Ultrafast electron diffraction and structural dynamics:: Transient intermediates in the elimination reaction of C2F4I2

We give a full account of our earlier report oil the structural dynamics in the elimination reaction of C2F4I2, as studied with the newly constructed third-generation apparatus for ultrafast electron diffraction (UED3) at Caltech (Ihee, H. Lobastov, V. A.; Gomez, U., Goodson B. M.; Srinivasan, R.; Ruan, C.-Y.; Zewail, A. H. Science 2000. 291, 458). Improvements in experimental stability, sensitivity, resolution, and versatility provided by UED3 permitted the reaction to be probed with spatial and temporal resolution of similar to1 ps and similar to0.01 Angstrom. respectively, and with a sensitivity to chemical change of similar to1%. In addition to the improvements in apparatus design. a number of advancements in data processing, analysis, and modeling were developed which are described here in detail for the first time. With UED, we directly observed the prompt (within 5 ps) formation of the C2F4I transient structure, and its subsequent decay with a rate of (25 +/- 7 ps)(-1). The molecular structure of the C2F4I radical was determined to be classical, not bridged-and was refined via a least-squares fitting procedure. These experimental results are compared with recent predictions obtained from ab initio and density functional theory calculations (Ihee, H.; Kua, J.; Goddard, W. A.; Zewail A. H. J. Phys. Chem. A 2001, 105, 3623), yielding quantitative agreement to within 0.03 Angstrom between the experimental and theoretical structures for the transient intermediate.