Unusual solid state properties of cubane discovered

Scientists at NIST and the University of Chicago recently have achieved a great advance in understanding the properties of solid cubane (C^sub 8^H^sub 8^), an immensely strained molecule that represents an atomicscale replica of a cube. First synthesized in 1964 by Eaton and Cole at the University of Chicago, the unique geometry of the molecule imposes an angle of 90deg on the C-C-C bond instead of the 109.5deg normally found in other hydrocarbons. Cubane, therefore, possesses a tremendous amount of strain energy, roughly 6.5 eV per molecule. Despite its exceptional structure, cubane remained an academic curiosity until the 1980's when it was recognized that its high heat of formation and very high density made certain derivatives of cubane the potential to be used as fuels or superexplosives. Remarkably, recent results have shown that certain complex cubane derivatives also may have biomedical applications. One, a cubane molecule substituted with keto, cyano, and amide groups, has exhibited activity against the AIDS virus, and another, phenylcubane, has exhibited moderate anticancer activity. Most recently, researchers at Chicago have found some cubanes are active against bone marrow cancer while other researchers at Northwestern have suggested that activity of yet other cubanes toward monoamineoxidase might presage a use against Parkinson's disease.

Cubane belongs to the class of solids known as molecular solids in which molecules, rather than individual atoms, make up the underlying lattice. Below their melting points, such solids often transform into a different phase, known as a "plastic"' phase where these molecules rotate about one or more axes. The crystal structure of such plastic phases is almost always cubic. The recent NIST/Chicago research has determined the structure of the plastic phase of cubane for the first time and probed the details of cubane dynamics. The results of these x-ray and neutron scattering measurements show that the plastic phase of cubane is not cubic but rhombohedral. The researchers were able to explain this surprising result by successfully modeling the potential energy of solid cubane in both the plastic and lowtemperature phases. Future work will focus on tetrahedrally substituted cubanes as well as other cubane derivatives to determine how the intermolecular interactions vary with substitution. It is hoped that this will yield valuable insight into the engineering of future cubane-based compounds for specific applications.