SLAC's high-speed electron chamber films a molecular film in HD

SLAC's high-speed electron chamber films a molecular film in HD

The researchers created the first atomic resolution film of the ring opening reaction of 1,3-cyclohexadiene (CHD) with an 'electron chamber'. called UED. Bottom: the UED electron beam precisely measures the distances between pairs of atoms in the CHD molecule as the reaction progresses. The distance between each pair is represented by a colored line in the graph. The variations in distances as the molecule changes shape represent the molecular film. Upper part: visualization of the molecular structure corresponding to the distribution of the distance measured in approximately 380 femtoseconds in the reaction (dashed line in the lower part). Credit: David Sanchez / Stanford University

With an extremely fast "electron chamber" at the National SLAC Accelerator Laboratory of the Department of Energy, researchers have made the first high-definition "film" of ring-shaped molecules that open in response to light. The results could improve our understanding of similar reactions with vital functions in chemistry, such as the production of vitamin D in our bodies.

A previous molecular film of the same reaction, produced with the Coherent Light Source X-ray laser (LCLS) from SLAC, recorded for the first time major structural changes during the reaction. Now, using the lab's ultra-fast electronic diffraction (UED) instrument, these new results provide high resolution details, which show, for example, how a bond in the ring breaks and the atoms move for long periods of time.

"The details of this ring opening reaction have already been resolved," said Thomas Wolf, a Stanford Pulse Institute scientist at SLAC and Stanford University and leader of the research team. "The fact that we can now directly measure changes in bond distances during chemical reactions allows us to ask new questions about the fundamental processes stimulated by light."

SLAC scientist Mike Minitti, who participated in both studies, said: "The results demonstrate how our unique instruments for studying ultra-fast processes complement each other, where LCLS stands out in capturing snapshots with extremely fast shutter speeds of only a few few femtoseconds, or millionths of a billionth of a second, UED increases the spatial resolution of these snapshots.This is a great result, and the studies validate the results of others, which is important when completely new measurement tools are used. "

LCLS Director Mike Dunne said: "We are now making the SLAC UED instrument available to the scientific community, as well as enhancing the extraordinary capabilities of LCLS by doubling its energy range and transforming its repetition rate. both tools position us in a unique way to allow the best possible studies of fundamental processes in ultra-small and ultra-fast scales ".

The team reported its results today in Chemistry of nature.

Visualization of a molecular film made with the SLAC electronic camera, in which researchers have captured in atomic detail how a ring-shaped molecule is opened in the first 800 millionths of a billionth of a second after being reached by a laser flash. Ring-opening reactions like this play an important role in chemistry, such as the synthesis of vitamin D in our bodies. Credit: Thomas Wolf / PULSE Institute

Molecular film in HD

This particular reaction has been studied many times before: when a ring-shaped molecule called 1,3-cyclohexadiene (CHD) absorbs light, a bond breaks and the molecule develops to form the quasi-linear molecule known as 1, 3,5-hexatriene. (HT). The process is an example of a ring-opening reaction textbook and serves as a simplified model for studying the processes driven by light during vitamin D synthesis.

In 2015, the researchers studied the reaction with LCLS, which resulted in the first detailed molecular film of its kind and revealed how the molecule changed from a ring to a cigar-like shape after being struck by a laser flash. The snapshots, which initially had a limited spatial resolution, were focused more through computer simulations.

The new study used UED, a technique in which researchers send a beam of electrons with high energy, measured in millions of electron volts (MeV), through a sample, to accurately measure distances between pairs of atoms. Taking snapshots of these distances at different intervals after an initial laser flash and tracking how they change allows scientists to create a motion-stop film of the structural changes induced by light in the sample.

The electron beam also produces strong signals for highly diluted samples, such as the CHD gas used in the study, said SLAC scientist Xijie Wang, director of the MeV-UED instrument. "This allowed us to follow the ring's opening reaction for much longer periods of time than before."

Amazing details

The new data revealed several surprising details about the reaction.

SLAC's high-speed electron chamber films a molecular film in HD

This illustration shows snapshots of the light-activated transition of the 1,3-cyclohexadiene (CHD) molecule ring-shaped (background) to its extended 1,3,5-hexatriene (HT) form (foreground) . The snapshots were taken with the high-speed "electron chamber" of SLAC, an instrument for ultra-fast electron diffraction (UED). Credit: Greg Stewart / SLAC National Accelerator Laboratory

They showed that the movements of the atoms accelerated when the CHD ring broke, helping the molecules to get rid of excess energy and accelerating their transition to the stretched HT form.

The film also captured how the two ends of the HT molecule moved as the molecules became more and more linear. These rotational movements lasted at least one picosecond, or one trillionth of a second.

"I never would have thought that these movements would last so long," Wolf said. "It shows that the reaction does not end with the opening of the ring and that there is a much more lasting movement in the processes induced by light than previously thought."

A method with potential.

The scientists also used their experimental data to validate a recently developed computational approach to include the movements of atomic nuclei in simulations of chemical processes.

"UED provided us with data that has the high spatial resolution needed to test these methods," said Stanford chemistry professor and PULSE researcher Todd Martinez, whose group led the computational badysis. "This document is the most direct test of our methods, and our results are in excellent agreement with the experiment."

In addition to improving the predictive power of computer simulations, the results will help deepen our understanding of the fundamental chemical reactions of life, Wolf said: "We have high hopes that our method will pave the way for studies of more complex molecules that are even closer to Those used in the processes of life. "

New & # 39; molecular film & # 39; reveals an ultra-fast chemistry in motion

More information:
The photochemical ring opening of 1,3-cyclohexadiene by ultrafast electron diffraction image, Chemistry of nature (2019). DOI: 10.1038 / s41557-019-0252-7,

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National Laboratory of SLAC Accelerators

SLAC's high-speed electron chamber films the molecular film in HD (2019, April 15)
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