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Different chemical bonds in hexabenzocoronene revealed for the first time (Image: Leo Gross/IBM)
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Sharing more leads to tighter bonds – even in the
world of molecules. The most detailed images yet made of the chemical
bonds in a molecule vividly show what large-scale models had long
assumed: the more electrons two atoms share, the shorter the bond. Bonds
that are more electron-dense also appear brighter in the new images.
In molecules, the atoms can share one or more of their outermost electrons in a covalent bond.
Whether they share two, four or six electrons determines the bond's
strength, which is an important factor in predicting the molecule's
geometry, stability and reactivity.
The new pictures, taken with a
modified atomic force microscope, marks the first time that scientists
have been able to observe the true physical differences between these
bond types, which could give a deeper understanding of chemical
reactions. It may also help researchers size up molecules for use as
electrical components in tiny circuits.
"We have seen bonds before, but we could not differentiate between bonds," says Leo Gross
of IBM Research in Zurich, Switzerland. "Now we can image these very
tiny differences between different bonds. This is really exciting to
me."
Buckyball portrait
In 2009, Gross and colleagues imaged
the individual bonds between the atoms of a molecule for the first time.
They used a type of atomic force microscopy, in which a vibrating
needle-like tip is scanned over a surface, and differences in
vibrational frequency due to the presence of electrons below are
recorded at different spots. The result was a picture of the bonds
linking the carbon and hydrogen atoms that make up the flat molecule pentacene.
But though the team noticed that some
of the bonds looked brighter and longer than others, they weren't sure
if they were seeing true physical differences, or just artefacts of the
imaging process.
Now they have used the same technique to image buckyballs,
cage-like molecules made of 60 carbon atoms each. They also imaged two
flat molecules, hexabenzocoronene and DBNP, which were synthesised
specially for the imaging.
Structural symmetry in these
carbon-containing molecules let the researchers distinguish actual
differences in their bonds from background effects caused by the imaging
method.
In addition to differences in
brightness, Gross and colleagues found that bonds that are more
electron-dense actually appear shorter than bonds that share fewer
electrons – though only by a few picometres, or 10-12 of a metre.
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