The observation of frictionless motion in water could change chemical models of liquid solutions.

frictionless.jpg Each rotating cyanide ion creates a shock wave that throws back the surrounding water molecules, allowing it to spin for a time with essentially no friction. Credit: Nicolle Rager-Fuller, National Science Foundation

It’s accepted among chemists that a single molecule in a liquid at room temperature usually has a lot of obstacles in its path, namely the solvent’s molecular components, which act on it in the form of friction. But researchers at the University of Southern California and Brown University say they have developed a method to observe a molecule spinning in water, like a propeller, with near-frictionless motion.

By using short laser pulses, Stephen Bradforth, associate professor of chemistry at USC, was able to spin a cyanide ion in water for 10 complete rotations with almost no friction to slow it down. While the discovery has no immediate practical use, it does change the way chemists conceptualize and model reactions that occur in liquid.

“This whole concept was not something that people thought was possible,” said Bradforth.

The few thousand femtoseconds that pass during this rotation is enough time for a chemical reaction to take place, said Bradforth. Since a majority of chemical reactions occur in liquid, the researcher’s findings could impact how chemists try to control certain reactions.

“If I want to try and manipulate how a chemical reaction happens by grabbing on to the atoms, which we can do with laser forces—if the reactive molecules are not so coupled to their surroundings—I can do that more effectively,” said Bradforth. “The ability of the solvent to randomize what I’m trying to do is not as effective as we thought.”

In 2003, when Bradforth began this line of research, which was published in the March 31 issue of Science, many of his colleagues thought he would never be able to prove that a molecule could rotate with minimal friction in water.

But after two experiments supported the notion, Bradforth and co-author Richard Stratt, professor of chemistry at Brown, have shown that a central theory, which describes chemical reactions in solution, needs to be modified to include the phenomena they’ve observed.

“The coupling between the molecular motions of the reactive system and the solvent aren’t quite as intimate as we thought,” said Bradforth. “It was pleasing to see that we had the right picture from the beginning.

“Not every body accepts it now, but more people accept it now than two or three years ago.”

Originally published April 13, 2006


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