Ultrafast electron diffraction and direct observation of transient structures in a chemical reaction

Ultrafast electron diffraction is a unique method for the studies of struct ural changes of complex molecular systems. In this contribution, we report direct ultrafast electron diffraction study of the evolution of short-lived intermediates in the course of a chemical change. Specifically, we observe the transient intermediate in the elimination reaction of 1,2 diiodotetraf luoroethane (C2F4I2) to produce the corresponding ethylene derivative by th e breakage of two carbon-iodine, C-I, bonds. The evolution of the ground-st ate intermediate (C2F4I radical) is directly revealed in the population cha nge of a single chemical bond, namely the second C-I bond. The elimination of two iodine atoms was shown to be nonconcerted, with reaction time of the second C-I bond breakage being 17 +/- 2 ps. The structure of the short-liv ed C2F4I radical is more favorable to the classical radical structure than to the bridged radical structure. This leap in our ability to record struct ural changes on the ps and shorter time scales bodes well for many future a pplications in complex molecular systems.