The water hexamer, shown, is the smallest assembly of water molecules that adopts a three-dimensional structure displaying hydrogen bonding patterns characteristic of liquid water and ice.
is the most abundant and one of the most frequently studied substances
on Earth, yet its geometry at the molecular level—the simple two
hydrogen atoms and one oxygen atom, and how they interact with other
molecules, including other water—has remained somewhat of a mystery to
understanding at that level is theoretical, requiring the use of
supercomputers to make innumerable calculations over periods of weeks to
make educated guesses as to the arrangements and structure of water
clusters before they form into liquid water or ice.
a new study, using experimentation with a highly advanced spectrometer
for molecular rotational spectroscopy, has removed some of the mystery
and validates some very complex theory involving the way water molecules
bond. It is published in the May 18 issue of the journal Science.
set out to determine quantitatively the structure that small assemblies
of water adopt, and then compare them to theory to see how well current
quantum chemistry predicts the properties of molecules,” said Brooks
Pate, a chemist in the University of Virginia’s College of Arts &
Sciences who led the study. “We found experimentally that modern quantum
chemistry has reached the point where its theories are proving out in
the lab regarding the unusual directional bonding properties of water
properties of water, and how it interacts with itself and other
molecules, is the basis for many processes in biology, and likely played
a major role in the development of life on Earth. But understanding how
those bonds form at the molecular level has been largely guesswork.
the first time, now we have an actual physical picture of what water’s
molecules put together look like, and it turns out they adopt three
different geometries,” Pate said. “This is in agreement with theory.”
and his U.Va. team identified and imaged a three-dimensional geometry
that a water molecule takes on that is the likely precursor structure
for forming liquid water and ice.
found that the bonding strengths of liquid water actually begin to
emerge even in a tiny cluster,” Pate said. “The challenge is figuring
out how it interacts with other molecules and how the forces between two
molecules of water can be described quantitatively, because the
orientation of how the waters come at each other makes a big difference
in the binding.”
are innumerable possibilities for how this happens, and theorists,
including Pate’s colleague on the paper, quantum chemist George Shields
of Bucknell University, have been working on the details for years
without direct validation from experiments.
difficulty has been in developing techniques that are sensitive enough
to image the tiny water molecules and how they orient themselves when
interacting with other water molecules. The breakthrough came earlier
this year when Pate’s U.Va. team used a new tool, a molecular rotational
spectrometer developed during the last two years, to make precise
measurements that ultimately validated what theory has expressed.
improved sensitivity of the instrument comes from advances in
high-speed digital electronics that provide unprecedented data
throughput in the measurement. This core technology is being
commercialized for applications in chemical analysis by a
Charlottesville start-up venture, BrightSpec.
will allow chemists to transfer what we’ve learned to larger systems,”
Pate said. “We are checking to see if theory can get right the
structures of the arrangements of water molecules so that that
information can be used to see how water interacts in larger systems.”
larger systems could include bio-molecules, such as protein in DNA, and
how surrounding water molecules might interact with those molecules
through hydrogen bonds.
is very satisfying to see that the experimental work we did, completely
independently of theory, came together so perfectly with the theory,”
said his research is the behind-the-scenes workings of chemistry that
ultimately makes up the big picture of how things come together at much
may not want to know how a bridge was designed, but you sure want to
know it was done right,” he said. “Likewise, if a theory is used to
predict, for example, how a medicine might work, you ideally would want
to be able to test the theory to make sure it’s right before making the
medicine. That would be the ultimate goal—to have theory and
experimentation in sync.”
Source: University of Virginia