12:33 29 June 2007
NewScientist.com news service
Stephen Battersby

A single hydrogen atom has been snipped off a molecule and then added back on again, marking the first time a single chemical bond has been broken and reforged in a controlled, reversible way.

The researchers used a scanning tunnelling microscope (STM) for their cutting tool, which works by manoeuvring a sharp metal tip close to an object, applying a small voltage, and measuring the trickle of electrons that flow between the two.

The team first used their STM to locate a methylaminocarbyne (CNHCH3) molecule that was fixed to a platinum surface.

Then they turned up the voltage, increasing the flow of electrons. That was enough to break one bond – between the molecule's nitrogen and hydrogen atom – but not to disturb any of the other bonds, leaving a molecule of methylisocyanide (CNCH3).

Reversed process
To reverse the process, the group simply bathed the sample in hydrogen gas. The platinum surface catalysed the splitting of the hydrogen molecules into their hydrogen atoms, which reacted with nitrogen in the methylisocyanide molecule to re-form methylaminocarbyne.

This kind of reversible alteration could be used in molecular electronics, says Yousoo Kim at the Surface Chemistry Laboratory in Wako, Japan, who carried out the experiment with colleagues.

Changing the bonding of a molecule like this also changes its electrical contact with the metal surface – if it could be reversibly changed from conducting to insulating, it would become a molecular switch.

Lucky break?
But it is not yet clear how to extend this result to other systems. When researchers have attempted molecular surgery with an STM in the past, it has usually either broken other bonds (often completely destroying the molecule), or resulted in a chemical change that cannot readily be reversed.

The key to the new experiment was in the choice of methylaminocarbyne, which turned out to be a much more stable subject.

The team calculates that electrons added by the STM tend to hover relatively closely to the nitrogen-hydrogen bond in the molecule, although they are still unsure why that makes the bond break so neatly.

"We've done a cute experiment and found a nice effect, but we don't fully understand why" says Michael Trenary of the University of Illinois at Chicago, another member of the team. "To a large extent we just got lucky."

Journal reference: Science (vol 316, p 1883)

http://www.newscientist.com/article/dn12161-molecular-surgery-snips-off-a-single-atom.html