Femtosecond Elimination Reaction Dynamics

Zhong, Dongping; Ahmad, Saleem; Zewail, Ahmed H.

doi:10.1021/ja9710013

Cited by 40 publications

(61 citation statements)

References 10 publications

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“…5). The first bond breakage must occur on a time scale shorter than 10 ps, and this is consistent with the spectroscopic detection of I atoms on the fs time scale (26,27); it occurs in Ϸ200 fs. In Fig.…”

Section: Resultssupporting

confidence: 83%

“…27); the barrier for the second bond breakage is Ϸ15 kcal͞mol (27). By using state detection on the fs time scale, a typical reaction time was measured to be 25 ps at 277 nm (27) and 69 ps at 307 nm (26). At our two-photon energy, the 17-ps time is entirely consistent with the range of available energy for the barrier crossing.…”

Section: Resultssupporting

confidence: 59%

Section: Resultsmentioning

confidence: 99%

See 2 more Smart Citations

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

Cao

Ihee

Zewail

1999

Proc. Natl. Acad. Sci. U.S.A.

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Ultrafast electron diffraction is a unique method for the studies of structural changes of complex molecular systems. In this contribution, we report direct ultrafast electron diffraction study of the evolution of shortlived intermediates in the course of a chemical change. Specifically, we observe the transient intermediate in the elimination reaction of 1,2-diiodotetraf luoroethane (C 2 F 4 I 2 ) to produce the corresponding ethylene derivative by the breakage of two carbon-iodine, COI, bonds. The evolution of the ground-state intermediate (C 2 F 4 I radical) is directly revealed in the population change of a single chemical bond, namely the second COI bond. The elimination of two iodine atoms was shown to be nonconcerted, with reaction time of the second COI bond breakage being 17 ؎ 2 ps. The structure of the short-lived C 2 F 4 I radical is more favorable to the classical radical structure than to the bridged radical structure. This leap in our ability to record structural changes on the ps and shorter time scales bodes well for many future applications in complex molecular systems.Significant progress has already been made in the probing of chemical reactions on the fs time scale. In these femtochemistry (1-4) experiments, the nuclear motions on the time scale of bond breaking and bond making are monitored by using an initiation pulse to establish the zero-of-time (clocking) and probing pulses to view the motion; typical probes are optical and IR spectroscopy, photoelectron spectroscopy, mass spectrometry, and nonlinear optical techniques (5). In this laboratory, efforts have been made to include diffraction techniques to map out ultrafast structural changes, especially in complex molecular systems (6, 7). Electron diffraction of molecules in their ground state has been a powerful tool over the past 50 years (8), and both electron and x-ray methods are now being advanced in several laboratories (6,7,(9)(10)(11)(12)(13)(14)(15)(16) for the studies of structural changes. Our focus here and before has so far been on gas-phase ultrafast electron diffraction (UED) of isolated chemical reactions.Elsewhere, we have reported the latest advance in UED (7), by which major challenges were surmounted: the very low number densities of gas samples, the absence of the long-range order that is present in crystals, which enhances coherent interference, and the challenging task of determining in situ the zero-of-time when diffraction changes are on the ps and sub-ps time scale. With UED, molecular structures (17) and branching ratios (18) of final products have been determined on the ps time scale. The diffraction change before and after the chemical reaction was observed (7). However, no direct observation of transient structural changes in the course of the reaction has so far been reported. In this article, we report such observation of the temporal evolution of short-lived intermediates probed with ultrafast electron diffraction. EXPERIMENTALThe UED experiments were performed in the secondgeneration apparatus dev...

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Section: Resultssupporting

confidence: 83%

Section: Resultssupporting

confidence: 59%

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

Cao

Ihee

Zewail

1999

Proc. Natl. Acad. Sci. U.S.A.

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“…Of particular importance are the diradical intermediates ͑see, e.g., Ref. 1͒, whose fs dynamics in many reactions, such as the Norrish, 2 Diels-Alder, 3 elimination 4 and other reactions, 5 are critical to the branching of product yields, to the stereochemistry, and to the reaction mechanism ͑see, e.g., Refs. 6 and 7͒.…”

Section: Introductionmentioning

confidence: 99%

Femtosecond β-cleavage dynamics: Observation of the diradical intermediate in the nonconcerted reactions of cyclic ethers

Scala

Diau

Kim

et al. 1998

The Journal of Chemical Physics

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Femtosecond ͑fs͒ dynamics of reactions of cyclic ethers, symmetric and asymmetric structures, are reported. The diradical intermediates and their ␤-cleavages, which involve simultaneous C-C, C-H -bond breakage and C-O, C-C -bond formation, are observed and studied by fs-resolved mass spectrometry. To compare with experiments, we present density functional theory calculations of the potential energy surface and microcanonical rates and product distributions.

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“…Importantly, the former step is dissociative and occurs within ∼200 fs, whereas the latter step is a barrier-crossing process involving energy redistribution. 31,32 Knowing the time scales involved, two structures were considered for the intermediate C 2 F 4 I ‡ : a classical structure, in which primary halide (I) resides predominantly on one -CF 2 moiety, and a bridged structure, in which the primary halide is shared equally between the two -CF 2 moieties (Figs. 2(c) and 2(d)).…”

Section: Uedmentioning

confidence: 99%

Perspective: 4D ultrafast electron microscopy—Evolutions and revolutions

Shorokhov

Zewail

2016

The Journal of Chemical Physics

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In this Perspective, the evolutionary and revolutionary developments of ultrafast electron imaging are overviewed with focus on the “single-electron concept” for probing methodology. From the first electron microscope of Knoll and Ruska [Z. Phys. 78, 318 (1932)], constructed in the 1930s, to aberration-corrected instruments and on, to four-dimensional ultrafast electron microscopy (4D UEM), the developments over eight decades have transformed humans’ scope of visualization. The changes in the length and time scales involved are unimaginable, beginning with the micrometer and second domains, and now reaching the space and time dimensions of atoms in matter. With these advances, it has become possible to follow the elementary structural dynamics as it unfolds in real time and to provide the means for visualizing materials behavior and biological functions. The aim is to understand emergent phenomena in complex systems, and 4D UEM is now central for the visualization of elementary processes involved, as illustrated here with examples from past achievements and future outlook.

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Femtosecond Elimination Reaction Dynamics

Cited by 40 publications

References 10 publications

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

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

Femtosecond β-cleavage dynamics: Observation of the diradical intermediate in the nonconcerted reactions of cyclic ethers

Perspective: 4D ultrafast electron microscopy—Evolutions and revolutions

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