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X-ray imaging of chemically active valence electrons during a pericyclic reaction.

Bredtmann T, Ivanov M, Dixit G - Nat Commun (2014)

Bottom Line: Here we demonstrate an effective and robust method, which emphasizes the information encoded in weakly scattered photons, to image chemically active valence electron densities.The degenerate Cope rearrangement of semibullvalene, a pericyclic reaction, is used as an example to visually illustrate our approach.Our work also provides experimental access to the long-standing problem of synchronous versus asynchronous bond formation and breaking during pericyclic reactions.

View Article: PubMed Central - PubMed

Affiliation: Max Born Institute, Max-Born-Strasse 2A, 12489 Berlin, Germany.

ABSTRACT
Time-resolved imaging of chemically active valence electron densities is a long-sought goal, as these electrons dictate the course of chemical reactions. However, X-ray scattering is always dominated by the core and inert valence electrons, making time-resolved X-ray imaging of chemically active valence electron densities extremely challenging. Here we demonstrate an effective and robust method, which emphasizes the information encoded in weakly scattered photons, to image chemically active valence electron densities. The degenerate Cope rearrangement of semibullvalene, a pericyclic reaction, is used as an example to visually illustrate our approach. Our work also provides experimental access to the long-standing problem of synchronous versus asynchronous bond formation and breaking during pericyclic reactions.

No MeSH data available.


Time-resolved scattering patterns and corresponding total electron densities.(a) Scattering patterns, and (b) total electron densities during the Cope rearrangement of semibullvalene via tunnelling. (c) Scattering patterns, and (d) total electron densities for the reaction over the barrier. The time-resolved scattering patterns are in the Qy−Qz plane (Qx=0) and the corresponding total electron densities are in the y−z plane (integrated along x-direction) at pump-probe delay times 0, T/4, T/2, 3T/4 and T. The time-resolved patterns are calculated until /Qmax/=10 Å−1, which corresponds to an incident X-ray pulse with 20 keV photon energy and the detection of photons scattered up to 60°. The intensities of the patterns are shown in units of the differential scattering probability, dPe/dΩ, in both cases with . The electron density is given in units of number of electrons per Å2.
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f2: Time-resolved scattering patterns and corresponding total electron densities.(a) Scattering patterns, and (b) total electron densities during the Cope rearrangement of semibullvalene via tunnelling. (c) Scattering patterns, and (d) total electron densities for the reaction over the barrier. The time-resolved scattering patterns are in the Qy−Qz plane (Qx=0) and the corresponding total electron densities are in the y−z plane (integrated along x-direction) at pump-probe delay times 0, T/4, T/2, 3T/4 and T. The time-resolved patterns are calculated until /Qmax/=10 Å−1, which corresponds to an incident X-ray pulse with 20 keV photon energy and the detection of photons scattered up to 60°. The intensities of the patterns are shown in units of the differential scattering probability, dPe/dΩ, in both cases with . The electron density is given in units of number of electrons per Å2.

Mentions: Time-resolved scattering patterns in the Qy−Qz plane (Qx=0) and the corresponding electron densities of space-fixed semibullvalene in the y–z plane as a function of the delay time at times 0, T/4, T/2, 3T/4 and T are presented in Fig. 2. Here T is the reaction time for the Cope rearrangement of semibullvalene, ranging from T=24.2 fs (1 fs=10−15 s) for the over-the-barrier reaction to T=970 s for the reaction via tunnelling at cryogenic temperatures31. The static structure of semibullvalene in the gas phase has been measured using electron scattering32. Figure 2a,c shows that the time-resolved scattering patterns distinguish the over-the-barrier reaction from tunnelling. In the tunnelling case, the intensity of scattered photons, associated with high Q-values, diminishes from the reactant to the reaction intermediate at T/2 and then increases again between T/2 and T (see Fig. 2a). The opposite behaviour is observed for the over-the-barrier reaction. The difference between the two reaction paths is most prominent at T/2. The reader is referred to Supplementary Fig. 1 for further details of the total scattering patterns. The origin of these differences is in the different rearrangement dynamics of the carbon core electrons for the two paths, which dominate the total electron density as visually shown by localized yellow-green circles in Fig. 2b,d for tunnelling and over-the-barrier pathways, respectively. The contribution of the valence electrons is very diffuse.


X-ray imaging of chemically active valence electrons during a pericyclic reaction.

Bredtmann T, Ivanov M, Dixit G - Nat Commun (2014)

Time-resolved scattering patterns and corresponding total electron densities.(a) Scattering patterns, and (b) total electron densities during the Cope rearrangement of semibullvalene via tunnelling. (c) Scattering patterns, and (d) total electron densities for the reaction over the barrier. The time-resolved scattering patterns are in the Qy−Qz plane (Qx=0) and the corresponding total electron densities are in the y−z plane (integrated along x-direction) at pump-probe delay times 0, T/4, T/2, 3T/4 and T. The time-resolved patterns are calculated until /Qmax/=10 Å−1, which corresponds to an incident X-ray pulse with 20 keV photon energy and the detection of photons scattered up to 60°. The intensities of the patterns are shown in units of the differential scattering probability, dPe/dΩ, in both cases with . The electron density is given in units of number of electrons per Å2.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4263170&req=5

f2: Time-resolved scattering patterns and corresponding total electron densities.(a) Scattering patterns, and (b) total electron densities during the Cope rearrangement of semibullvalene via tunnelling. (c) Scattering patterns, and (d) total electron densities for the reaction over the barrier. The time-resolved scattering patterns are in the Qy−Qz plane (Qx=0) and the corresponding total electron densities are in the y−z plane (integrated along x-direction) at pump-probe delay times 0, T/4, T/2, 3T/4 and T. The time-resolved patterns are calculated until /Qmax/=10 Å−1, which corresponds to an incident X-ray pulse with 20 keV photon energy and the detection of photons scattered up to 60°. The intensities of the patterns are shown in units of the differential scattering probability, dPe/dΩ, in both cases with . The electron density is given in units of number of electrons per Å2.
Mentions: Time-resolved scattering patterns in the Qy−Qz plane (Qx=0) and the corresponding electron densities of space-fixed semibullvalene in the y–z plane as a function of the delay time at times 0, T/4, T/2, 3T/4 and T are presented in Fig. 2. Here T is the reaction time for the Cope rearrangement of semibullvalene, ranging from T=24.2 fs (1 fs=10−15 s) for the over-the-barrier reaction to T=970 s for the reaction via tunnelling at cryogenic temperatures31. The static structure of semibullvalene in the gas phase has been measured using electron scattering32. Figure 2a,c shows that the time-resolved scattering patterns distinguish the over-the-barrier reaction from tunnelling. In the tunnelling case, the intensity of scattered photons, associated with high Q-values, diminishes from the reactant to the reaction intermediate at T/2 and then increases again between T/2 and T (see Fig. 2a). The opposite behaviour is observed for the over-the-barrier reaction. The difference between the two reaction paths is most prominent at T/2. The reader is referred to Supplementary Fig. 1 for further details of the total scattering patterns. The origin of these differences is in the different rearrangement dynamics of the carbon core electrons for the two paths, which dominate the total electron density as visually shown by localized yellow-green circles in Fig. 2b,d for tunnelling and over-the-barrier pathways, respectively. The contribution of the valence electrons is very diffuse.

Bottom Line: Here we demonstrate an effective and robust method, which emphasizes the information encoded in weakly scattered photons, to image chemically active valence electron densities.The degenerate Cope rearrangement of semibullvalene, a pericyclic reaction, is used as an example to visually illustrate our approach.Our work also provides experimental access to the long-standing problem of synchronous versus asynchronous bond formation and breaking during pericyclic reactions.

View Article: PubMed Central - PubMed

Affiliation: Max Born Institute, Max-Born-Strasse 2A, 12489 Berlin, Germany.

ABSTRACT
Time-resolved imaging of chemically active valence electron densities is a long-sought goal, as these electrons dictate the course of chemical reactions. However, X-ray scattering is always dominated by the core and inert valence electrons, making time-resolved X-ray imaging of chemically active valence electron densities extremely challenging. Here we demonstrate an effective and robust method, which emphasizes the information encoded in weakly scattered photons, to image chemically active valence electron densities. The degenerate Cope rearrangement of semibullvalene, a pericyclic reaction, is used as an example to visually illustrate our approach. Our work also provides experimental access to the long-standing problem of synchronous versus asynchronous bond formation and breaking during pericyclic reactions.

No MeSH data available.