After 200 years, a key physical chemistry mystery may have been solved: ScienceAlert

The question of exactly how protons move through water in an electric field has fascinated scientists for centuries. Now, more than 200 years after the last significant image of the phenomenon, scientists have some clarity.

In 1806, Theodor Grotthuss formulated a hypothesis, which became known as the Grotthuss mechanism for “proton hopping”, about how a charge can flow through a water solution.

While Grotthuss’s hypothesis was very advanced for its time – before protons or even the true structure of water were known – modern researchers have long known that it did not provide a complete understanding of what happened at the molecular level.

The latest findings on the subject may have solved the mystery by solving the electronic structures of hydrated protons that have so long remained elusive.

The findings suggest that protons move through water in “trains” of three water molecules, with “tracks” built ahead of the train as it goes and pulled up as it passes.

This loop can continue indefinitely to transport protons through the water. While the idea has been proposed before, the new study yields a different molecular structure that better fits the solution proposed by Grotthuss, according to the study’s authors.

“Debates about the Grotthuss mechanism and the nature of proton solvation in water have increased, as this is one of the most fundamental challenges in chemistry,” says chemist Ehud Pines of Ben-Gurion University of the Negev in Israel.

The new study is exciting because it combines a theoretical approach with physical experimentation made possible by recent technological advances. The researchers used an X-ray absorption spectroscopy (XAS) experiment to monitor how the proton charges affected the electrons in the individual oxygen atoms of water.

As predicted, the effect was greatest in three water molecules, although to a different degree in each individual molecule within the ternary complex. The researchers found the groups of three molecules that form chains with the proton.

The researchers also incorporated chemical simulations and calculations at the quantum level to determine the interactions between protons and neighboring water molecules as the protons move through the liquid.

“Understanding this mechanism is pure science, pushing the boundaries of our knowledge and changing one of our fundamental understandings of one of nature’s most important mass and charge transport mechanisms,” says Pines.

The discovery plays a role in many other chemical processes, including photosynthesis, cellular respiration and energy transfer in hydrogen fuel cells.

It’s not just the solution that’s remarkable, but how the researchers were able to arrive at it—testing and validating theoretical predictions against experimental results and vice versa, in a long, involved process that took nearly two decades from start to finish. end.

“Everyone had been thinking about this problem for over 200 years, so that was quite a challenge for me to decide to tackle,” says Pines. “Seventeen years later, I’m happy that I probably found and demonstrated the solution.”

The research has been published in Angewandte Chemie International Edition.

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