Film chemistry in real time with the high-speed x-ray camera

Chemistry is omnipresent. In chemical reactions, atoms are deposited in or between molecules while chemical bonds are formed and broken. These chemical bonds consist of valence electrons.

Accordingly, the motion of valence electrons is central to any chemical reaction. It should be noted that only a fraction of these valence electrons - often only a small part of the charge of an electron - actively participate in chemical reactions. And this happens extremely fast: the duration of many important chemical processes such as The first steps of vision and light collection in biological systems are only a few femtoseconds (1 femtosecond = 10-15 seconds). The recording of these chemically active valence electrons is therefore extremely demanding. First, you need a camera with excellent time and spatial resolution. Secondly, a very sensitive camera is needed, because you are not only interested in how the atoms move, but also in breaking chemical bonds and forming new bonds - and that means taking the fraction of active valence electrons that are in the ocean of all electrons move, which are bound to the atoms in molecules.

Fig. 1 Recording the fracture and the formation of chemical bonds during a pericyclic reaction: We show theoretically that the ultrafast X-ray camera is not only sensitive to chemically inert core electrons, but can also visualize the movement of chemically active valence electrons.

An x-ray camera easily meets the first requirement. The scattering of X-rays through matter has been an indispensable tool for the resolution of structures with atomic spatial resolution since the discovery of X-rays. Thanks to enormous technological progress, ultrashort X-ray flashes can now also be generated, which can extend the preliminary investigations by time resolution in the femtosecond range. These x-ray flashes promise to generate stroboscopic snapshots of chemical and biological processes in individual molecules.

Fulfilling the second prerequisite - sensitivity to active valence electrons - is not one of the strengths of an X-ray camera. The scattering of X-rays by molecules is always dominated by core electrons and inert valence electrons. Therefore, it is generally assumed that the small part of valence electrons, which is actively involved in chemical reactions, is lost in the total scattering signal and thus the epoch of the ultrafast rearrangement of active valence electrons by means of an X-ray camera is not possible.

Fig. 2 The combination of the routine analysis of X-ray scattering patterns (A, B) with the additional analysis of that range, which is limited to a relatively small momentum transfer, makes it possible to visualize the movement of chemically active valence electrons during a pericyclic reaction (C, D) , The formation and breakage of chemical bonds along different pathways can be directly recorded and analyzed.

Our work published in Nature Communications suggests a way to solve this challenge. Theoretically, we demonstrate a robust and effective method that allows information on chemically active valence electrons to be extracted from the X-ray scattering patterns of a single molecule - a key step in the effort to record the formation and breaking of chemical bonds in real time at atomic spatial resolution. Our work shows how the movement of chemically active valence electrons can be visualized by a combination of the routine analysis of X-ray scattering images with the additional analysis of that region of the scattering images which is limited to a relatively small momentum transfer.

In addition to demonstrating how chemically active valence electrons can be captured by X-rays, the work provides experimental access to the much discussed problem of synchronous versus asynchronous bond formation and bond breaking in chemical reactions. The ultra-fast X-ray camera confirms that the answer depends on whether the atoms have enough energy to override the energy barrier that separates reactants from products, or whether the atoms need to rely on the quantum phenomenon of the tunnel through the energy barrier. In the first case we confirm a lag time between breaking old and forming new bonds. In the second case, we observe no delay: the breakage of the old and the formation of new bonds is synchronous. We hope that our work will provide new insights into the initialization and control of complex chemical and biological reactions.

Original publication

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

T. Bredtmann, M. Ivanov, G. Dixit

Nature Communications 5 (2014) 5589/1-7