in materials science/chemistry. As seen in this review, the technique of intercalation depends on the layered structure that is being investigated. It should also be pointed out that there is a huge repertoire of layered systems that is at the disposal of the researcher, providing the opportunity to create a wide range of nanostructured materials with specific applications. 

Transputers. At higher levels of coimectedness there is a wide variety of parallel computers. A great many parallel computers have been built using INMOS Transputer chips. Individual Transputer chips mn at 2 MELOPS or greater. Transputer chips have four communication channels, so the chips can readily be intercoimected into a two-dimensional mesh network or into any other interconnection scheme where individual nodes are four-coimected. Most Transputer systems have been built as additions to existing host computers and are SIMD type. Each Transputer has a relatively small local memory as well as access to the host s memory through the interconnection network. Not surprisingly, problems that best utilize local memory tend to achieve better performance than those that make more frequent accesses to host memory. Systems that access fast local memory and slower shared memory are often referred to as NUMA, nonuniform memory access, architecture. 

Fig. 32a and b. Schematic illustration of intermolecular arrangements in the crystalline complexes of host 25 (taken from Ref.25>) a the two-dimensional hydrogen bonding pattern parallel to the ab plane (the shaded area represents the 1,1-diphenylcyclohexane framework) b the van der Waals type packing of the hydrogen bonded layers along the c axis (R represents the cyclohexyl ends of the host species)... 

Fig. 6. Diagrammatic (two-dimensional) representation of different modes of lattice inclusions involving coordinative (H-bond) interactions (indicated by broken lines) (a) cross-linked matrix type of inclusion (host-host interaction, true clathrate) (b) coordinatoclathrate type of inclusion (coordinative host-guest interaction, coordination-assisted clathrate)...

Fig. 9. Diagrammatic representation of a bulky host constitution (A) and (a)—(c) of crystal lattice-analogous arrangements of A (two-dimensional versions shaded areas represent the lattice voids)...

Two-dimensional systems layered host, layers of sites for guests... 

Owing to the relatively good crystallinity and the large number of OOZ reflections observed for the hydro calumite derivative, the electron density distribution along the c axis can be estimated using a series of OOZ reflections, in accordance with previous literature [82,83]. One-dimensional electron density calculations based on X-ray diffraction are often carried out to probe the structure of the intercalated species in two-dimensional inorganic hosts [33,84-86]. This yields specific information about the orientation and structure of the intercalated species or at least ehminates certain conformational possibihties, which are incompatible with the diffraction data. In LDH systems, however, such calciflations are usually impossible because the X-ray diffraction patterns of hybrid materials are often very ill defined [87,88]. 

Yamamoto and coworkers used two-dimensional, nuclear Over-hauser effect experiments (NOESY) to determine the proximity of particular protons situated on an included p-nitrophenolate ion to particular protons of a host alpha cyclodextrin molecule. The experiments showed cross-peaks connecting the H-3 resonance of alpha cyclodextrin to both meta and ortho proton resonances of the p-nitrophenolate ion, whereas H-5 of the alpha cyclodextrin gave a cross-peak only with the resonance of the meta proton thereof. As a consequence, it was unequivocally confirmed that the p-nitrophenolate ion is, in solution, preferentially included with its nitro group oriented to the narrow end of the alpha cyclodextrin... 

Note 1 Intercalation can refer to the insertion of a guest species into a one-, two- or three-dimensional host structure. 

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