Supermolecules hydrogen-bond complexes

Clearly, the most satisfactory way to decide between conflicting concepts of the structure and nature of the hydrogen bond is to treat quantum-mechani-cally a hydrogen-bonded complex as a single large molecule entity with no truncation and to compare the results obtained for this supermolecule to those obtained for the separated molecules treated in the same approximation. This mode of approach is now possible, and a number of such computations using both all-valence electrons methods and the SCF MO non-empirical procedure have recently appeared. The references pertinent to biochemistry have been listed in Tables I and II. These concern only various hydrogen-bonded amides and the base pairs of the nucleic acids. 

The well-established perturbation theory of intermolecular interaction [53 59] can be applied to hydrogen-bonded systems in combination with the frozen molecule approximation, when the interaction is either sufficiently weak [60 62], or when the interaction is treated at a more qualitative level. When the interaction becomes larger, structural relaxations become sizable. Then the more usual approach to treat the hydrogen-bonded complex or cluster as a supermolecule becomes more practical and also more appropriate. However, also in this case, the detailed analysis of the interaction energy is often done with the aid of different variants of energy partitioning techniques [63,64] which closely follow the lines of intermolecular perturbation theory. 

A ferris wheel assembly involving a 1 1 complex of 19 and metallated [18]crown-6 is found in the cationic supermolecule [La(H20)3([ 18]crown-6)] (19+2H) + [48]. The lanthanum ion is coordinated by one calixarene sulfonate group, the [18] crown-6 and three aquo ligands, and the metallated crown sits inside the calixarene cavity. A helical hydrogen bonded chain structure is formed between the cationic assembly, water and chloride ions. The ferris wheel structural motif is also found in Ce3+ complex which simultaneously contains a Russian Doll assembly [44]. 

The crystal structure 57 of the strong and selective complex formed by the terephthalate dianion with a hexaprotonated macrobicyclic polyamine shows that it is a molecular cryptate 56 with the dianion tightly enclosed in the cavity and held by formation of three hydrogen bonds between each carboxylate and the ammonium groups [4.19]. Both structures 53 and 57 illustrate nicely what supermolecules really are they show two covalently built molecules bound to each other by a set of non-covalent interactions to form a well-defined novel entity of supramolecular nature. Acyclic [4.20a,b] and macrobicyclic [4.20c] hydrogen bonding receptors... 

V=9.267(1) nm, Z=4, Ri=0.0S3 and Rw=0,058. The complex is composed of copper cations, nitrate anions, 1,10-phenanthroline, protocatechuic acid and lattice water molecules. The structure of H3PCA, N03 and waters comprises packing of three-dimensional network by hydrogen bonds with cavities. The complex can be considered as a model of host/guest supermolecule. The three-dimensional hydrogen-bonding network is the host species. The Cu(phen)3 cations, guest species, occupy the cavities of the host. 

In this study, we have chosen the supermolecule approach and have used the semi-empirical quantum mechanical method called PCILO (Perturbative Configuration Interaction using Localized Orbitals) (16) to calculate intermolecular interactions. This method has recently been used successfully to calculate the intermolecular energies and geometries of hydrogen-bonded dimers of hydrocarbons and water (17,18). H-bonded complexes are particularly well characterized by this method (19). 

At this point we are essentially left with two families of compounds binary charge-transfer complexes and hydrogen-bonded or halogen-bonded co-crystals. This section will focus almost exclusively upon representatives from the latter categories [67]. The terms binary/ternary supermolecule indicates a discrete species with predictable and desirable connectivity, constructed from two /three different molecular species and assembled via directional non-covalent forces [68-78]. 

Supramolecular chemistry may be defined as "chemistry beyond the molecule." bearing on the organized entities of higher complexity that result from the association of two or more chemical species held together by intermo-lecular noncovalent forces, such as metal ion coordination, electrostatic forces, hydrogen bonding, van der Waals interactions, and others.These supramolecular entities derived from supramolecular association are generally called supermolecules or supramolecular compounds. Thus, one may say that "supramolecules are to... 

---