Scientists used ultrafast electron diffraction technology to image the structure of the pericyclic minimum, the "transition state" of the electrocyclic reaction. In chemical reactions, molecules pass through critical geometries as they transform from reactants to reaction products. In chemistry, geometry refers to the arrangement of atoms in a molecule. Scientists often refer to the critical geometry in a reaction as a transition state. The lifespan of this state is almost incomprehensibly short, less than a millionth of a second.
Artist's illustration of the observed photochemical "transition state" structure (middle). This state lasts less than a millionth of a second. Image source: Courtesy of Greg Stewart, SLAC National Accelerator Laboratory
Scientists recently captured critical geometries using SLAC's ultra-high-speed "electronic camera." Combined with quantum simulations of the reaction, the researchers identified the critical structure as one end of the molecule bent away from the rest of the molecule.
Chemists use the reaction investigated in this study, a so-called electrocyclic reaction, because it produces very specific reaction products. These products can be predicted by the Woodward-Hoffman rule. These rules won the Nobel Prize in Chemistry in 1981 and are taught in every organic chemist's undergraduate education.
However, these rules do not explain in detail why a reaction produces only a specific reaction product. The new results help resolve this open question. Additionally, they open the way for researchers to create new rules for other types of reactions. This helps make organic chemistry a more powerful tool.
Electrocyclic reactions are characterized by the simultaneous formation and dissociation of multiple chemical bonds through a critical geometry. In the molecule studied in this project, alpha-terpinene, two double bonds and one single bond are converted into three double bonds. The synchrony and single critical configuration of these processes ensure their stereospecificity, a property that makes them important tools in synthetic chemistry. Stereospecificity can be predicted by the well-known Woodward-Hoffman rule.
This study combines ultrafast electron diffraction and simulation of the reaction kinetics of α-terpinene to investigate a photochemical (i.e., light-triggered) electrocyclic ring-opening reaction. Stereospecificity of reactions in α-terpenes is guaranteed by the two ends of the emerging chain reaction product rotating away from each other in the same clockwise or counterclockwise direction, as predicted by the Woodward-Hoffmann rule.
The new results show that the origin of stereospecificity does not lie in the exact nature of the motion. Rather, stereospecificity is determined by the fact that the change from two to three double bonds has already occurred to a large extent when the molecule assumes a critical geometry. The single-bond dissociation that leads to the opening of the α-terpinene ring occurs during the transition of the molecule from its critical geometry to the reaction product.