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Molecular cage traps rare gases: organic compound could cull valuable xenon from the air.

With a squeeze, an organic molecule can snatch rare gases from the air. The compound contains cavities that are the right size to nab atoms of xenon and radon, and to a lesser extent krypton. These noble gases range from valuable to radioactive; they're largely inert, usually present in low or negligible concentrations in the air and extremely difficult to capture.

"I would die to have these compounds in my hands," says chemist Siegfried Waldvogel of Germany's Johannes Gutenberg University Mainz.

The new chemical cage was a "happy accident," admits materials chemist Andrew Cooper of the University of Liverpool in England. Cooper and colleagues had set out to create a polymer. Instead, they produced a 3-D cage by reacting four aldehyde molecules with six nitrogen-containing ones. The cages clumped together to form a multichambered atomic jail.

The cages are around 0.44 nanometers wide. A krypton atom is 0.37 nanometers wide, xenon is 0.41 and radon 0.42. However, openings to the cages are only about 0.36 nanometers, small enough to possibly prevent gas atoms from slipping in or out. But the cage molecules wiggle, expanding and contracting the entryways.

The researchers estimate xenon could slip in and out just 7 percent of the time; for radon, a door would be open 3 percent of the time. Smaller krypton could come and go easily.

In experiments, the researchers tested the cage's selectivity, blowing a mix of common elements found in air and trace amounts of xenon and krypton through the cage for 45 minutes. The nitrogen, oxygen and carbon dioxide passed through. Krypton was held only briefly while xenon got trapped for around 15 minutes. The results appear July 20 in Nature Materials.

The finding suggests that the molecular trap can strip out valuable xenon from other gases. Xenon, which sells for around $5,000 per kilogram, is useful in lighting, medical imaging and anesthesia. Cooper and colleagues think the molecules could also detect radon in homes, which can cause lung cancer.

Caption: Molecular cages such as the one illustrated in gray and blue have a central gap (purple) perfectly sized to hold a rare gas atom.

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Title Annotation:MATTER & ENERGY
Author:Mole, Beth
Publication:Science News
Geographic Code:4EUUK
Date:Aug 23, 2014
Words:371
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