Forces within molecules can strengthen extra-long carbon-carbon bonds

Sep 16, 2011 by Mike Ross
A team of European and SLAC scientists joined two tiny diamond-like “diamondoid” structures to create the longest carbon-carbon bond (highlighted in this image by the star) ever seen in an alkane, a class of molecules made of only carbon and hydrogen connected by single covalent bonds. What was surprising, however, was that the molecule was much more stable than had been expected, since longer bonds are usually weaker. The researchers determined that attractive forces between the diamondoids enhanced the stability of this molecule. Image by Peter R. Schreiner, Justus-Liebig University, Germany.

( -- The strength of a chemical bond between atoms is the fundamental basis for a molecule’s stability and reactivity. Tuning the strength and accessibility of the bond can dramatically change a molecule’s properties. For example, a bond’s strength is directly related to its length: stretching a bond beyond its normal length makes it weaker.

But new research by a team from two European universities and SLAC National Accelerator Laboratory shows that between other parts of a molecule can make a stretched bond joining two much more stable than expected. This result should lead to improvements in how scientists design new , materials and catalysts.

“We provide an understanding for why molecules with exceptionally long bonds do not necessarily have to be unstable,” said Jeremy Dahl, a scientist with the Stanford Institute for Materials & Energy Science (SIMES), an institute jointly run by SLAC and Stanford University. Collaborators were Peter R. Schreiner from the Justus-Liebig University in Germany and Andrey A. Fokin from the Kiev Polytechnic Institute in the Ukraine. The results are published today in Nature.

Though applicable to all types of molecules, the new research involves alkanes, a class of molecules composed of just carbon and hydrogen atoms connected by single covalent bonds, and diamondoids, which are molecule-sized diamonds pioneered by SIMES researchers. Ethane, propane and octane are familiar alkanes that have backbones of two, three and eight carbon atoms, respectively, all joined by single bonds. The carbon atoms in the extremely rigid diamondoids are arranged in the same tetrahedral shape as diamond.

In their new research, the scientists joined pairs of diamondoids to create three new alkanes that had an ultra-long carbon-carbon bond in the middle. To accommodate the bulky diamondoids, the central bond had to stretch far beyond the normal carbon-carbon bond length of 1.54 angstroms. (An angstrom, or Å, is a tenth of a nanometer.) One of the new molecules had the longest carbon-carbon bond ever measured in an alkane: 1.704Å.

Surprisingly, these new dual-diamondoid molecules turned out to be much more stable than expected. Earlier research by other groups had shown that an alkane with a 1.65Å carbon-carbon bond survived less than an hour at 167C (333F). In contrast, the central carbon-carbon bond in two of the new linked-diamondoid molecules broke apart only after being heated above 300C (572F). The third one, with the 1.704Å carbon-carbon , lasted until it was heated to 220C (428F).

“Based on carbon-carbon bond-length alone, I expected these diamondoid molecules to be much less stable than they proved to be,” Schreiner said. “Something else had to be going on that was keeping these new molecules together.”

What made the difference? X-ray crystal structure, nuclear magnetic resonance, and thermogravimetric studies made by Dahl’s European colleagues showed that even as the bonds stretched, attractive forces between the two diamondoids were pulling them closer together. These attractive forces are usually seen between separate molecules, where they are called van der Waals forces after the Dutch physicist who first described them in 1873. “Scientists usually don’t consider van der Waals attractions when analyzing the stability of a single molecule, but it now looks like they should,” Dahl said.

The group’s research findings were supported by sophisticated computations that allowed the scientists to switch the attractive forces on and off when they evaluated the new molecules’ stabilities.

Schreiner added that this finding may explain why conventional analysis predicts branched alkanes – which contain large groups of attached to the alkane backbone – to be much less stable than they actually are, and why diamondoids have far higher melting points than expected.

These results are the latest in several interesting research findings and applications for diamondoids since Dahl and his colleague Robert Carlson developed a way of isolating significant quantities of them from crude oil in 2003.

Among their interesting properties, diamondoids emit electrons extremely efficiently. In work published in Science in 2007, SIMES scientists Zhi-Xun Shen, Wanli Yang and Nick Melosh – in addition to Dahl, Carlson and the Schreiner Group – showed that diamondoids readily emit electrons over a very narrow energy range, a property that could improve the imaging capabilities of several types of electron microscopes as well as electron-beam patterning used to make computer chips.

Diamondoids are also highly customizable, meaning different varieties can be produced and modified to meet different specifications: three-dimensional crystals for applications requiring a powdered material, two-dimensional films for coating other materials, and possibly even one-dimensional nanowires for transferring charge or light.

“These latest results show that making molecules in new shapes and sizes can lead to surprising discoveries,” Dahl said.

Explore further: In-situ nanoindentation study of phase transformation in magnetic shape memory alloys

Related Stories

Converting Nitrogen to a More Useful Form

Jan 09, 2007

Nitrogen-containing organic compounds are important products as well as intermediates for many pharmaceuticals, agrochemicals, and chemicals used in electronics. Air contains plenty of nitrogen, but it is in a form that cannot ...

New carbon allotrope could have a variety of applications

Apr 22, 2011

( -- Carbon comes in many different forms, and now scientists have predicted another new form, or allotrope, of carbon. The new form of carbon, which they call T-carbon, has very intriguing physical ...

Recommended for you

'Exotic' material is like a switch when super thin

Apr 18, 2014

( —Ever-shrinking electronic devices could get down to atomic dimensions with the help of transition metal oxides, a class of materials that seems to have it all: superconductivity, magnetoresistance ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Sep 16, 2011
Scientists usually dont consider van der Waals attractions when analyzing the stability of a single molecule, but it now looks like they should "

I thought van der Waals force was also the sum of the forces between parts of the same molecule as well as the sum of the forces between seperate molecules...

More news stories

NASA's space station Robonaut finally getting legs

Robonaut, the first out-of-this-world humanoid, is finally getting its space legs. For three years, Robonaut has had to manage from the waist up. This new pair of legs means the experimental robot—now stuck ...

Ex-Apple chief plans mobile phone for India

Former Apple chief executive John Sculley, whose marketing skills helped bring the personal computer to desktops worldwide, says he plans to launch a mobile phone in India to exploit its still largely untapped ...

Filipino tests negative for Middle East virus

A Filipino nurse who tested positive for the Middle East virus has been found free of infection in a subsequent examination after he returned home, Philippine health officials said Saturday.

Egypt archaeologists find ancient writer's tomb

Egypt's minister of antiquities says a team of Spanish archaeologists has discovered two tombs in the southern part of the country, one of them belonging to a writer and containing a trove of artifacts including reed pens ...