X-ray snapshots reveal how neighboring molecules influence radical movement

Researchers have identified how the structure of neighboring molecules dictates where radicals reside and the range of their movement. By observing the "first contact" between a radical and its surrounding environment, the study provides insights into how these highly reactive molecules behave before chemical bonds are formed (311).

The research team, led by professors from the Pohang University of Science and Technology (POSTECH) and the Korea Advanced Institute of Science and Technology (KAIST), used time-resolved X-ray solution scattering (TR-XSS) to capture these interactions (311). This technique allows for the recording of molecular motion through a series of snapshots taken at extremely short intervals (311). Using a synchrotron facility in Japan, the team used laser pulses to split iodine molecules into radicals within solutions of benzene and mesitylene (311). The researchers observed that "contact complexes," in which radicals are in close proximity to neighboring molecules, had already formed within 100 picoseconds (311).

The study revealed distinct movement patterns based on the solvent used (311). In benzene, which has a hexagonal ring structure, the radicals occupied positions near the edge of the ring and moved around its perimeter (311). In mesitylene, which has methyl group side branches attached to the ring, the radicals behaved differently (311). In this environment, the radicals favored directions that lacked side branches and were restricted to a much narrower range of movement (311).

These findings suggest that the surrounding molecular environment can be engineered to improve reaction selectivity (311). Because the placement of a single atom can significantly impact the effectiveness of a drug or the performance of a material, controlling radical movement is vital (311). Professor Kyung Hwan Kim stated that the study moves beyond inferring processes from final products to identifying the structure of short-lived contact states (311). He added that applying this work to reactants in practical synthesis could help design environments that guide desired chemical reactions (311).