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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.


Reconstructed valence electron densities and the pericyclic electron densities.(a) Reconstructed valence electron densities obtained via the restricted Q-reconstruction method, and (b) the pericyclic electron densities (ρperi(r; R)) obtained via partitioning the total electron densities for the Cope rearrangement of semibullvalene via tunnelling. (c) Reconstructed valence electron densities, and (d) ρperi(r; R) for the over-the-barrier reaction. The electron densities are in the y−z plane at pump-probe delay times 0, T/4, T/2, 3T/4 and T. ρperi(r; R) accounts for the six rearranging valence electrons according to the Lewis structures (cf. Fig. 1b). The reconstruction of the densities is performed using scattering intensity information up to /Qlimited/=3.4 Å−1 from the full time-resolved scattering patterns as shown in Fig. 2. The electron density is given in units of number of electrons per Å2.
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f4: Reconstructed valence electron densities and the pericyclic electron densities.(a) Reconstructed valence electron densities obtained via the restricted Q-reconstruction method, and (b) the pericyclic electron densities (ρperi(r; R)) obtained via partitioning the total electron densities for the Cope rearrangement of semibullvalene via tunnelling. (c) Reconstructed valence electron densities, and (d) ρperi(r; R) for the over-the-barrier reaction. The electron densities are in the y−z plane at pump-probe delay times 0, T/4, T/2, 3T/4 and T. ρperi(r; R) accounts for the six rearranging valence electrons according to the Lewis structures (cf. Fig. 1b). The reconstruction of the densities is performed using scattering intensity information up to /Qlimited/=3.4 Å−1 from the full time-resolved scattering patterns as shown in Fig. 2. The electron density is given in units of number of electrons per Å2.

Mentions: We now demonstrate how the chemically active valence electron densities, which carry invaluable information about chemical reactions and hence electronic bond-to-bond fluxes, can be directly accessed from the full scattering patterns: although there is no strict separation of the contributions from the core, inert valence and the chemically active valence electrons to the total scattering pattern, their relative contributions might be different in different regions of the Q-space. The delocalized valence electrons scatter weakly and should show their fingerprints mostly in the low Q-region of the scattering pattern. The well-localized core electrons, on the other hand, scatter strongly and contribute across the full scattering pattern. For the Cope rearrangement of semibullvalene, representing a whole range of pericyclic reactions, the chemically active valence electrons can indeed be brought to the fore to a remarkable extent by restricting the reconstruction to the relatively small momentum transfer, corresponding to the low Q-region, Qmax>Qlimited, as shown in Fig. 4. Performing this (restricted-Q) fourier transform from the Q-space back to the coordinate space requires knowledge of the phases associated with the full scattering pattern. Several phase-retrieval procedures exist to obtain the phase and reconstruct the total electron density from the scattering pattern for both crystalline and non-crystalline samples3940. While in Fig. 4 the phase information is inferred from the theoretical calculations, we show in Supplementary Figs 2 and 3 that virtually identical results are obtained if the phase information is retrieved using a hybrid-input-output (HIO) algorithm, based on iterative fourier transformations, back and forth between the momentum space and real space41.


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

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

Reconstructed valence electron densities and the pericyclic electron densities.(a) Reconstructed valence electron densities obtained via the restricted Q-reconstruction method, and (b) the pericyclic electron densities (ρperi(r; R)) obtained via partitioning the total electron densities for the Cope rearrangement of semibullvalene via tunnelling. (c) Reconstructed valence electron densities, and (d) ρperi(r; R) for the over-the-barrier reaction. The electron densities are in the y−z plane at pump-probe delay times 0, T/4, T/2, 3T/4 and T. ρperi(r; R) accounts for the six rearranging valence electrons according to the Lewis structures (cf. Fig. 1b). The reconstruction of the densities is performed using scattering intensity information up to /Qlimited/=3.4 Å−1 from the full time-resolved scattering patterns as shown in Fig. 2. 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

f4: Reconstructed valence electron densities and the pericyclic electron densities.(a) Reconstructed valence electron densities obtained via the restricted Q-reconstruction method, and (b) the pericyclic electron densities (ρperi(r; R)) obtained via partitioning the total electron densities for the Cope rearrangement of semibullvalene via tunnelling. (c) Reconstructed valence electron densities, and (d) ρperi(r; R) for the over-the-barrier reaction. The electron densities are in the y−z plane at pump-probe delay times 0, T/4, T/2, 3T/4 and T. ρperi(r; R) accounts for the six rearranging valence electrons according to the Lewis structures (cf. Fig. 1b). The reconstruction of the densities is performed using scattering intensity information up to /Qlimited/=3.4 Å−1 from the full time-resolved scattering patterns as shown in Fig. 2. The electron density is given in units of number of electrons per Å2.
Mentions: We now demonstrate how the chemically active valence electron densities, which carry invaluable information about chemical reactions and hence electronic bond-to-bond fluxes, can be directly accessed from the full scattering patterns: although there is no strict separation of the contributions from the core, inert valence and the chemically active valence electrons to the total scattering pattern, their relative contributions might be different in different regions of the Q-space. The delocalized valence electrons scatter weakly and should show their fingerprints mostly in the low Q-region of the scattering pattern. The well-localized core electrons, on the other hand, scatter strongly and contribute across the full scattering pattern. For the Cope rearrangement of semibullvalene, representing a whole range of pericyclic reactions, the chemically active valence electrons can indeed be brought to the fore to a remarkable extent by restricting the reconstruction to the relatively small momentum transfer, corresponding to the low Q-region, Qmax>Qlimited, as shown in Fig. 4. Performing this (restricted-Q) fourier transform from the Q-space back to the coordinate space requires knowledge of the phases associated with the full scattering pattern. Several phase-retrieval procedures exist to obtain the phase and reconstruct the total electron density from the scattering pattern for both crystalline and non-crystalline samples3940. While in Fig. 4 the phase information is inferred from the theoretical calculations, we show in Supplementary Figs 2 and 3 that virtually identical results are obtained if the phase information is retrieved using a hybrid-input-output (HIO) algorithm, based on iterative fourier transformations, back and forth between the momentum space and real space41.

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.