The ionic bond and the covalent bond hold atoms together; however, weaker, albeit very important, types of forces provide weaker, noncovalent bonds between molecules. These types of intermolecular forces can be classified as:

1. Hydrogen bonds
2. Charge-charge interactions and charge-dipole interactions
3. Attractive van der Waals forces

The natural world contains many spontaneously assembled supermolecules in which the individual molecular components are held together by these forces. For example, the two strands of double-helical DNA are held together by hydrogen bonds between corresponding base pairs in opposite strands as well as by the attractive van der Waals interactions (often called “stacking interactions”) between adjacent base pairs. In another example, the coats of virus particles are aggregates of small proteins held together largely by noncovalent forces. These coats are closed three-dimensional objects that in many cases have great beauty and symmetry.

Chemists have wondered whether they could harness such attractive forces to produce “designer” supermolecules–large, discrete assemblies of smaller molecules–that would form spontaneously from their constituents. A great deal of recent research has been devoted to explorations of these ideas.

In one example, British groups from the Universities of Nottingham and York (1) recently designed a molecule that would act as a corner in a hydrogen-bonded hexagonal aggregate. Their compound is a “hybrid base” that incorporates the acceptor-acceptor-donor hydrogen-bonding pattern of the DNA base cytosine on one side and the donor-donor-acceptor hydrogen-bonding pattern of the DNA base guanine on the other side.

The crucial point is that hydrogen bonds on the different sides are oriented at 120°.

This means that a hexagonal aggregate of six hydrogen-bonded molecules should spontaneously form. Preparation of the compound in fact yielded crystals that were shown by x-ray crystallography to contain the predicted aggregates.

The hexagonal arrays contains large holes, or channels, corresponding to the empty hole in the hexagonal array shown above, through which solvent molecules could freely diffuse. These channels run throughout the entire crystal. As expected, the large channels result in a very low density for the crystals.

Another quite different example of a supermolecule involves compounds in which a cyclic compound is spontaneously threaded over a long chain (an analogy is a ring inserted onto a finger). Such compounds are called in general rotaxanes and pseudorotaxanes. Professor J. Fraser Stoddart and his colleagues at the University of Birmingham in the United Kingdom have been exploring this family of compounds systematically.

One of the more spectacular examples of this type of compound is a supermolecule, shown below as the “[3 + 2] adduct,” formed from two molecules of compound 1 and three molecules of compound 2. As shown below in the schematic, the cyclic molecules 2 served as molecular “belts” to hold the arms of the 1 molecules.

The adduct is held together by hydrogen bonds between the hydrogens of the N-H groups on compound 1 and the unshared electrons on the oxygens of compound 2. The British scientists postulate that van der Waals attractions between the two layers probably also play a role.

Another intriguing example in this field is a “two-dimensional” pseudorotaxane polymer in which cyclic compounds are threaded onto a two-dimensional coordination polymer network (3).

What lies in the future for such supermolecules? The hope is that the principles learned in the design of such species will enable scientists to design molecular receptors or molecule-selective channels, among other things. Reference (1) contains a large number of leading references to work in the supermolecule field.

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