Supramolecular unit bonding

D2) a supramolecular unit is an aggregate of molecular units that is linked by a topologically connected network of noncovalent bonds equivalently, an electronic distribution that links a collection of molecular units by a contiguous network of noncovalent bonds. 

As in the molecular case (Dl), the definition (D2) allows the supramolecular unit(s) to be determined by an operational NBO search of a given electron distribution ir(fi, r2,..., Fjv ) 12. Given the NBO molecular units of the distribution, we can search the intermolecular interactions (e.g., the table of perturbative donor-acceptor stabilizations) to determine the connecting noncovalent bonds that satisfy the required thermal threshold, and thereby determine the contiguously bonded supramolecular unit(s) by (D2). 

Of these crystal structures, 1-phenylethylammonium 2-phenylpropionate (NMACEP, PMACEP, Figure 4.38)115 and 2-phenylbutyrate (PEAPEA, PBUPEA)115,116 are the earliest examples reported by Brianso. The less soluble salt is the combination of the 5-acid with the R-amine (PMACEP and PEAPEA) in each case. The crystal structures of both pairs of the diastereomeric salts (less and more soluble salts) have been determined to show that these two systems have quite similar crystalline characteristics. In both less and more soluble salts a columnar hydrogen-bond network, formed by the carboxylate and ammonium groups, is commonly built around a two fold screw axis two molecules of the carboxylate part and two molecules of the ammonium part form a unit, which is piled up along a 2i-axis to form a supramolecular hydrogen-bond column. 

The crystal structures of six 3,6-diaryl-l,2,4,5-tetrazines have been determined and their molecular packing has been compared to the supramolecular architecture observed in related carboxylic acid dimers. In the tetrazines, covalent N-N bonds are considered to replace the intermolecular O-H- - -O hydrogen bonds of the carboxylic acids. In the system investigated, the covalent six-membered ring of the tetrazine was an appropriate replacement for the carboxylic acid synthon. This apparent interplay between molecular and supramolecular units may have applications in the crystal engineering of new materials (Figure 1) <2003HCA1205>. 

Figure 1 Schemes for (A) linear molecular (small units, strong covalent bonds) and supramolecular (units of various size and shape, secondary bonds) polymerization (B) supramolecular binding of small units to a molecular polymer (C) supramolecular binding of two polymer chains.

The crystal stmcture of PPT is pseudo-orthorhombic (essentially monoclinic) with a = 0.785/nm b = 0.515/nm c (fiber axis) = 1.28/nm and d = 90°. The molecules are arranged in parallel hydrogen-bonded sheets. There are two chains in a unit cell and the theoretical crystal density is 1.48 g/cm. The observed fiber density is 1.45 g/cm. An interesting property of the dry jet-wet spun fibers is the lateral crystalline order. Based on electron microscopy studies of peeled sections of Kevlar-49, the supramolecular stmcture consists of radially oriented crystaUites. The fiber contains a pleated stmcture along the fiber axis, with a periodicity of 500—600 nm. 

Other supramolecular structures such as dendrimers have also been synthesized with zinc-containing porphyrins. Sixteen free base and sixteen zinc porphyrin units were added at the fifth generation of dendritic poly(L-lysine) and intramolecular fluorescence energy transfer was observed.823 Assembly of supramolecular arrays in the solid state has been achieved with the incorporation of an amide group for hydrogen bonding. Zinc meso-tetra(4-amidophenyl)porphyrin... 

Here, the concept of linkage implies only that each intermolecular noncovalent bond is sufficiently large compared with kTto withstand ambient thermal collisions. Thus, for near-standard-state conditions (where kT 0.6kcal mol-1), even weak noncovalent interactions of 1-2 kcal mol-1 may be adequate to yield supramolecular complexes with stable equilibrium populations, thereby becoming true constituent units of the phase of lowest free energy. 

At shorter distances, particularly those characteristic of H-bonded and other charge-transfer complexes, the concepts of partial covalency, resonance, and chemical forces must be extended to intramolecular species. In such cases the distinction between, e.g., the covalent bond and the H-bond may become completely arbitrary. The concept of supramolecular clusters as fundamental chemical units presents challenges both to theory and to standard methods of structural characterization. Fortunately, the quantal theory of donor-acceptor interactions follows parallel lines for intramolecular and intermolecular cases, allowing seamless description of molecular and supramolecular bonding in a unified conceptual framework. In this sense, supramolecular aggregation under ambient thermal conditions should be considered a true chemical phenomenon. 

J. H. K. K. Hirschbeig, F. H. Beijer, H. A. van Aert, P. C. M. M. Magusin, R. P. Sijbesma, E. W. Meijer, Supramolecular polymers from linear telechelic siloxanes with quadruple hydrogen bonded units , Macromolecules 1999, 32, 2696-2705. 

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