Scientists capture a ‘quantum tug’ between neighboring water molecules

The Science

Water is the most abundant liquid in nature, but it’s also one of the least understood. Water has unusual properties. For example, most materials get denser when they get colder, but water is densest a few degrees above freezing. That’s why ice floats. Scientists suggest that water’s so-called “proton quantum effect” may be at the heart of many of water’s strange properties. This experiment with ultrafast electron diffraction marks the first time that scientists have directly observed this quantum effect in water. Scientists observed how the hydrogen atoms in water molecules tug and push neighboring molecules when water is excited with laser light. The results reveal quantum effects that could underpin key aspects of the microscopic origin of water’s strange properties. This could lead to a better understanding of how water helps proteins function in living organisms

The Impact

Each water molecule contains one oxygen and two hydrogen atoms. Water molecules are held together by a web of hydrogen bonds. In these webs, hydrogen and oxygen link in two ways. The hydrogen protons that are covalently bound (bonds involving atoms that share electrons) to oxygen in one molecule are also weakly attracted electrically to another oxygen in neighboring molecules. This intricate network drives many of water’s strange properties. Researchers used short pulses of high-energy electrons to observe these interactions. This quantum effect could be the missing link in theoretical models describing the strange properties of water. It could also help to develop renewable energy methods using polymer membranes that transport hydrogen.

Summary

Many strange properties of liquid water, such as its density, which is greatest at 4 degrees Celsius and causes ice to float, originate from water’s well-connected hydrogen bond network. A complete unveiling of the intermolecular dynamics of water requires direct time- and structure-resolved measurements. Neither X-rays nor neutron scattering can be used to study water’s hydrogen bond structure dynamics due to the lack of scattering power (X-rays) or time resolution (neutrons). This research, by a team that included SLAC National Accelerator Laboratory, Stanford University, and Stockholm University, employed the MeV-UED instrument at the Linac Coherent Light Source (LCLS), a Department of Energy (DOE) user facility at SLAC. The MeV-UED instrument (for Megaelectronvolt Ultrafast Electron Diffraction) is a high-speed “electron camera,” a unique tool capable of investigating water’s hydrogen bond structure dynamics. The experimental results reveal unique effects, such as how the contraction of the hydrogen bond emerges upon laser excitation. Revealing key aspects of the microscopic origin of water’s strange properties could lead to a better understanding of how water helps proteins function in living organisms. Understanding the quantum behavior also could be important in modeling the many critical reactions that involve hydrogen, including carbon-hydrogen bond activation, water oxidation, carbon dioxide reduction, and acid-base chemistry critical for energy-related phenomena.



Funding

Development and operation of the Ultrafast Electron Diffraction (UED) instrument and the research reported here were supported by DOE’s Office of Science, Basic Energy Sciences (Chemical Sciences, Geosciences, and Biosciences Division, Materials Sciences and Engineering Division, and Scientific User Facilities Division), and Fusion Energy Sciences as well as Stanford University fellowships. The UED is part of the Linac Coherent Light Source (LCLS), a DOE Office of Science user facility at SLAC National Accelerator Laboratory.