Isomerism, architectural

Keywords Architectural Isomerism m Clathrates m Crystal Engineering... 

The architectural isomerism described above, in which a framework based on a specific pillar exhibits two different forms depending upon the guest size, suggests that GS hosts with longer pillars would revert to the bilayer framework for the larger guests. Indeed, we demonstrated that the bilayer-to-brick isomerism observed for G2VI with 1,4-dibromobenzene, 1,4-divinylbenzene, 1-nitro-... 

J. A. Swift, A. M. Pivovar, A. M. Reynolds, M. D. Ward, Template-directed architectural isomerism of open molecular frameworks engineering of crystalline clathrates , J. Am, Chem. Soc 1998,120, 5887-5894. 

The existence of more than one type of network super-stracture for the same molecular building block represents supramolecular isomerism. Therefore, it is related to stractural isomerism at the molecular level. Supramolecular isomerism is the existence of different architectures (i.e., architectural isomerism) or superstructures. Polymorphism is a type of supramolecular isomerism but not vice versa. Supramolecular isomerism can be classified as structural (the same components result in different network superstructures), conformational (different conformations of a flexible molecule generate different, but often related, network architectures), catenane (the different maimer and degrees in which networks interpenetrate or interweave), and optical (chirai networks that... 

A large number of terms have been used to describe forms of isomerism in extended supramolecular arrays observed in the solid state. These include architectural isomerism,topological isomerism,and older terms that have specific and often well-defined meanings, in-... 

Frechet and coworkers synthesized porphyrin cored dendrimers (Figure 12) and their architecturally isomeric linear analogues which were... 

In a somewhat different adaption, CAHBs were used to assemble host frameworks based on the cationic metal complex [Co(en)2(ox)]+ and 2,6-naphthalenedisulfonate pillars (Fig. 14) [54]. These compounds exhibited the same kind of architectural isomerism that had been observed in the guanidinium... 

Chain models capture the basic elements of the amphiphilic behaviour by retaining details of the molecular architecture. Ben-Shaul et aJ [ ] and others [ ] explored the organization of tlie hydrophobic portion in lipid micelles and bilayers by retaining the confonuational statistics of the hydrocarbon tail withm the RIS (rotational isomeric state) model [4, 5] while representing the hydrophilic/liydrophobic mterface merely by an... 

That dendrimers are unique when compared with other architectures is confirmed by an investigation on porphyrin core dendrimers and their isomeric linear analogues [63]. The isomers displayed dramatically different hydrodynamic properties, crystallinity, and solubility characteristics when compared to those of their dendritic analogues, and photophysical studies showed that energy transfer from the poly(benzylether) backbone to the core was more efficient in the dendrimer because of the shorter distance between the donor units and the acceptor core. 

Based on the various hybridization states of carbon, (Figure 1.2) at least four major carboskeletal architectures are known [6, 15]. They are recognized as (I) linear, (II) bridged (2D/3D), (III) branched and (IV) dendritic. In adherence with skeletal isomerism principles demonstrated by Berzelius (1832) these major architectural classes determine very important differentiated physicochemical properties that define major areas within traditional organic chemistry (e.g. linear versus branched hydrocarbons). It is interesting to note that analogous... 

The affect of architecture on small molecular properties has been recognized since the historical Berzelius (1832) discovery that defined the following premise substances of identical compositions but different architectures - skeletal isomers - will differ in one or more properties [15]. These effects are very apparent when comparing the fuel combustion benefits of certain isomeric octanes or the dramatic property differences observed in the three architectural isomers of carbon namely graphite, diamond and buckminsterfullerene. 

Dendritic polymers are most often reported to be amorphous, which can be anticipated from their highly branched architecture. However, some exceptions are presented in the literature. Percec et al. [34,35] reported on liquid crystalline (LC) hyperbranched polymers where the LC-phase was achieved by conformational isomerism. Various repeat units of A2B type have been used where a flex-... 

Tertiary amides, such as those associated with prolyl amide bonds frequently influence turn architectures. The importance of the cis Xaa-Pro bond on activity was recognized and proposed to be the source of differentiation in biological activity [86] therefore, isomerization of the prolyl amide bond is central to regulation of protein folding, immunosuppression, and mitosis. These functions are not surprisingly associated with several disease states and thus substitution of the acyl-proline amide bond with the fluoroolefin isostere has received considerable attention. 

Another important concept from organic chemistry that has an impact on polymer structure is that of isomerism. Recall that structural isomers are molecules that have the same chemical formula, but different molecular architectures. For example, there are two different types of propyl alcohols, both with the same formula, depending on where the —OH functional group is placed on the carbon backbone (see Figure 1.53). Another type of isomerism results in stereoisomers, in which the functional groups are... 

Calculations by the MNDO (modified neglect of diatomic overlap) method with full optimization of geometry were carried out for [Zn, N, C, H]+ ions indicating the possible existence of four stable isomers (Figure 7). According to the same semi-empirical method, [Zn, N, C2, H3]+ ions can form nine stable isomeric stmctnres (Fignre 7) In a separate study by density functional theory, self-assembled helicate architectures have been proposed for ions of the [Zn (CN)2 +i] series. ... 

Another example of many interesting architectures prepared by the Newkome group are the isomeric metallodendrimers 5 and 6. The values (versus Fc/Fc + in lOOmM Et4NBF4/CH3CN, GCE) of the two waves of 5 were slightly (ca. 30 mV) more negative than that of 6. This difference might result from differences in solvent environment between the two isomers or differences in the relative interaction of the counterions with the electroactive moieties.96 97... 

In this instance, Ziegler-Natta polymerization yields a soluble, linear polymer 2, containing a six-membered cyclic ring fused at each repeat unit. Unfortunately, this polymer undergoes isomerization to form a non-conjugated polymer, disrupting the electronic properties of the backbone [31]. It was found that this isomerization could be prevented by the introduction of heteroatom functionality into the diyne architecture, as exemplified by the polymerization of propiolic anhydride 3, which yielded a stable polymer 4 as shown in Scheme 11 [32]. 

Supramolecular isomerism Supramolecular isomerism has been defined by Zaworotko64 as the existence of more than one type of network superstructure for the same molecular building blocks, and hence he adds that it is therefore related to structural isomerism at the molecular level. In cases where the molecular building blocks are capable of forming more than one type of supramolecular synthon then supramolecular isomerism is identical to polymorphism. Zaworotko defines another kind of supramolecular isomerism, however, in which the same building blocks exhibit different network architectures or superstructures. We will see examples of this phenomenon in chapter 9, particularly regarding interpenetrated networks. 

The relationship between chirality and motion in interlocked architectures is not limited to translational (or rotational) isomerism. In some cases, the characteristics of the mechanical bond can also help express or suppress the expression of chirality. 

Many other examples of outwardly complex molecular structures, whose salient architectural features appear to self-assemble from their constituent building blocks, have been documented [16]. The formation of the DNA double helix from its constituent chains is perhaps the quintessential example, whilst the perfect reconstitution of the intact tobacco mosaic virus from its constituent RNA and protein monomers also exhibits all the hallmarks of a cooperative self-assembly process [17]. The same is true of ribonuclease. Reconstitution of this enzyme in the presence of mercaptoethanol, to allow reversible exchange of the four disulfide bridges, proceeds smoothly to generate eventually only the active conformation from many possible isomeric states [18], In each of these cases, the thermodynamic stability of the product is vital in directing its synthesis. These syntheses could therefore be termed product-directed. 

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