Ultrafast Electron Diffraction and Structural Dynamics: Transient Intermediates in the Elimination Reaction of C_2F_4I_2

Abstract

We give a full account of our earlier report on the structural dynamics in the elimination reaction of C_2F_4I_2, 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 ∼1 ps and ∼0.01 Å, respectively, and with a sensitivity to chemical change of ∼1%. 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 C_2F_4I transient structure, and its subsequent decay with a rate of (25 ± 7 ps)^(-1). The molecular structure of the C_2F_4I 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 Å between the experimental and theoretical structures for the transient intermediate.

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