Noncovalent bond interactions

Mechanical movements in supramolecular structures rely on modulation of noncovalent-bonding interactions. Such changes occur when charge-transfer interactions between electron-donor and electron-acceptor groups are weakened. 

When a rotaxane contains two different recognition sites in its dumbbell component, it can exist as two different equilibrating coconformations, the populations of which reflect their relative free energies as determined primarily by the strengths of the two different sets of noncovalent bonding interactions. In the schematic representation shown in Fig. 13.8, it has been assumed that the molecular ring... 

The two possible coconformational isomers of such catenanes can be interchanged by appropriate stimuli. In a diagram of potential energy against rotation angle of the asymmetric macrocycle, the two coconformations correspond to energy minima, provided by the intercomponent noncovalent bonding interactions. The... 

In precursor processing, shelf life is rarely a problem as noncovalent bonding interactions between molecules and solvent content define processability. Furthermore, atomic mixing and multimetallic compounds are readily accessible as will be amply demonstrated below. Thus, there is recent interest in developing oxide precursors for processing... 

Fig. 3.1 Noncovalent bonding interactions, models, examples and dependence of energy on distance (r)...

Over the last decade, noncovalent bonding interactions between appropriate Jt-elec-tron-deficient and Jt-electron-rich recognition sites have been exploited in our laboratories for the synthesis of catenanes and rotaxanes. 271 In the example illustrated in Figure 10, the bis(hexafluorophosphate) salt 11-2PF6 was treated 28 with trons-bis(pyridine)ethylene (12) in the presence of the previously formed macrocyclic polyether bis-p-phenylene-34-crown-10 (13). The resulting [2]catenane 14-4PF6 was isolated in 43 % yield after counterion exchange. 

The essential realization in this spontaneous ordering process is the importance of noncovalent bonding interaction between molecules, that is, supramolecular chemistry. These conformation-specific interactions are governed by weak forces including hydrogen bonding, metal coordination, van der Waals forces, pi-pi interactions, and electrostatic Coulombic effects. The cooperative action of multiple noncovalent interaction forces is precisely the path nature takes to produce shape and form. 

Molecules incorporating interlocked [refs. 6, 7] components (Fig. 10.1) and their supramolecular analogs are suitable candidates for the generation of bistable chemical systems. A [2]pseudorotaxane is a supramolecular complex composed of a macrocyclic host encircling a linear guest. The two components are held together solely by noncovalent bonding interactions and they can... 

Allow for the denotation of features that structural diagrams and formulas lack (e.g., noncovalent bonding interactions)... 

Fig. 18 Different representations of the same donor-acceptor [2]catenane. Line drawings (a) and condensed structural formulas (b) were the order of the day until the modem era of MIMs gave way to structural diagrams (c) and crystal structures (d). Graphical representations, also called cartoons (e), can be a helpful compromise between these representations when attempting to emphasize the topology, noncovalent bonding interactions, shape, beauty, or function of a MIM...

We live in an era that beautifies machines, from automobiles to computers. Arguably the most compelling aspect of MIMs - at least those templated by noncovalent bonding interactions - is their susceptibility to controlled mechanical motion of... 

Complexation/decomplexation of metal ions or of neutral organic molecules, protonation/deprotonation reactions, and oxidation/reduction processes can all be exploited to alter reversibly the stereoelectronic properties of one of the two recognition sites, thus affecting its ability to sustain noncovalent bonds [30-34, 41]. These kinds of switchable [2 catenanes can be prepared following the template-directed synthetic strategy illustrated in Figure 5, wherein one of the two macrocyclic components is preformed and then the other one is clipped around it with the help of noncovalent bonding interactions. 

The difficulties somehmes encountered in the synthesis of polytopic hgands might be overcome by the development of efficient synthehc protocols (such as click chemistry) or the use of noncovalent bonding interactions (H bond or coordinahon). tn addihon, the development of conhnuous-flow reachon systems might prove advantageous for the practical apphcations of MOCP catalysts. 

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