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Exquisite Control

Roald Hoffmann

Guidance May Lead in Unexpected Directions

One can also affect a reaction's outcome by choosing the way reactants approach. This is done by firing beams of gaseous reagents at each other, in which one or both of the reaction partners is at least partially aligned in a specific way. The control one may thus achieve on reaction geometry is impressive; it is also something one cannot do in the liquid state. Let me show you an example that blends molecular beams and selective excitation.

The first reaction encountered in organic chemistry texts is likely to be the chlorination of hydrocarbons. The important, rate-limiting "chain-propagation" step in that reaction (for, say, methane) is Cl + CH4 → CH3 + HCl. Zare's group recently found a remarkable result in their study of this old chlorination reaction. When the reactants just collide, even if the Cl atoms move in quickly (actually they are fired in), the reaction is difficult. To make the reaction go, a direct hit (in the trade this is called a "small impact parameter") is needed, and this is hard to arrange, for the molecular bullets are very small, metaphorically speaking.

That HCl comes off "backwards" relative to the incoming Cl is understandable. Says Zare, "It's just like three billiard balls of the same mass all colliding in a line, in which the incoming ball nearly stops and the one at the other end takes off in the same direction." In what we call a center-of-mass system, the HCl (made up of the Cl billiard ball coming in and the H hit first) would be going "back," rebounding, viewed from the CH3 fragment.

To make the collisions more effective, the laser chemists excite selectively and strongly the C–H stretching vibration of methane, CH4. Now a lot of HCl is formed (the rate of the reaction increased by more than a factor of a hundred); obviously even glancing collisions are effective. And much of the HCl product moves in the same direction as the incoming Cl. What has changed? Well, the highly excited C–H bond is stretched a lot. It is more likely to be on the "outside" of the rotating CH4 target that presents itself to the incoming Cl atom (Figure 3). The target for the arrow is effectively bigger, much bigger. And the Cl sweeps along the hydrogen whose bond had been weakened by vibrational excitation, stripping it from the methane.

Figure 3. Contrasting reaction dynamicsClick to Enlarge Image

The preceding examples have shown how energy specifically deposited in the vibrations of a bond or a molecule may dramatically alter the course of a reaction. What follows is a fascinating example of the "microscopic reverse," molecules vibrating in finely defined ways as a consequence of a chemical reaction.

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