Molecular 'X-ray' shows first anatomical differences

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.

Molecular transistor

Gross is most interested in using the imaging technique to ask fundamental questions about the way bond type influences the properties of a molecule. For example, questions such as "What happens to the rest of the bonds in a chain of carbon rings if you pinch a hydrogen atom off the end?" are very difficult to address unless you can resolve small differences between bonds.

The findings could also have applications in molecular electronics, a potential future version of electronics where individual molecules serve as transistors and switches.

"The ability to determine fundamental properties of individual chemical bonds could affect many technologically relevant fields," writes Ruben Perez of the Autonomous University of Madrid, Spain, who was not involved in the work, in an accompanying article.

Journal reference: Science, DOI: 10.1126/science.1225621

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