Mapping of Electron Distribution in Hypervalent Atom Advances our ...

Tag : Organic Intermediate

The chemical bonds in organic compounds are made from pairs ofelectrons, whose negative charge sticks atoms together. Atoms withmore than four chemical bonds are called hypervalent, which iscommon in elements such as phosphorus and sulfur, but very rare incarbon.
Hypervalent carbon atoms are not just a curiosity, explainsteam-member Daisuke Hashizume of RIKENs Advanced Science Institute in Wako . Carbon is the most important atom in chemistry, he says. Whencarbon-based organic molecules react, the atomic rearrangementinvolved usually creates short-lived hypervalent intermediates.Studying more stable hypervalent compounds can help to explain whycertain chemical reactions proceed in particular ways.
Scientists at Hiroshima and Waseda Universities first created acompound containing a central carbon atom connected to two flatanthracene groups that carried a total of four dangling oxygenatoms. A chemical reaction then added positive charges to theanthracene groups, drawing electrons from the oxygen atoms to forma bond with the central carbon atom, making it hypervalent.
Then Hashizumes teamworking with colleagues at the University ofCalifornia, Riverside, and the Rigaku Corporationanalyzed exactlyhow electrons were distributed in the molecule, to confirm itsbonding pattern.
The team used x-ray diffraction at RIKENs SPring-8 Center to mapthe positions of atoms and the electron density in the molecule,and confirmed that the oxygen atoms were close enough to the carbonto form genuine chemical bonds. They also found that electronsoriginally located around the oxygen atoms had shifted towards thecentral carbon atom. These bonding electron pairs are spreadbetween three atoms, as opposed to the usual two, says Hashizume.
Although some hexavalent carbon compounds are already known, thisis the first time that such a detailed analysis of how theelectrons are distributed around the molecule has been done. Wehave confirmed all six bonds on the carbon atom experimentally, nottheoretically, explains Hashizume (Fig. 1).
The results will help scientists to synthesize related compounds,says Hashizume, and to study reaction intermediates in more detail.The next stage is to isolate and see the electronic structure ofreaction intermediate species, to give us a deeper understanding ofchemical reactions. Yamaguchi, T., Yamamoto, Y., Kinoshita, D., Akiba, K., Zhang, Y.,Reed, C.A., Hashizume, D. & Iwasaki, F. Synthesis and structure ofa hexacoordinate carbon compound. Journal of the American ChemicalSociety 130, 68946895 (2008).
Posted October 3rd, 2008