Supramolecular weak interactions

Liquid crystal behavior is a genuine supramolecular phenomenon based on the existence of extended weak interactions (dipole-dipole, dispersion forces, hydrogen bonding) between molecules. For the former two to be important enough, it is usually necessary for the molecules to have anisotropic shapes, able to pack efficiently so that these weak interactions can accumulate and co-operate, so as to keep the molecules associated in a preferred orientation, but free enough to move and slide, as they are not connected by rigid bonds. 

Halogen-halogen interaction as the supramolecular synthon has a significant potential in the presence of other weak interactions (for example, CH/n inter-... 

Finally, we wish to emphasize that, to the best of our knowledge, only the present theory can consistently explain the whole molecular recognition systems and present a global and unified view to understand the sophisticated supramolecular interactions in chemistry and biology. It is also said that the molecular recognition phenomenon through cooperative weak interactions is synonymous to entropy-governed chemistry. 

Since the metal center in gold complexes usually displays acid character, Au- -NM contacts will be more easily formed as the basicity of the non-metal increases. Thus, there are many gold- -halogen weak interactions between cationic gold complexes and halides or anions of which the halogen forms part, but most are individual contacts between ions that do not generate supramolecular structures. For example, about 50% of Au- -Cl contacts are isolated cation- -anion interactions. 

Although selenium derivatives of gold occur less frequently than sulfur ones, the presence of Au- Se intermolecular weak interactions has also been reported in a number of selenium complexes. For example, the cations of the tetranuclear complex [Se(AuPPh3)4](CF3S03)2 [40] are paired across symmetry centers to form loose dimers via an [Au- -Se]2 unit (Figure 5.21), which resembles the supramolecular motif usually found in sulfur compounds of gold. The intermolecular Au---Se... 

Three different viewpoints on the humic substances structural conformation are actually reported in the literature. One suggests that HS are macromolecular and assume random coil conformations in solution (Swift, 1999) a second proposes that HS are molecular associations of relatively small molecules held together by weak interaction forces, thus forming a supramolecular structure (Piccolo and Conte, 1999) a third considers that HS are in solution as micelles or pseudomicellar structures (Wershaw, 1999). Viewpoints two and three could be broadly considered to be under the same umbrella (Clapp and Hayes, 1999). 

Since the original definition of supramolecular chemistry was coined by Lehn several corollaries have emerged. One that has risen to great importance is the idea of a dynamic combinatorial library of molecular components that self-sort to generate supramolecules with reactive termini which are then predisposed to form covalent bonds. The effects of weak interactions together with geometric and steric constraints lead to the formation of far fewer products than would be predicted by pure statistics. This development will be discussed in greater detail later. 

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