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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.


Density differences for the Cope rearrangement of semibullvalene.The total electron density at time zero is subtracted from the electron densities at later delay times during the course of the reaction via (a) tunnelling, and (b) for the over-the-barrier reaction. The density differences are presented in the y−z plane at pump-probe delay times T/4, T/2, 3T/4 and T.
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f3: Density differences for the Cope rearrangement of semibullvalene.The total electron density at time zero is subtracted from the electron densities at later delay times during the course of the reaction via (a) tunnelling, and (b) for the over-the-barrier reaction. The density differences are presented in the y−z plane at pump-probe delay times T/4, T/2, 3T/4 and T.

Mentions: Although the full time-resolved scattering patterns distinguish tunnelling from the over-the-barrier reaction, the changes in chemical bonding are hardly visible since the corresponding electron densities do not peak at the positions of the bonds33343536. Frequently one analyses total electron density differences to gain insight into valence electron rearrangements (see for example, refs 6, 11, 12). These are obtained by subtracting the total electron density at time zero from the successive snapshots of the total electron density. Such electron density differences in the y–z plane at times T/4, T/2, 3T/4 and T are shown in Fig. 3a,b for tunnelling and the over-the-barrier reaction, respectively. The contributions of core and inert valence electrons are much more pronounced in the electron density differences, hiding information from chemically active electrons. It is worth to mention that the analysis of the normalized density difference yields identical conclusions.


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

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

Density differences for the Cope rearrangement of semibullvalene.The total electron density at time zero is subtracted from the electron densities at later delay times during the course of the reaction via (a) tunnelling, and (b) for the over-the-barrier reaction. The density differences are presented in the y−z plane at pump-probe delay times T/4, T/2, 3T/4 and T.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: Density differences for the Cope rearrangement of semibullvalene.The total electron density at time zero is subtracted from the electron densities at later delay times during the course of the reaction via (a) tunnelling, and (b) for the over-the-barrier reaction. The density differences are presented in the y−z plane at pump-probe delay times T/4, T/2, 3T/4 and T.
Mentions: Although the full time-resolved scattering patterns distinguish tunnelling from the over-the-barrier reaction, the changes in chemical bonding are hardly visible since the corresponding electron densities do not peak at the positions of the bonds33343536. Frequently one analyses total electron density differences to gain insight into valence electron rearrangements (see for example, refs 6, 11, 12). These are obtained by subtracting the total electron density at time zero from the successive snapshots of the total electron density. Such electron density differences in the y–z plane at times T/4, T/2, 3T/4 and T are shown in Fig. 3a,b for tunnelling and the over-the-barrier reaction, respectively. The contributions of core and inert valence electrons are much more pronounced in the electron density differences, hiding information from chemically active electrons. It is worth to mention that the analysis of the normalized density difference yields identical conclusions.

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.