Scientists have directly imaged a photochemical “transition state” during a ring-opening reaction in the molecule terpinene using a high-speed “electron camera” at the Department of Energy’s SLAC National Accelerator Laboratory and cutting-edge quantum simulations. A specific configuration of a molecule’s atoms that determines the chemical outcome. Through a photochemical ring-opening reaction that occurs when the molecules of a substance absorb light energy, scientists have for the first time precisely tracked the structure of molecules.
The outcomes, distributed in Nature Correspondences, could promote how we might interpret comparable responses with essential jobs in science, like the creation of vitamin D in our bodies.
The majority of chemical reactions that are sparked by heat rather than light involve transition states. For molecules involved in a chemical reaction, they are like a point of no return: before completing their transformation into new molecules, the molecules rearrange themselves into a fleeting configuration as they acquire the energy required to fuel the reaction.
“The ability to compare the predictive capability of theoretical and computational approaches and obtain a very accurate picture of how molecules behave is made possible by comparing our experimental data with quantum simulations of the reaction.”
SLAC scientist and co-author Yusong Liu.
“Progress states truly educate you a great deal regarding how and why responses occur,” said co-creator and SLAC researcher Thomas Wolf. “Photochemical reactions that play important roles in chemistry and biology might be better understood if similar critical configurations are investigated. We actually must now take a gander at a few explicit qualities of such responses utilizing our diffraction methods.”
There was never a method that was sensitive enough to capture these brief states, which last only one millionth of a billionth of a second, up until this point. The researchers used the SLAC instrument for ultrafast electron diffraction, MeV-UED, to precisely measure the distances between the atoms in the gas’s molecules by passing a high-energy electron beam through the gas at millions of electronvolts (MeV). Taking depictions of these distances at various stretches after an underlying laser streak permits researchers to make a stop-movement film of the light-prompted nuclear modifications in the particles.
“These responses are significant for understanding the quantum mechanics supporting photochemistry,” said SLAC researcher and co-creator Yusong Liu. “We can get a very clear picture of how molecules behave and measure the predictive power of theoretical and computational methods by comparing our experimental results to quantum simulations of the reaction.
MeV-UED enabled the team to capture the coordinated dance between electrons and nuclei in a previous study of a related reaction. A set of rules about the stereochemistry, or three-dimensional arrangement, of the final product’s atoms that have been around for half a century were finally confirmed for the first time by the results.
The fact that some of the atomic rearrangements occur earlier than others, as the researchers found in this experiment, explains the specific stereochemistry produced by the reaction.
“I as of late thought back on a few old introductions I did in school about these kinds of responses and the well-known set of decisions that foresee the results. However, these guidelines do not actually explain why or how reactions occur.” Wolf said. “What’s more, presently I’m returning to that and can begin responding to these inquiries, and that makes it unbelievably invigorating for me.”
Wolf stated that the fact that the same reaction occurs in biological processes like the biosynthesis of vitamin D in human skin is another major reason for conducting these experiments. The researchers intend to further investigate this connection through subsequent studies.
More information: Y. Liu et al, Rehybridization dynamics into the pericyclic minimum of an electrocyclic reaction imaged in real-time, Nature Communications (2023). DOI: 10.1038/s41467-023-38513-6
