Credit: ©2008 ACS

The conformation of a molecule can be strongly influenced by attractive interactions between arene rings present in the compound. Such interactions have an important role in the structure of proteins and polynucleotides, and are often invoked when explaining the stereoselectivity of some organic transformations. Small-molecule modelling is one of the most effective tools for quantifying the strength of these arene–arene interactions, but it has often been hampered by the inability to distinguish between the different approach geometries, that is, face-to-face or edge-to-face.

Now, Ken Shimizu and co-workers from the University of South Carolina have designed1 a molecular 'balance' that enables the face-to-face interaction between two arene rings to be measured definitively. The design of the balance is based on a highly rigid bicyclic structure that presents one member of the arene–arene pair in a fixed orientation, and the second in one of just two possible orientations — 'open' with no interaction with the other arene group, and 'closed' where steric constraints ensure that only a face-to-face interaction between the arenes is possible. The strength of the face-to-face interaction can then be measured by determining the ratio of these two conformations by 1H NMR spectroscopy.

These molecular balances are highly soluble, and enable examination of the solvent effect on the arene–arene interaction. Future studies will look at the effect of substituents on the arene rings. It is conceivable that this could lead to relative interaction values analogous to the often used 'A values', which describe stereoelectronic effects based on the conformations of substituted cyclohexanes.