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Lasers offer surgical control over reactions.

With lasers instead of scalpers, eye surgeons perform ever more precise operations. Now, laser-wielding chemists have extended their surgical skills in severing chemical bonds and controlling chemical reactions.

Richard N. Zare and his colleagues at Stanford University have demonstrated that they can excite and then cleave water molecules in specific places. They begin by using an infrared laser to induce vibrations in one bond of a water molecule. Then they knock off one of the molecule's hydrogen atoms by bombarding it with other hydrogen atoms. They describe the new work in the Dec. 1 JOURNAL OF CHEMICAL PHYSICS.

In the same journal, another group describes the theoretical basis for using lasers to make either left-or right-handed versions of mirror-image molecules. This "coherence chemistry" technique could prove important in drug development, says Paul Brumer of the University of Toronto. Many compounds exist in both mirror-image forms, but in most cases, only one of those versions proves therapeutic, he notes.

Brumer and Moshe Shapiro of the Weizmann Institute of Science in Rehovot, Israel, suggest that sequences of laser pulses can split a large molecule in such a way that the reaction is likely to produce the desired mirror-image version. Their calculations show that because of quantum mechanics -- in which particles exhibit wavelike properties -- "you're canceling out the stuff you don't want," Brumer says.

Scientists only recently gained the ability to control the distribution of energy involved in chemical reactions -- a step toward selecting the resulting products (SN: 4/20/91, p. 245).

Zare and his co-workers now find that they can choose which hydrogen atom to cleave from a water molecule by taking advantage of slight differences in the quantum-mechanical properties of chemical bonds. The researchers start with "heavy" water containing one atom of hydrogen and another of deuterium (a heavier hydrogen isotope). The bond between the oxygen and the deuterium resonates at a lower energy level than the bond between the oxygen and the normal hydrogen. This enables the scientists to selectively tune their infrared laser to excite one or the other bond, explains study coathor Michael J. Bronikowski.

After bombarding the excited molecules with hydrogen atoms, the Stanford chemists immediately confirm which product they produced by scanning their reaction chamber with an ultraviolet laser, which causes each product of the water-splitting reaction to emit a charateristic fluorescent "signature."

When they excited the link between oxygen and hydrogen, this bond stretched and weakened so much that in a subsequent collision with a fast-moving hydrogen atom, the water molecule let go of its own hydrogen. This left behind an oxygen-deuterium molecule. Conversely, when the researchers excited the oxygen-deuterium link, they produced a high proportion of hydroxyl (OH) molecules, Bronikowski says. "It's somewhat surprising that it increases the chance of a reaction that much," he adds.

The Stanford work builds on experiments done by F. Fleming Crim of the University of Wisconsin-Madison, who used visible-light laser beams to break heavy water's hydrogen-oxygen bond. But Crim could not shake the link between oxygen and deuterium. "We're really pleased to see the [Stanford] work," he says. "It's a very important result."

He and the Stanford researchers emphasize, however, that the technique applies only to molecules with very specific characteristics. "It's certainly not something that's going to work for [just] any general reaction," says Bronikowski.

Indeed, "there are much more sophisticated ways of controlling reactions," says Graham R. Fleming, a physical chemist at the University of Chicago. Like Brumer and others, he believes chemists will eventually figure out how to harness quantum mechanics to inhibit the production of unwanted products, not just to control which bonds break.
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Author:Pennisi, Elizabeth
Publication:Science News
Date:Dec 7, 1991
Words:601
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