12/05/91
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STANFORD -- Since the days of alchemy, chemists have searched for ways to control chemical reactions precisely.
For alchemists, the idea was to turn base metals into gold. Modern chemists have a goal that's less glittering, but has its own economic and environmental rewards. They want to be able to start a reaction between substance A and substance B and end up with compound C -- and only compound C.
That sounds simple, but it's not.
For example, when a mixture of molecules and atoms is warmed, the heat energy speeds up not only the desired reaction, but all the possible reactions that the mixture can produce. The side- reactions create unwanted waste products and impurities.
With the invention of the laser, many chemists thought they could use focused laser energy for "bond-specific" chemistry. The laser light would speed up the reaction of some specific part of the molecule, perhaps selectively breaking a single bond. The result would be to produce a single desired reaction product.
In almost 20 years of trying, experimentalists have been unable to do this. "It's been a boulevard of broken dreams," said Richard Zare, professor of chemistry at Stanford University.
Now Zare and three graduate and postdoctoral students have trained a set of lasers to control the reaction of a hydrogen atom with a water molecule. For the first time, they have achieved complete control of branching in a chemical reaction.
Normal water (H2O) has an oxygen atom in the center bonded to two hydrogen atoms. The form of water the Stanford scientists used is HOD: an oxygen atom bonded on one side to an atom of hydrogen, and on the other side to an atom of deuterium, a heavy isotope of hydrogen.
Zare's team tuned an infrared laser to energize selectively only one of those bonds, "stretching" the link between the atoms so it breaks when they start a reaction.
Depending on which bond is excited, the reaction produces a completely different final product.
Zare said bond-specific chemistry so far has only been demonstrated for the hydrogen-oxygen-deuterium (HOD) system. If this is the only system of bond-specific elements, it probably would have little practical application.
"It could be that we end up with only a curiosity," he said.
If it works with molecules that are important for industry, however, "it could open up a whole new area of how to do chemistry," Zare said. "You might be able to control which path a reaction follows, among many paths. It would be very important for building things or making things happen, yielding purer compounds and perhaps even new compounds." He said such control still appears far in the future.
A communication describing the National Science Foundation-funded experiment has been published in the December 1st issue of the Journal of Chemical Physics. Zare's co-authors are Stanford graduate students Michael J. Bronikowski and William R. Simpson, and postdoctoral fellow Bertrand Girard, who recently accepted a faculty position at the Universite <Grace: e needs accent> Paul Sabatier in Toulouse, France.
The experiment begins by bombarding HOD molecules with "fast" hydrogen atoms, energized by an ultraviolet laser. The high-velocity hydrogen atoms normally would react randomly, some atoms with the hydrogen and some with the deuterium in HOD.
A second, tunable infrared laser is simultaneously focused into the chamber. Depending on how this laser is tuned, it vibrationally excites either the oxygen-hydrogen or the oxygen- deuterium bond on the HOD molecule, stretching and weakening the selected link. A third laser is used to analyze the results.
If the infrared laser is tuned to stretch the oxygen- hydrogen bond, the bombarding hydrogen atom reacts with the loosened hydrogen to form a hydrogen-hydrogen (H2) molecule and leave behind an oxygen-deuterium (OD) molecule.
If the infrared laser stretches the oxygen-deuterium bond, the bombarding hydrogen atom reacts with the deuterium to form a hydrogen-deuterium (HD) molecule and leave behind an oxygen-hydrogen (OH) molecule.
Only one product is observed depending on which bond is stretched; the reaction of fast H atoms with HOD yields exclusively OD or exclusively OH.
Other scientists have performed some of the same steps. At the University of Wisconsin-Madison, Prof. F. Fleming Crim and co-workers have used a higher-energy visible laser to enhance the reactivity of the oxygen-hydrogen bond in HOD. Zare said that as far as he knows, his own lab is the first to excite one molecule in two different ways and see two different complete yields.
The next step -- to see whether this bond-specific chemistry works for other reactions -- is hampered by the same difficulties that have conspired against the idea all along. Not only is it hard to energize one part of a molecule and not another; it's hard to keep the energy from spreading throughout the molecule, either before or during the reaction.
"It's not clear this can be generalized to other molecules," said J. Francis Wodarczyk, program director for experimental physical chemistry with the National Science Foundation. "The hope is that maybe you could. We haven't even been able to do it for molecules containing hydrogen up until now."
So is Zare's experiment the bond-specific "Holy Grail" that laser chemists have been seeking?
"It is a great technical accomplishment, a tour de force," says James L. Kinsey, a professor of chemistry and Dean of Natural Sciences at Rice University. Still, he agrees with Zare that it will take more experimentation -- and eventually some practical uses for bond-specific chemistry -- before they can say whether it has far- reaching implications.
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