Mock atoms prove attractive: Researchers added first pseudo atoms to electronegativity scale

Electron attraction and repulsion determine how atoms and pseudo-atoms behave in different environments. This study provides scientists with the information they need to better predict, manipulate and control those behaviors, whether in batteries for solar farms or catalysts for bio-fuel production.

This study began when Alexander Whiteside, a student at Heriot-Watt University, came to PNNL as part of the 2008 Summer Research Institute for Interfacial and Condensed Phase Physics. During that summer visit, Whiteside, his advisor Maciej Gutowski, and Laboratory Fellow Sotiris Xantheas started to determine the electronegativity of the ammonium radical.

The team calculated the electronic structure of the neutral ammonium molecule, NH₄, and its positive and negative ions. Next, they computed the properties of ammonium complexes, specifically combining ammonium with astatine or selected borohydrides; the latter are promising materials for hydrogen storage.

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For bio-fuel production, this study provides scientists with the information they need to better predict, manipulate and control the behaviors of atoms and pseudo-atoms.

"These results clarified the properties of NH₄ and placed it in the proper scale compared to the alkali metals," said Xantheas. "In comparison with alkali atoms, ammonium's electronegativity punches above its effective cationic radius. Nobody had really put it into the scale yet. This study did just that."

The results graced the cover of Chemistry: A European Journal and were highlighted in the Royal Society of Chemistry's Chemistry World.

"Past generations of physical and theoretical chemists were intrigued by the properties ammonium. Alex was standing on the shoulders of giants—Berzelius, Pauling, Mulliken, Herzberg—whose pictures are highlighted in the cover of the journal, while Sotiris and I helped him to keep his balance," joked Gutowski.

"This work opens up the opportunity to developing a comprehensive view on other pseudo-alkali metal species, pseudo-halogens and other pseudo-atoms" says Alexander Boldyrev of Utah State University.

Indeed, the team is expanding their study to the properties of the nearly ubiquitous hydronium and methyl groups, which contain a single oxygen or carbon atom and three hydrogen atoms. In addition, they are examining cyanide, a carbon and nitrogen atom combination that could provide new insights into that pseudo-atom's behavior.

More information: Whiteside A, SS Xantheas, and M Gutowski. 2011. "Is Electronegativity a Useful Descriptor for the Pseudo-Alkali Metal NH4?" Chemistry: A European Journal 17:13197-13205. DOI:10.1002/chem.201101949

Provided by Pacific Northwest National Laboratory (news : web)