MARGINALIA
Pulse, Pump & Probe
Roald Hoffmann
The Intermediate Detected
Ahmed Zewail of the California Institute of Technology has provided us with many of the concepts of femtochemistry, exemplifying them through a series of beautifully designed experiments. I mention here but one of them, relevant to a kind of organic reaction of interest to me.
On photolysis, halogen-substituted ethanes fall apart to ethylenes and diatomic halogen, X2 (X is iodine, I, in the case studied). The overall reaction is shown in Figure 2. But how exactly does this happen? Does the reaction proceed in one fell swoop, the iodine atoms coming off in a concerted manner? Or is there an "intermediate," as in the mechanism shown in Figure 3? The geometrical details of the CF2I–CF2 are left undefined, as they were when the experiment was first done.
Zewail and coworkers showed unambiguously that the two C–I bonds do not break synchronously. Furthermore they defined for us the time scales of the various processes. First one C–I bond is broken, in approximately 200 fs. This is followed by a much slower cleavage of the second C–I bond, in about 32 picoseconds, which equals 32,000 fs.
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