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Exquisite Control

Roald Hoffmann

The chemical reaction should go, but refuses to do so? Bang on it—with heat, with light, with pressure. Or, so much better, find a catalyst—an ingenious partner in a reaction that cuts down an energy hill that is in the way, or gets involved with the reactant molecules, takes them in hand, so to speak, and guides them in a path around that hill. And then the catalyst is regenerated.

Figure 1. Giacomo Balla’s 1912 oil on canvasClick to Enlarge Image

The giant chemical industries of the world offer testimony to our skill at devising ways to make reactions go in ton lots. And yet we would like to exercise still greater control over what happens. For instance, it's no sweat to break the weakest bond in a molecule. But what if the bond we want to break, and no other one, is not the weakest? In this, the last of four Marginalia on contemporary chemical dynamics, I look at some fascinating developments in precisely guiding chemical reactions at the microscopic, molecular level.

The direction may take different forms. Energy in molecules is distributed in their motion (translation), and also in their internal vibrations and rotations. Molecular beams and lasers allow chemists to select very precisely the way a molecule is tweaked—for instance the excitation may be neatly deposited in one vibration. Having prodded the reactants in a very specific way, we can look at what happens to the reaction products. Which bond is energized? Which bond is broken, which formed? Let me begin with one remarkable and simple reaction that has been studied in this way.

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