Subjective Suboxide

Carbon monoxide and carbon dioxide are probably the best known molecules containing just carbon and oxygen, but they do form others, such as carbon suboxide (C₃O₂), which is one of the most stable.

Carbon suboxide was discovered in 1906 and is a linear molecule in which all five atoms are bound to each other with double bonds in the order O=C=C=C=O. The gas-phase compound reacts spontaneously, forming a solid polymer-macromolecule composed of many individual C₃O₂ monomers. However, a detailed structure for this macromolecule has remained elusive for the best part of a century. Now, German chemists have at last confirmed the suspected ribbon-like structure for the polymer of carbon suboxide.

Research during the 1960s led to the conclusion that polymeric C₃O₂ has a poly-alpha-pyrone structure. Such a structure would resemble a long ribbon of connected rings in which pairs of C₃O₂ are fused into an alpha-pyrone ring consisting of five carbon atoms and one oxygen atom. The second oxygen atom is connected by a double bond to one of the carbon atoms directly neighboring the oxygen in the ring. The sixth carbon atom and the two remaining oxygen atoms form part of the next ring. However, analytical limitations at the time meant scientists never proved this conclusively nor did they determine the length of any ribbon.

Polymer Unit (Diagram drawn in ChemSketch by David Bradley)

Now, Matthias Ballauff and Sabine Rosenfeldt of the University of Bayreuth, Johannes Beck and Petra Krieger-Beck of the University of Bonn, and Nico Dingenouts of the University of Karlsruhe have succeeded in confirming the proposed ribbon-like structure of polymeric C₃O₂. They used X-ray scattering experiments on a polymer solution, which also revealed the ribbons to contain about forty monomers. "This work presents a nice example on how scattering techniques such as small-angle X-ray scattering can be used to elucidate the structure of complicated molecules," Baullauf told Reactive Reports.