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

Timm Bredtmann,Gopal Dixit,Misha Ivanov

doi:10.1038/ncomms6589

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.

Highlights

Time-resolved imaging of chemically active valence electron densities is a long-sought goal, as these electrons dictate the course of chemical reactions
We describe an effective and robust approach that allows us to extract changes of that part of the total electron density directly related to bond making and bond breaking during chemical reactions, that is, chemically active valence electron density, from the overall X-ray scattering pattern, which itself is dominated by the core electrons
Depending on the reactant energy, the Cope rearrangement can proceed by tunnelling or over the barrier. Such alternative pathways are ubiquitous in many chemical reactions

Summary

Introduction

Time-resolved imaging of chemically active valence electron densities is a long-sought goal, as these electrons dictate the course of chemical reactions. We describe an effective and robust approach that allows us to extract changes of that part of the total electron density directly related to bond making and bond breaking during chemical reactions, that is, chemically active valence electron density, from the overall X-ray scattering pattern, which itself is dominated by the core electrons. This allows us to image the flow of valence electrons in space and time during a chemical reaction and solve one of the major problems, which has hampered the progress of time-resolved imaging of chemical reactions. Our example addresses another important and long-debated issue: is the breaking of old bonds during pericyclic reactions synchronous with the formation of new bonds? We show that our method distinguishes these two alternatives

Methods

Results

Conclusion