Pinning down Transients

CHEMICAL PHYSICS

At a town hall meeting, talking about transients conjures up images of fly-by-night residents of dubious character. To chemists, however, the term brings to mind short-lived species that are often invoked in theoretical reaction models but tough to pin down in the lab. The fleeting chemical entities don't hang around long enough to be studied by conventional means. But by using ultrafast diffraction methods, an international team of scientists has just succeeded in elucidating structures of transient species present in reactions of haloalkanes.

Hyotcherl Ihee, an assistant chemistry professor at Korea Advanced Institute of Science & Technology in Daejeon, and his coworkers in Korea, France, and Italy report direct experimental evidence for CH₂ICH₂, a solution-phase bridged radical species formed during dissociation of 1,2-diiodoethane (Science, published online July 14, dx.doi.org/10.1126/ science1114782).

The study broadens the range of liquid-phase samples that can be probed with time-resolved X-ray diffraction methods and may lead to similar studies of complex biomolecules in solution, the researchers say.

The bridged structure of the iodoethane radical, as opposed to bent alternatives in which iodine is bonded to only one carbon atom, was first hypothesized more than 40 years ago to explain the stereochemical outcome of free-radical addition reactions involving halogenated alkanes. Until now, the species has not been detected directly.

To examine the reaction dynamics, Ihee and coworkers used a pump-and-probe method, in which the analyte is excited with short bursts of ultraviolet laser light and then interrogated with X-ray pulses of picosecond duration from a synchrotron source.

The group finds that just picoseconds after firing the laser, diiodoethane is stripped of one of its iodine atoms. The product, CH₂ICH₂, adopts the bridged structure with a triangular CIC frame. Then within nanoseconds, CH₂ICH₂ binds to an iodine atom to form an isomer of diiodoethane, C₂H₄II. Finally, roughly 1 s after the laser pulse excites the molecules, the isomer dissociates into C₂H₄ and I₂.