Scientists capture a `quantum tug' between neighboring water molecules


Date:
August 25, 2021
Source:
DOE/SLAC National Accelerator Laboratory
Summary:
Researchers have made the first direct observation of how hydrogen
atoms in water molecules tug and push neighboring water molecules
when they are excited with laser light.



FULL STORY ========================================================================== Water is the most abundant yet least understood liquid in nature. It
exhibits many strange behaviors that scientists still struggle to
explain. While most liquids get denser as they get colder, water is most
dense at 39 degrees Fahrenheit, just above its freezing point. This is
why ice floats to the top of a drinking glass and lakes freeze from the
surface down, allowing marine life to survive cold winters. Water also
has an unusually high surface tension, allowing insects to walk on its
surface, and a large capacity to store heat, keeping ocean temperatures
stable.


==========================================================================
Now, a team that includes researchers from the Department of Energy's
SLAC National Accelerator Laboratory, Stanford University and Stockholm University in Sweden have made the first direct observation of how
hydrogen atoms in water molecules tug and push neighboring water molecules
when they are excited with laser light. Their results, published in
Nature today, reveal effects that could underpin key aspects of the
microscopic origin of water's strange properties and could lead to a
better understanding of how water helps proteins function in living
organisms.

"Although this so-called nuclear quantum effect has been hypothesized
to be at the heart of many of water's strange properties, this
experiment marks the first time it was ever observed directly," said
study collaborator Anders Nilsson, a professor of chemical physics at
Stockholm University. "The question is if this quantum effect could be
the missing link in theoretical models describing the anomalous properties
of water." Each water molecule contains one oxygen atom and two hydrogen atoms, and a web of hydrogen bonds between positively charged hydrogen
atoms in one molecule and negatively charged oxygen atoms in neighboring molecules holds them all together. This intricate network is the driving
force behind many of water's inexplicable properties, but until recently, researchers were unable to directly observe how a water molecule interacts
with its neighbors.

"The low mass of the hydrogen atoms accentuates their quantum wave-like behavior," said collaborator Kelly Gaffney, a scientist at the Stanford
Pulse Institute at SLAC. "This study is the first to directly demonstrate
that the response of the hydrogen bond network to an impulse of energy
depends critically on the quantum mechanical nature of how the hydrogen
atoms are spaced out, which has long been suggested to be responsible for
the unique attributes of water and its hydrogen bond network." Love thy neighbor Until now, making this observation has been challenging because
the motions of the hydrogen bonds are so tiny and fast. This experiment overcame that problem by using SLAC's MeV-UED, a high-speed "electron
camera" that detects subtle molecular movements by scattering a powerful
beam of electrons off samples.



==========================================================================
The research team created 100-nanometer-thick jets of liquid water -
about 1,000 times thinner than the width of a human hair -- and set the
water molecules vibrating with infrared laser light. Then they blasted
the molecules with short pulses of high-energy electrons from MeV-UED.

This generated high-resolution snapshots of the molecules' shifting
atomic structure that they strung together into a stop-motion movie of
how the network of water molecules responded to the light.

The snapshots, which focused on groups of three water molecules, revealed
that as an excited water molecule starts to vibrate, its hydrogen atom
tugs oxygen atoms from neighboring water molecules closer before pushing
them away with its newfound strength, expanding the space between the molecules.

"For a long time, researchers have been trying to understand the
hydrogen bond network using spectroscopy techniques," said Jie Yang,
a former SLAC scientist and now a professor at Tsinghua University in
China, who led the study. "The beauty of this experiment is that for the
first time we were able to directly observe how these molecules move."
A window on water The researchers hope to use this method to gain more
insight into the quantum nature of hydrogen bonds and the role they play
in water's strange properties, as well as the key role these properties
play in many chemical and biological processes.

"This has really opened a new window to study water," said Xijie Wang,
a SLAC distinguished staff scientist and study collaborator. "Now that
we can finally see the hydrogen bonds moving, we'd like to connect
those movements with the broader picture, which could shed light on how
water led to the origin and survival of life on Earth and inform the development of renewable energy methods." MeV-UED is an instrument of
the LCLS user facility, operated by SLAC on behalf of the DOE Office of Science, which funded this research.

========================================================================== Story Source: Materials provided by
DOE/SLAC_National_Accelerator_Laboratory. Original written by Ali
Sundermier. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Yang, J., Dettori, R., Nunes, J.P.F. et al. Direct observation of
ultrafast hydrogen bond strengthening in liquid water. Nature,
2021 DOI: 10.1038/s41586-021-03793-9 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210825113614.htm

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