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A Really Moving Story

Roald Hoffmann

Chemistry is A + B → C + D, substances and their transformations. Change, symbolized by the arrow in the equation, was always of real value to people, whether it was in the protochemistries of food preparation, gunpowder, dyes or metallurgy. The hold was psychological as well as economic; it was fascinating, frightening, thrilling—all of these—to see color changes, healing and explosions. As chemistry became a molecular science, the magic of transformation did not abate. And the value of adding value to the natural by human transformation never left us, it only changed scale; aside from airplanes and computers, the chemical industry is pretty much the major positive contributor to the U.S. trade balance.

But all along it has been too easy to focus on the before and after, on the reactants and products, on the noun and not the verb. Nor did much change as we learned of the molecules within macroscopic matter—to capture change itself, to visualize it and harness its special features was never easy. The blinking of an eye is glacially slow compared with what happens in the first few of dozens of reactions in photosynthesis, or in the controlled explosions in your car engine.

There is a change in chemistry today, for the arrow is now well on its way to being understood. (See "Reaction Dynamics in Organic Chemistry," March–April 1997.) We have femtosecond spectroscopies that allow us to freeze the arrow's motion in neo-Zeno ways, as in the remarkable work of Ahmed Zewail of Caltech. The pictures we obtain of molecular motion, albeit indirect, are truly as startling, and as decisive of argument, as the photographs that Eadweard Muybridge obtained of galloping horses in the late 19th century (a point made to me by my Stanford colleague John Brauman).

And, in the true test of understanding, we can bend the arrow, direct the reaction elsewhere. Why should we wish to do so? Because we desire that which is not easily given unto us. Or we may profit from this capability. But mainly we wish to gain confidence in our understanding of how transformation takes place by affecting the transformation, by ringing the changes on change itself. In the first installment of Marginalia on the new kinetic art, I tell here two contemporary stories of reaction dynamics.

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