The way protons move through water is one of the most important and mysterious processes in chemistry.
It’s at the heart of how our eyes work, how energy is stored in batteries, and even how rocket fuel functions.
Scientists have known about this phenomenon for more than two centuries—but no one had ever directly measured it on a microscopic scale.
Now, researchers at Yale University have done just that. Using a highly specialized instrument developed over decades, the Mark Johnson lab has, for the first time, set benchmarks for how long it takes protons to move through a network of six water molecules.
The work, published in Science, provides long-sought experimental data that will serve as a crucial guide for theoretical models used around the world.
“We show what happens in a tiny molecular system where there is no place for the protons to hide,” said Mark Johnson, senior author of the study and the Arthur T. Kemp Professor of Chemistry at Yale.
“We’re able to provide parameters that will give theorists a well-defined target for their simulations.”
Protons, the tiny positively charged particles at the center of atoms, don’t behave in ways that are easy to capture.
They move according to the rules of quantum mechanics, which means they shift rapidly and unpredictably, almost like dancers passing from partner to partner. Scientists believe they carry charge through water using a relay-like process, hopping from one water molecule to another.
But until now, there had been no direct experimental measurement of how long this “hop” takes.
To study the process, the team used a small organic molecule, 4-aminobenzoic acid, which can attach an extra proton in two distinct spots.
Each site absorbs a different color of light, allowing the researchers to see when a proton moves from one to the other. The only way this can happen is if the proton “catches a ride” on the surrounding water molecules, making the molecule an ideal probe for proton transfer.
The real breakthrough, however, came from the lab’s custom-built mass spectrometer. Stretching 30 feet through Yale’s Sterling Chemistry Laboratory, the machine combines lasers, piping, electronics, and a cooling chamber that chills molecules to nearly absolute zero.
The device allows researchers to build small assemblies of water molecules, trigger reactions with laser light, and measure the results—ten times every second.
After years of refinement, the instrument finally revealed what scientists had been seeking: the timeframe for a proton’s journey. The researchers could not capture the proton mid-hop, but they now know where it starts, where it ends, and how long the trip takes.
“This study marks the first time we’ve been able to measure the rate of this fundamental chemical process,” said Johnson. “It’s a small system, but it’s a big step toward understanding one of chemistry’s most essential mysteries.”
