In laboratories at the Pacific Northwest National Laboratory and the SLAC accelerator center, scientists have for the first time obtained molecular images of how water rearranges around a molecule during a key chemical reaction. Light triggers a simultaneous transfer of an electron and a proton, and the surrounding water instantly changes its structure to support this process.
The reaction, known as proton-coupled electron transfer, underlies photosynthesis, the operation of fuel cells, and many catalytic processes. A molecule absorbs a photon, the electron redistributes, and a proton attaches to a specific site. Until now, no one could observe all these changes simultaneously with atomic precision.
The researchers applied a combination of X-ray techniques: absorption spectroscopy to track electron movement and X-ray scattering to record the rearrangement of water molecules. Experiments were conducted at the Linac Coherent Light Source at SLAC, where ultrashort pulses allowed them to 'freeze' events on the picosecond timescale.
As a model, they chose a ruthenium complex that does not complicate the signals with extraneous rearrangements. Theoretical calculations and molecular dynamics simulations helped decipher the data and link local changes in electronic structure to the global rearrangement of the water network.
The results open the way to creating more efficient catalysts, flow batteries, and energy conversion devices. Understanding how the solvent environment affects charge transfer will allow engineers to better control these processes in real systems.
The scientists note that direct observation of the proton is not yet possible, but the agreement between experiment and calculations provides a reliable picture. The new technique is already ready for application to more complex biological and technological reactions.
This observation shows how closely the molecule and its aqueous environment act as a single unit, and opens new horizons for studying fundamental chemical transformations.
