Conformational isomers, supramolecular

The protein folding, notorious for an astronomic number of possible conformations, is only an example of the multiple minima problem, inherently connected to all applications of theory to structural chemistry (isomers, supramolecular structures etc.). The multiple minima problem is also virtually ubiquitous in other sciences, and whenever a mathematical description is used, the situation is encountered more and more often. Despite the complexity of the protein folding, remarkable achievements in the prediction of the 3D structure of globular proteins are possible nowadays. 

The photoisomerization of (Z)-cyclooctene (30) (Scheme 12) to the (E)-isomer (31) was sensitized by enantiopure alkyl benzenecarboxylates immobilized in zeolite to give modest ees. The use of an antipodal sensitizer pair of (R)-and (S)-1 -methyIheptyl benzoates, 32d and 32e, yielded enantiomeric 31 in — 5% and +5% ee, respectively, while the same sensitizers gave practically racemic 31 upon irradiation in homogeneous solutions. This small, but apparent, enhancement of the product ee observed upon irradiation in modified zeolite supercages is likely to arise from the decreased conformational freedom of the adsorbed sensitizer, the hindered approach of 30 to the sensitizer, and/or the different exciplex structure in confined media. In this context, it is interesting to examine the effect of temperature on the supramolecular photochirogenesis in modified zeolites and to compare the results with those obtained in the homogeneous phase. Such an examination will reveal the distinctly different role of entropy in confined media, which should be clarified in a future study. 

In some Instances, more than one structuie may be possible for a given set of molecular components, and such structures can be described as supramolecular isomers For example, three forms of [Co2(N03)4 I,2-Z /5-(4-pyridyI)ethane 3]n ha e been structurally characterized with different network structures This results from the conformational freedom of I,2-Z /y(4-pyridyI)ethane (Fig 4c), which unlike 4,4 -bipy can adopt gauche or anti conformations... 

Fig. 6 Class III supramolecular isomers of [Co2(N03)4(l,2-bis(pyrid-4-yl)ethane)3]. j(guest), (a) isomer exhibiting ligands with both gauche and anti ligand conformations to afford a one-dimensional polymer (b) isomer again containing both gauche and anti ligand conformations, but in this case, forming a two-dimensional bilayer structure. (View this art in color at WWW. dekke r. com.)...

Supramolecular stabilization means stabilization of certain chemical species, their different forms, and assemblies in a particular supramolecular environment. Supramolecular stabilization is responsible for the existence of "energetically unfavorable" conformations, different spatial isomers, unexpected oligomers, "unstable" molecules. highly reactive radicals, elements in unusual oxidation states, certain ionic species, and so on, in supramolecular phases. Such species decompose, change, or equilibrate with alternative forms once they are brought out of their "stabilizing" environment. The best-known chemical reaction based strictly on supramolecular stabilization is the so-called "blue reaction." where iodine reversibly polymerizes in the presence of starch. 

Temperature has long been used as a variable which influences the crystal form obtained. In the case of isomerism, the kinetic isomer at ambient temperature may become thermodynamically favored at a different temperature. One such example is provided by the coordination polymers with composition [Cu2(tcppda)(H20)2]n (tcppda = V, N, N, yV -tetrakis(4-carboxyphenyl)-l,4-phen-ylenediamine), 1. The tcppda ligand can adopt two conformations, either square-planar C2h or tetrahedral D2. On association with the square-planar Cu2 paddlewheels, these give rise to two supramolecular isomers in the form of... 

Reports of conformational supramolecular isomers of organic materials include that of the compound 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile, 5, which has at least six polymorphs. The main difference in the six phases is the variation in the torsion angle between the thiophene and the o-nitroaniline moieties. Another example involves the two components C-methylcalix[4]resorcin-arene (6) and 4,4 -bipyridine (bipy), which can form three supramolecular isomers (7) in the presence of different... 

Conformational flexibility in ligands can lead to conformational snpramolecular isomers in metal-organic network systems. For example, hydrothermal crystallization using an aminotriazole ligand afforded two 3D supramolecular... 

Luminescence is one such property which has yielded to the study of supramolecular isomers. The absorption and emission spectra of the supramolecular isomers of [Ni(4-bpd)2(NCS)2], 13, where 4-bpd = l,4-bis(4-pyridyl)-2,3-diaza-l,3-butadiene, showed distinct differences which could be attributed to the differing conformations of the bpd ligand in the square-grid framework versus the 6 8 framework structures." Both isomers also... 

In the case of supramolecular species [5, 6], spectroelectrochemical measurements are even more important. They, indeed, enable to establish the more stable isomer among the possible ones, and to evidence the occurrence of electro-chemically induced molecular movements or conformational rearrangements. In this regard, three studies performed on catenanes (supramolecular systems minimally formed by two interlocked rings) are illustrated in the following. 

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