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. Image: 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
attractive forces between other parts of a molecule can make a stretched bond
joining two carbon atoms much more stable than expected. This result should
lead to improvements in how scientists design new molecules, materials and
“We provide an understanding for why
molecules with exceptionally long bonds do not necessarily have to be
unstable,” says 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 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 Å. One of the new molecules had the longest carbon-carbon bond ever
measured in an alkane: 1.704 Å.
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 1.65 Å carbon-carbon bond survived
less than an hour at 167 C (333 F). In contrast, the central carbon-carbon bond
in two of the new linked-diamondoid molecules broke apart only after being
heated above 300 C (572 F). The third one, with the 1.704 Å carbon-carbon bond,
lasted until it was heated to 220 C (428 F).
“Based on carbon-carbon bond-length alone,
I expected these diamondoid molecules to be much less stable than they proved
to be,” Schreiner says. “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 says.
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’
Schreiner added that this finding may
explain why conventional analysis predicts branched alkanes—which contain large
groups of atoms attached to the alkane backbone—to be much less stable than
they actually are, and why diamondoids have far higher melting points than
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:
3D crystals for applications requiring a powdered material, 2D films for
coating other materials, and possibly even 1D nanowires for transferring charge
“These latest results show that making
molecules in new shapes and sizes can lead to surprising discoveries,” Dahl