The "buckyball" – an iconic soccer-ball shaped molecule made exclusively of carbon atoms – is not quite as special anymore. Chemists have discovered a similar hollow cage-like molecule made only of boron atoms.
Boron is best known for forming compounds with other elements that are used to make things like fibreglass, borax laundry powder, and homemade silly putty.
In the new molecule, named borospherene, 40 boron atoms join together to form a three-dimensional structure made up of 48 triangles, four seven-sided rings and two hexagonal rings, reported scientists from Brown University in Providence, R.I., and Shanxi University and Tsinghua University in China. The discovery was published in the latest issue of Nature Chemistry.
"We were very, very excited about it," said Lai-Sheng Wang, the Brown University chemist who led the research team, in an interview with CBCNews.ca.
While smaller cage-like structures have been made of heavy metals such as gold and tin, none have ever been made of common, lighter elements other than carbon, Wang said. "This is the first one."
Wang was a postdoctoral researcher in the Texas lab that won the 1996 Nobel Prize for its discovery of carbon "buckyballs."
The hollow, soccer-ball shaped molecules made up of 60 carbon atoms each were officially named fullerenes or buckminsterfullerenes after the American architect and inventor Richard Buckminster Fuller. He was known for designing geodesic domes of interlocking pentagons and hexagons that were similar to the structure of the new carbon molecules.
The atoms of boron, carbon's next-door-neighbour on the periodic table of elements, also form very strong bonds with each other, said Wang.
Same technique for making carbon buckyballs
He has been studying clusters of boron atoms since 2001, starting with three atoms and working his way up to 40. In order to create the clusters, he used the technique that was first used to generate carbon buckyballs.
"It's exactly the same experiment," he said.
'I knew since 2005 that this cluster was special.' - Lai-Sheng Wang, Brown University
A disc of pure boron was hit with a laser to vaporize the molecules in a chamber full of helium, which is very inert. The vaporized molecules then condense into clusters of different sizes.
Wang's team fed the resulting molecules into a machine called a mass spectrometer that separated them out by size. Interestingly, clusters of 40 atoms seemed to be unusually stable.
"I knew since 2005 that this cluster was special," Wang said.
But he had no way of knowing what the cluster actually was – what it looked like. It didn't help that there are an awful lot of ways you can arrange 40 atoms.
All Wang had was a "fingerprint" of the molecule collected using a technique called photoelectron spectroscopy. The technique involves firing a laser beam at a molecule and trying to knock off an electron. That generates a unique fingerprint that depends on how tightly the molecule holds onto its electrons. That, in turn, varies for each molecule depending on its shape and structure.
10,000 possible arrangements
Solving the puzzle took three teams of computational chemists in China who used powerful computers to generate computer models of more than 10,000 possible arrangements of the 40 atoms bonded to each other. Each model included a calculation of how stable the structure was – only the most stable ones can exist in real life.
The models showed that neutral boron molecules made up of 40 atoms are most stable when they formed a cage made up of 48 triangles, four seven-sided rings and two six-membered rings. When the cluster was negatively charged – as it needs to be in order to be able to separate out molecules of different masses – the most stable forms were the cage shape and an almost flat molecule with two adjacent hexagonal holes.
The computer also calculated the theoretical photoelectron spectroscopy fingerprints that the molecules would generate.
When the researchers compared those to the fingerprint of the real-life cluster, they eventually realized that it looked like the fingerprints of both the cage and the almost flat molecule overlapping.
Wang says that's because the two kinds of molecules have identical masses, so they can't be separated from one another, and they have such similar stabilities that they're formed in equal quantities.
"There is no way you can only make one or the other."
Wang suspects in the future, there may be other kinds of boron fullerenes left to discover.
"But," he added, "the first one is always special."