New technique reveals details of photochemical reactions

Scientists at the Department of Energy’s SLAC National Accelerator Laboratory have made a groundbreaking discovery by directly imaging a photochemical “transition state” during a ring-opening reaction in the molecule α-terpinene. Using a high-speed “electron camera” and advanced quantum simulations, the researchers were able to track the molecular structure during the reaction triggered by the absorption of light energy. This achievement, documented in a publication in Nature Communications, has significant implications for understanding similar reactions that play vital roles in chemistry, including the production of vitamin D in the human body.

Transition states are crucial configurations in chemical reactions, typically induced by heat rather than light. They represent a point of no return for molecules involved in the reaction, where they transiently rearrange themselves before completing the transformation into new molecules, fueled by the energy gained. Co-author Thomas Wolf, a scientist at SLAC, emphasizes the importance of studying transition states, stating that they provide valuable insights into the mechanisms and reasons behind reactions. By investigating analogous critical configurations in photochemical reactions, scientists can enhance their understanding of reactions with key roles in chemistry and biology. The availability of diffraction techniques now allows for the examination of specific characteristics of such reactions.

Previously, no method existed with sufficient sensitivity to capture these fleeting transition states, which last for mere fractions of a second. However, at MeV-UED (MeV Ultrafast Electron Diffraction), an instrument at SLAC, researchers employed a high-energy electron beam to precisely measure atomic distances within gas molecules. By taking snapshots of these distances at different time intervals after an initial laser flash, scientists can create a stop-motion movie depicting the light-induced atomic rearrangements in the molecules.

The significance of these findings extends to the realm of quantum mechanics underlying photochemistry. Yusong Liu, a co-author and scientist at SLAC, highlights the ability to compare experimental results with quantum simulations, allowing for a highly accurate depiction of molecular behavior and providing a benchmark for the predictive power of theoretical and computational methods.

In a previous study involving a related reaction, MeV-UED enabled the capture of the coordinated dance between electrons and nuclei, confirming a set of rules about the final product’s stereochemistry that had been proposed half a century ago. Building upon this success, the current experiment revealed that different parts of the atomic rearrangements occur at distinct time points, offering an explanation for the specific stereochemistry produced by the reaction.

Thomas Wolf reflects on his excitement in addressing long-standing questions regarding these reactions and the renowned rules that predict their outcomes. He acknowledges that while these rules provide predictions, they do not explain the underlying reasons and mechanisms, which the current research aims to elucidate.

Wolf further emphasizes that beyond fundamental research, another significant motivation for these experiments is the relevance of the same reaction in biological processes, such as the biosynthesis of vitamin D in human skin. The researchers plan to conduct follow-up studies to explore this connection further.

In summary, scientists have achieved a major breakthrough by directly imaging a photochemical transition state during a ring-opening reaction. This advancement not only enhances our understanding of reactions with critical roles in chemistry and biology but also has potential implications for the production of vitamin D in the human body.

Source: SLAC National Accelerator Laboratory

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