How to rip apart molecules
Brute force added to the list of ways to do clever chemistry.
Here's a new trick for making molecules — chemists have succeeded in literally ripping the bonds between atoms apart, rather than using the usual suspects of heat, pressure, light or electricity to drive a chemical reaction.
The clever experiment proves that mechanical forces can activate reactions in a controlled way, and so could open up a whole new avenue for synthetic chemists to explore.
Jeffrey Moore, at the University of Illinois at Urbana-Champaign, Urbana, and colleagues performed their trick with a specially designed polymer. Their polymer chain has inserted halfway along it a molecular fragment called a mechanophore, which includes a tiny ring of just four carbon atoms. Because carbon finds it hard to form bonds at such sharp angles, this ring is strained, and is inclined to burst open if given a nudge.
Moore's team provided the nudge with ultrasound — a blast of high-frequency sound that cannot be heard by the human ear. This does two things to a solution containing tangled strings of the special polymer. First, the sound waves create a flow in the solution that untangles each polymer string, beginning at one end. This provides a tugging force as one end of the polymer is pulled. Second, the ultrasound creates bubbles in solution many times bigger than the polymer molecules. As these bubbles burst, they both speed up the flow and the untangling of the polymers, and add a bang of mechanical energy.
Together, these forces rip apart the four-carbon ring. "We're pulling the atoms apart as opposed to just letting them do what they would like to do thermally," Moore says.
Pulled into place
There is another clever twist in this tangled tale, the researchers report in Nature1. In heat- or light-triggered reactions, the energy provided allows bonds to break, but there is no directional element to the reaction.
In this case, though, the tugging of the polymer strand means that the molecule is being stretched out. "We have a force that's applied in a specific direction," says Moore. This helps to determine the shape of the final, broken product.
The mechanophore can be inserted into the polymer in two different ways, making two different shapes out of the long, unbroken molecule (see diagram). But the pulling force straightens out the polymer in such a way that the snapped version always looks the same.
If heat had broken the bond, this wouldn't be true — there would be two differently-shaped snapped polymers. And these could react with other molecules in different ways, making different products. But with the mechanical break, only one final product can be made.
This is selectivity "at the highest possible level", says Virgil Percec, from the University of Pennsylvania, Philadelphia.
Ultrasound and other mechanical techniques have been used to break bonds in polymers before, notes Percec, but only in a very primitive way, severing the chain at a random point. "People would take polymer chains and smash them," he says. By breaking only one specific weak spot, Moore's directed route is nowhere near as brutal. "It's a very elegant way of approaching a single molecule," Percec says.
Sergei Sheiko, at the University of North Carolina at Chapel Hill, last year showed that some carbon-carbon bonds could be broken quite easily under mechanical strain2. He says that there is still debate about whether ultrasound works by causing electronic changes in molecules as well as mechanical stress; but Moore's single product is proof that in this case it is mechanical forces at work, he says. Chemists in future should think about using mechanical forces as a way to activate a reaction, he adds.
Both Sheiko and Percec envisage that chemists will find ways of using stress to trigger reactions in future. Percec suggests a molecular tweezer system to pick apart bonds one by one. Moore is testing other mechanical systems at the moment.
Coming soon: Watch for our reporter's blog from the American Chemical Society meeting in Chicago, starting Sunday 25 March.
- Hickenboth C. R., et al. Nature, 446 . 423 - 427 (2007).
- Sheiko S. S., et al. Nature, 440 . 191 (2006).
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