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September 29, 2008
Volume 86, Number 39
p. 9

Physical Chemistry

Solvation Observation

Experiment shows solvent molecules don't always obey theory

Elizabeth Wilson

TWO SIMILAR REACTIONS that have the same product can cause different behavior in surrounding solvent molecules, a new study shows. The finding adds experimental weight to the idea that, at least for some reactions, individual solvent molecules, rather than their collective average, can play an important role in dictating chemical reaction dynamics.

8633cov3_SiGNacxd2.tif Adapted From Science
Starting points In one experiment (top), an electron is added to Na+; in another, an electron is removed from Na. The reactions' paths to equilibrium (far right) are very different. Solvent molecules are gray.

The new work could help researchers better understand the mechanisms of certain types of reactions, such as those involving electron transfer, and certain properties such as friction.

Scientists generally treat solvents as an undifferentiated sea of molecules. But UCLA chemistry professor Benjamin J. Schwartz, postdoc Arthur E. Bragg, and graduate student Molly Cavanagh now find experimental evidence for individualistic behavior of solvent molecules during some reactions (Science 2008, 321, 1817).

The finding manifests itself as a "breakdown" of linear response theory, used to predict the behavior of numerous physical and chemical systems. It holds that the rates at which solvents relax into equilibrium should be the same when the end results are the same, regardless of how the system began.

The UCLA group studied the phenomenon for electron-transfer reactions of sodium atoms in a bath of tetrahydrofuran molecules. In one reaction, they removed an electron from Na, and in the other, they added an electron to Na+.

Both reactions generated neutral Na, with the surrounding tetrahydrofuran molecules eventually relaxing into equilibrium within picoseconds. But dynamics of the absorption spectra of the two processes were very different, showing that the solvent surrounding the Na starting material took about twice as long, 7.5 picoseconds, to relax.

The group suggests that because Na is a much larger species than Na or Na+, solvent molecules need more time to fill in the void left when an electron is removed to form Na than they do in the case of the Na+ reaction.

Although some theoretical studies have suggested that linear response theory doesn't always hold true, experimental evidence "has been scant, so this work is a very useful contribution," says Phillip Geissler, chemistry professor at UC Berkeley.

These few experimental studies, Richard M. Stratt, chemistry professor at Brown University, notes in a commentary accompanying the report, "offer welcome instances when the obscuring fog of [linear response theory] lifts, revealing those missing molecular details."

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Chemical & Engineering News
ISSN 0009-2347
Copyright © 2009 American Chemical Society


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