\n\t\t\t\t\t\t\t\t\t\tBYLINE: By Karyn Hede\t\t\t\t\t\t\t\t\t\t<\/p>\n
Newswise \u2014 RICHLAND, Wash.\u00a0\u2014\u00a0Some of nature\u2019s most important chemical reactions rely on the coupled movement of negatively and positively charged particles. These processes play central roles in photosynthesis, catalysis\u00a0and biological energy conversion\u00a0yet remain difficult to\u00a0observe.\u00a0<\/p>\n
Now, a research team led by the Department of Energy\u2019s Pacific Northwest National Laboratory, in collaboration with colleagues at\u00a0SLAC National\u00a0Accelerator\u00a0Laboratory\u00a0and several academic labs, has captured snapshots of these events that are triggered when light strikes a molecule.\u00a0<\/p>\n
The findings,\u00a0published in\u00a0Nature Communications<\/a>,\u00a0could help researchers better understand and\u00a0ultimately design\u00a0better flow batteries, fuel\u00a0cells\u00a0and catalysts.\u00a0<\/p>\n The research team focused on the coupled movement of positively charged particles called protons with negatively charged electrons. This coordinated energy transfer is among the most efficient\u00a0known to exist and in plants is used to capture the sun\u2019s energy and convert it\u00a0into stored energy, among other processes in nature.\u00a0<\/p>\n By moving electrons and protons in a coordinated fashion, molecules can bypass energetically costly intermediate steps. This makes reactions faster and dramatically more energy efficient. In this case,\u00a0the research team focused on how changes in a molecule\u2019s electronic structure, the addition of a proton\u00a0and the surrounding water environment are linked during the reaction.\u00a0<\/p>\n A first look\u00a0<\/p>\n Despite\u00a0studying this interplay\u00a0for decades, no one had previously captured\u00a0it\u00a0in a single study, with local and structural sensitivity. Advanced X-ray methods available at\u00a0the Linac Coherent\u00a0Light Source\u00a0at SLAC, combined with\u00a0state-of-the-art\u00a0quantum chemistry\u00a0calculations and molecular dynamics simulations, gave unprecedented insight into this crucial process.\u00a0<\/p>\n PNNL experimental chemical physicist Elisa Biasin, former PNNL scientist Abdullah Kahraman and PNNL theorists Niranjan (Niri) Govind and Amity Andersen, together with their collaborators, used a combination of ultrafast X-ray spectroscopy, scattering\u00a0and advanced simulations to capture key steps in a light-driven proton-coupled electron transfer reaction, or PCET. This combined approach reveals for the first time with structural sensitivity how gaining a proton reshapes a molecule\u2019s electronic structure at specific sites and reorganizes the surrounding water environment.\u00a0<\/p>\n \u201cWe have captured for the first time how electronic changes associated with proton transfer are coupled to reorganization of the surrounding solvent,\u201d said Biasin. \u201cThis gives us a new way to understand how molecules and their environments evolve together during fundamental chemical transformations.\u201d\u00a0<\/p>\n Why\u00a0this\u00a0reaction\u00a0matters\u00a0<\/p>\n PCET is a workhorse in nature, allowing plants to harvest light for photosynthesis and animals to efficiently metabolize food for energy, among other essential energy-conversion processes. In the experimental system the team focused on here, the basic mechanism is well understood.\u00a0<\/p>\n However, in some PCET reactions, there continues to be debate about the order of movement among protons and electrons.\u00a0<\/p>\n \u201cAre they happening together or not? At which molecular site? And how is the water network\u00a0facilitating\u00a0the proton hop?\u201d Biasin asked. \u201cThese are some of the possible open questions. To answer them, you need ultra-fast time resolution, chemical and structural sensitivity, and alignment with theory. We have made a step forward to shed light on these questions.”\u00a0<\/p>\n This\u00a0new approach\u00a0helps answer these questions. The study reveals how local changes in electronic structure are coupled with reorganization of the surrounding water network as the molecule gains a proton. By enabling researchers to probe these coupled molecular changes, the approach could help guide the design of more efficient catalysts, fuel cells, flow\u00a0batteries\u00a0and other energy-conversion technologies.\u00a0<\/p>\n Multiple techniques, one story\u00a0<\/p>\n Electrons move on timescales that make them difficult to track, and protons move\u00a0almost as\u00a0quickly. The water molecules bathing the reacting molecules reorganize themselves in ways that are notoriously difficult to\u00a0observe\u00a0directly.\u00a0Until now, experiments could provide\u00a0pieces of the picture\u00a0but not a combined view of molecular electronic changes and reorganization.\u00a0<\/p>\n To design the experiment, Biasin and her collaborators started with a well-studied ruthenium-based molecule that absorbs light and,\u00a0in acidic conditions,\u00a0captures\u00a0a proton from the surroundings.\u00a0<\/p>\n \u201cWe identified the metal complex used in this study because it does not undergo additional electronic and structural rearrangements that complicate interpretation of X-ray signals, allowing us to isolate signals associated with the electron, proton and solvent motion,\u201d said Christopher\u00a0Larsen,\u00a0a co-investigator and senior lecturer at the University of Aukland,\u00a0New Zealand.\u00a0<\/p>\n The research team then used time-resolved characterization techniques available at the University of Geneva to\u00a0determine\u00a0the\u00a0optimal\u00a0conditions and timescales for the X-ray experiments.\u00a0<\/p>\n To watch it in action, the team paired element-specific X-ray absorption spectroscopy\u00a0from\u00a0the\u00a0chemRIXS instrument<\/a>, which reveals how electrons\u00a0move between molecular sites, with time-resolved X-ray scattering\u00a0from\u00a0the\u00a0X-ray Correlation Spectroscopy (XCS)<\/a> instrument, which tracks the rearrangement of atoms, including the motions of solvent molecules.\u00a0<\/p>\n The project then drew on the complementary theoretical\u00a0expertise\u00a0of Govind and Andersen. They performed time-dependent density functional theory and molecular dynamics simulations, respectively, that were essential for interpreting the complex signals produced by the X-ray measurements and revealing the underlying proton-electron dynamics.\u00a0<\/p>\n First author Abdullah Kahraman said the combination of techniques was crucial to the findings.<\/p>\n “Understanding the photochemistry of this complex required us to push the limits of our data analysis. By combining X-ray absorption spectroscopy with precise theoretical modeling, we gained an unprecedented look into the real-time electronic changes driving these reactions,\u201d said Kahraman, who worked on the project at SLAC while he was a PNNL postdoctoral associate.<\/p>\n Govind highlighted the theoretical aspect of the research.<\/p>\n \u201cWhile this was an experiment-driven discovery, our theoretical work provided the molecular-level interpretation needed to translate the X-ray measurements into a detailed picture of the underlying coordination between proton, electron and solvent motion.\u201d\u00a0<\/p>\n Biasin notes that the study is limited in that the team could not directly\u00a0observe\u00a0the proton.\u00a0<\/p>\n \u201cX-ray scattering mostly sees atoms that are rich with electrons, and so the proton is not seen directly,” she said. \u201cBut we observe the local reorganization of the electronic structure, together with the global reorganization of the water networks, and we can draw conclusions based on the agreement between data and calculations.”\u00a0<\/p>\n Nonetheless, the multimodal X-ray technique lays out a framework that other researchers can now use to study PCET reactions in more complex chemical systems.\u00a0<\/p>\n \u201cMany of the most important chemical reactions involve electrons, protons, and their surrounding environment moving together on ultrafast timescales,\u201d\u00a0said Roberto Alonso Mori, senior scientist at SLAC and a coauthor on the study.\u00a0\u201cBy combining complementary X-ray techniques at LCLS, this work provides a uniquely complete view of these coupled processes, opening new opportunities to understand and ultimately control the chemistry that underpins energy conversion and catalysis.\u201d\u00a0<\/p>\n Coauthor and\u00a0SLAC\u00a0staff scientist David Hoffman\u00a0added,\u00a0\u201cThis is an important first step in combining X-ray scattering and spectroscopy to study these complicated processes in a model system. With the better signal-to-noise offered by the LCLS-II upgrade, we can use these methods to solve real problems in catalysis and energy harvesting.\u201d\u00a0<\/p>\n