Scientists ‘strummed’ a molecule’s chemical bonds like guitar strings

Like plucking a tiny guitar string, scientists have “strummed” chemical bonds.

Plucking the bonds, which connect two carbon atoms separated by just 140 billionths of a millimeter, required a minuscule “pick.” A single molecule of carbon monoxide attached to the extremely thin tip of an atomic force microscope did the trick, researchers report in a paper accepted in Physical Review Letters.

Atomic force microscopes image materials by measuring the forces a material exerts on the microscope’s slender tip, which is scanned across the surface (SN: 8/1/19). Physicist Jay Weymouth and colleagues used such a microscope to study molecules of perylenetetracarboxylic dianhydride, or PTCDA. They were chosen for their flat shape, which allows them to be laid on a surface for inspection. The researchers oscillated the atomic force microscope’s tip back and forth over the surface, measuring how fast the tip jiggled and how much energy was required to keep it moving.

When the microscope tip was dragged over a bond, more energy was required to keep the tip jiggling. That’s because the carbon monoxide tip flexed upward due to the forces exerted on the tip by the bond and the attached atoms, before snapping back down on the other side of the bond. Because forces in physics come in “equal and opposite” pairs, the force on the microscope tip means that the tip likewise exerted a force on the atoms and the bond. That force from the tip, the researchers say, is akin to strumming the bond.

Investigating chemical bonds in this way could help atomic force microscopes better characterize molecules. “It’s a new way to look at these chemical bonds,” says Weymouth, of Universität Regensburg in Germany.