Traveling at the speed of protons

For the first time, researchers have measured the speed at which protons move through charged water molecules, shedding light on one of the most fundamental chemical reactions.

Researchers at Yale University (CT, USA) have measured how quickly protons move through charged water, setting the benchmark for this key event, which could enable more accurate chemical simulations in energy storage, rocket science and more.

A key property of water is how networks of water molecules deform in the presence of an electrical charge. However, how water transports positive charge via protons has remained a mystery until now.

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N-O room to hide

In this work, led by Mark Johnson, the team studied the proton transfer that occurs when six molecules are attached to 4-aminobenzoic acid carrying an extra proton. They developed a special mass spectrometer and a ‘pump-probe’ method to study how the N-isomer changed into the O-isomer.

“To monitor the movement of the charge, you need a special type of organic molecule that can attach a proton in two different locations that are easily differentiated by the color of light they absorb,” commented Payten Harville (Yale University). “It’s designed so that the only way for protons to get from one docking site to the other is to hitch a ride on a water network ‘taxi’.”

Johnson and his team applied an IR laser to the N-isomer, pumping energy in, then utilized a UV laser to probe for the presence of the O-isomer. Over time, they were able to measure the presence of the O-isomer by how the UV absorption increased.

Their set-up allowed for this reaction to be analyzed 10 times a second and revealed that the conversion of one isomer to another followed a single-exponential decay that took 0.9 microseconds.

“We show what happens in a tiny molecular system where there is no place for the protons to hide. We’re able to provide parameters that will give theorists a well-defined target for their chemical simulations, which are ubiquitous but have been unchallenged by experimental benchmarks…we know where the proton started and where it ended up, and now we know how long it takes to get there,” Johnson concluded.