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December 2008

Boron Far From Boring

University, national lab team uncovers new arrangement of reactive atoms

Graphic: Boron structures The structures of two 16-atom boron clusters were discovered. On the left is the flat structure created by 16 boron atoms plus 2 electrons. On the right is the rippled form created by 16 boron atoms with 1 electron. Enlarged View

Results: Widely used by industry and one of the most reactive elements on earth, boron atoms form different structures depending on how many atoms and electrons are present. The shape of these structures is difficult to determine because of the sheer number of possible configurations. For example, 16 atoms can theoretically be arranged in about 20 trillion different ways, but only 1 really exists in nature. Which arrangement is right?

"The more atoms, the more ways you can arrange them," said lead investigator Dr. Lai-Sheng Wang of Washington State University and Pacific Northwest National Laboratory. "Progress is slower than we'd like."

That's the challenge scientists from Utah State University, Washington State University, and Pacific Northwest National Laboratory faced when they were studying boron ions, clusters of atoms with extra electrons.

The team found that the ion with 16 boron atoms and one electron formed a rippled structure that resembled a warped snowshoe. Interestingly, when another electron was added, the ion became utterly flat. But, more interesting to the team was the location of the electrons swarming around the outer edge of the atoms, known as the pi electrons. These electrons are in the same orbit as the hydrocarbon naphthalene, responsible for the smell of mothballs, making this ion a hydrocarbon analogue.

Why does this matter? "This is entirely discovery science," said Dr. Bruce Garrett, Director of the Chemistry & Materials Sciences Division at PNNL. "Like the discovery of buckyballs, it could lead to new ways of doing things." Buckyballs, a naturally occurring hollow sphere of 60 carbon atoms, were discovered in 1985. At the time, it was an interesting discovery. Today, buckyballs or more formally fullerenes are the foundation of the nanotechnology industry, used in everything from waterproofing clothing to crack-resistant concrete.

Methods: The scientists combined photoelectron spectroscopy and theoretical calculations to determine which of the trillions of options for boron structures had the lowest stable energy. Why does the stable energy matter? Because nature favors the structures with the lowest energy.

The team began by creating negatively charged boron clusters with 16 atoms. They created clusters with one added electron, B16-, and with two added electrons, B162-. Then, they injected the clusters into the Department of Energy's EMSL photoelectron spectrometer. The spectrometer provided detailed information on the stability and electronic structure of the clusters.

With the data from the spectrometer, the team performed ab initio calculations to translate the stability and structure information into an actual arrangement of atoms. Ab initio (Latin for "from the beginning") calculations rely on basic and established laws of nature without additional assumptions or special models.

The results showed that for the cluster with one added electron, B16-, the structure was a rippled form. When another electron was added, the form became perfectly flat.

What's Next? The team and other groups at PNNL and the University of Utah continue to unlock the secrets of boron. The team is actively searching for analogues of larger more complex hydrocarbons.

Acknowledgments: The National Science Foundation funded the research by Alina P. Sergeeva, Dmitry Zubarev, and Alexander I. Boldyrev at Utah State University; Hua-Jin Zhai of Washington State University, and Lai-Sheng Wang of Washington State University and Pacific Northwest National Laboratory.

The team performed the experimental work and the theoretical calculations in DOE's EMSL, a national scientific user facility at PNNL.

This work supports PNNL's mission to strengthen U.S. scientific foundations for innovation by developing tools and understanding required to control chemical and physical processes in complex multiphase environments.

Reference: Sergeeva AP, DY Zubarev, HJ Zhai, AI Boldyrev, and LS Wang. 2008. "Photoelectron Spectroscopic and Theoretical Study of B16- and B162-: An All-Boron Naphthalene." Journal of the American Chemical Society 130(23):7244-7246.

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