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Exquisite Control

Roald Hoffmann

One Bond at a Time

For the longest time chemists had been foiled in trying to put energy into a molecule in a selective way. It's clear that just heat, which is the totality of collisions between molecules, is indiscriminate. But one might have thought that using light to excite one particular vibration of a molecule would work. Which vibrations? In Figure 2 are shown the three "normal modes" of vibration of a triatomic molecule, water. It looks like the asymmetric stretch might break an OH bond. But no, inject a dollop of energy (from an infrared laser) into just that vibration, and the energy is just quickly randomized among all the vibrations.

Figure 2. The normal modes of vibration of a water moleculeClick to Enlarge Image

In 1989 Fleming Crim and his coworkers at the University of Wisconsin overcame that. They used the singly deuterated water molecule, HOD; the deuterium served to distinguish two modes of vibration: In one, the O–H bond is mostly vibrating on its own; in the other, the O–D bond is. By tuning a laser to a specific frequency (color), one could excite the third overtone of the O–H stretching motion; by tuning to another color, one could put that energy into the fourth overtone of the O–D bond. An overtone of water is just like an overtone of a vibrating guitar string—a sound emitted at a multiple of the fundamental frequency; one such overtone of a water vibration (the fifth O–H overtone) is responsible for the pale blue-green color of thick water layers.

The two vibrations—O–H stretch and O–D stretch—do not communicate well with each other in water. If you now put in (with still another laser!) a substantial further amount of energy via light, the "O–H excited" HOD breaks the O–H bond preferentially, and the "O–D excited" HOD favors breaking the O–D bond. Similar large selectivities are exhibited in a chemical reaction:

H + HOD → H2 + OD vs. H + HOD → HD + OH

Crim's experiments were the first demonstration of bond-selective laser chemistry—prior to his work people thought that even bonds as distinct as O–H and O–D would quickly randomize any energy put into one bond among the two. Crim and Richard Zare at Stanford have gone on to find several further spectacular selective bond-breaking reactions.

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