Getting beneath the surface of water

Water is everywhere on Earth and is crucially important to life, but there’s plenty that scientists don’t yet understand about its properties at the atomic scale. Now a group of researchers have made the first direct observation of how the atoms within water molecules move when excited, a finding that may lead to a more accurate picture of how energy flows during chemical reactions. (Nature, 2021, DOI: 10.1038/s41586-021-03793-9)

The researchers used the mega-electron-volt ultrafast electron diffraction instrument at SLAC National Accelerator Laboratory to capture the motion of molecular bonds within the water. They started with jets of water 100 nm thick, excited them with infrared laser light, and then fired bursts of high-energy electrons from the instrument into the stream to detect the vibrations.

The team found that when a water molecule starts to vibrate, its hydrogen atom initially pulls the oxygen atoms in neighboring molecules toward it. Then, as the system heats up, the atoms move apart again. In order to simulate this behavior correctly, the researchers had to use quantum mechanical simulations. Classical simulations don’t account for the position of the hydrogen atom with high enough resolution, and therefore resulted in too much error.

Understanding the weird properties of liquid water, such as the fact that it is densest a few degrees above freezing, while most materials get denser when they get colder, may depend on understanding how the hydrogen bonds in water behave. Unfortunately, says Kelly Gaffney, a professor of photon science at Stanford and one of the authors of the work, the impact of quantum mechanical effects on these behaviors of water may not be very large.

On the other hand, understanding the quantum behavior could be important in modeling the many critical reactions that involve hydrogen, he says, including carbon-hydrogen bond activation, water oxidation, carbon dioxide reduction, and acid-base chemistry.

“I think this experiment sets a baseline for the water behavior,” says study author Davide Donadio of the University of California, Davis. “So it gives us a tool to understand different environments,” like at an interface with another substance, “at that very fine atomistic scale,” he says.

Yuki Nagata, a chemical engineer at Max Planck Institute for Polymer Research, who was not involved in the study, calls capturing the ultrafast motion of water at high resolution a great technological leap. Scientists had known that the motion of a water molecule was closely tied to that of its neighbors, but until now they hadn’t known how, he says. The fact that the bond between oxygen and hydrogen first contracts before releasing its energy is counterintuitive, he says, but it shows how the vibration of the atoms is tied to the larger motion of the molecules. The work may not have much in the way of practical implications, he says, “but it is an impressive piece of fundamental research that helps understanding why water is so special.”