Capturing the Fleeting “Transition State” of a Photochemical Reaction in Real-Time: A Scientific Milestone

by Amir Hussein
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Transition State

An artist’s depiction shows the fleeting photochemical “transition state” configuration, which endures for less than a trillionth of a second. The image is credited to Greg Stewart of the SLAC National Accelerator Laboratory.

Researchers have utilized ultrafast electron diffraction techniques to visualize the pericyclic minimum, referred to as the “transition state,” in electrocyclic chemical reactions.

Within the realm of chemical reactions, molecules transition from their initial state as reactants to their final state as products through an essential geometrical configuration. In this context, geometry pertains to the spatial arrangement of atoms within the molecule. Such crucial configurations in chemical processes are commonly termed transition states and are notable for their extremely short lifespan, lasting less than a trillionth of a second.

Recently, the cutting-edge electron camera at SLAC was employed to capture this pivotal configuration. This, in tandem with quantum simulations, facilitated the identification of this critical structure as the point at which one end of the molecule deviates from its original orientation relative to the rest of the molecule.

Significance of the Study

Electrocyclic reactions, the type of reaction examined in this research, yield highly specific products that can be forecasted using the Woodward-Hoffmann rules. Established in 1981 with a Nobel Prize in chemistry, these rules are foundational to the education of every organic chemist. However, these rules fall short of providing an exhaustive explanation for why only particular reaction products are produced. The current findings contribute to resolving this unresolved issue and pave the way for scholars to formulate new rules for additional kinds of reactions, thereby augmenting the capabilities of organic chemistry.

Summary of Research Findings

Electrocyclic reactions are distinguished by the simultaneous making and breaking of several chemical bonds via a single critical configuration. In this specific study focused on the molecule alpha-terpinene, two double bonds and a single bond undergo transformation into three double bonds. The Woodward-Hoffmann rules assert that the specific spatial orientation of the resulting molecular structure is dictated by a coordinated rotation at the terminal ends of the emergent molecular chain.

Contrary to existing assumptions, the recent findings indicate that the root cause of this specific spatial orientation does not lie in the precise nature of molecular movements. Instead, the specificity is a consequence of the fact that the transition from two to three double bonds predominantly occurs as the molecule assumes this critical geometry. The subsequent dissociation of a single bond, which leads to the ring-opening of the alpha-terpinene molecule, takes place later in the transition from the critical configuration to the final reaction products.

Acknowledgments

This research received support from the AMOS program under the Department of Energy (DOE) Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. The MeV-UED instrument used is part of the Linac Coherent Light Source at the SLAC National Accelerator Laboratory. Additional support was provided by the DOE Office of Science, Office of Basic Energy Sciences, SUF Division Accelerator and Detector R&D program, the LCLS Facility, and SLAC. Study co-author David Sanchez received backing from the Lawrence Livermore National Laboratory. The study was published on 18 May 2023 in Nature Communications, with the DOI: 10.1038/s41467-023-38513-6.

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