The covalent bonds that hold complex molecules together can come in different forms. Atoms like carbon, nitrogen, and oxygen can form both single and double bonds, sharing two or four electrons. Nitrogen and carbon can even form a triple bond, sharing six. And those are some of the simpler ones. A mixed series of single and double bonds, like those found in benzene, can end up creating a diffuse electron cloud, so that each of the bonds has an odd number of electrons.

The exact strength of these bonds depends strongly on the context of the surrounding molecule. It's possible to get a variety of information about these bonds. We could calculate what their energy is, probe them with chemical reactions, and could even detect the difference in bond strength by imaging the structure of a crystalized population of molecules. But now, a consortium of researchers in Europe have figured out how to use a modified form of atomic force microscopy to examine the strength of chemical bonds in a single molecule.

The rules of covalent bonds are, at least on the surface, quite simple. The more electrons that are shared, the stronger the bond. And the stronger the bond, the closer together the two atoms on either end of it will be. As we said above, though, things get complicated when there is a mixture of single and double bonds, which can create a set of delocalized electrons. The simplest form of this is a benzene ring, which has six carbon atoms arranged in a circle, linked by three single and three double bonds. In this case, the bonds all become equivalent, and each atom is linked by what you could consider 1.5 bonds, instead of a single or double.

Things get even more complex when a benzene ring is embedded in a larger molecule, with other single and double bonds surrounding it. Take a buckyball, in which carbon atoms are linked in a set of interconnected five- and six-atom rings. The six atom rings have benzene-style alternating bonds, while the five atom ones are all linked through single bonds. So a given atom may be part of two benzene rings and a pentagon, all at the same time.

Instead of nice, clean 1.5 bonds, these arrangements create fractional shared electrons, leading to very small differences in energy and distance between adjacent atoms.

All of which makes detecting the difference that much more of an achievement. Standard atomic force microscopy relies on a needle with a single atom as a tip, and that's able to probe the electronic conditions in a molecule or surface. But it doesn't have enough resolution to detect differences in bond length. Instead, the team behind the new paper used a modified form, in which that single atom is capped by a carbon monoxide molecule, meaning the tip of the needle juts out by two extra atoms.

This turned out to be critical. In a number of the samples, the difference in bond length is expected to be at or below the best resolution of atomic force microscopy. As the tip gets pressed down, the carbon monoxide molecule can flex out of the way, behavior that appears to amplify the length of the bond. As a result, the authors were able to discriminate down to bonds that differed by only 0.03 Angstroms (3.0 × 10-12 meters).

The team started by imaging a buckyball, where they could see the difference in bonds between those in five and six membered rings. They then moved on to more complex molecules, such as the one shown on top.

Aside from being an impressive technical achievement, the technique should open up a number of potential opportunities. Molecules that don't form crystals easily could be imaged with this technique, and the extremely precise control could allow researchers to inject electrons or probe chemical conditions at specific points in a single molecule. All of which could help provide a better understanding of the reactions and catalysts that we rely on for many of the basic materials we use every day.

Science, 2012. DOI: 10.1126/science.1225621 (About DOIs).