Scientists have long known that molecules dance about as the temperature rises,
but now researchers know the exact steps that water takes with a certain molecule.
Results with small, electrically charged cyanide ions and water molecules reveal
that water zips around ions to a greater extent than expected. The findings
improve our understanding of a chemical interaction important in environmental
and atmospheric sciences.
 | | Water molecules (red and white) dance around cyanide ions (blue and gray) more actively than previously thought. Credit: Image courtesy American Chemical Society. |
"One of the cornerstones of Department of Energy nuclear cleanup missions
and climate research is a fundamental understanding of water and ions, one of
the most common chemical interactions in the environment," said chemist
Xue-Bin Wang of the DOE's
Pacific Northwest National Laboratory and Washington State University.
"We've developed a new instrument to probe the dynamics of ions in water,"
Wang said. "And we've combined theory and modeling to make sense of those
experiments, giving us a deeper fundamental understanding of what is happening
with this ubiquitous molecule – water."
Wang, PNNL physical chemist Sotiris S. Xantheas, physical chemist Lai-Sheng
Wang of PNNL and WSU, and their colleagues published the results in the Journal
of Physical Chemistry A. The journal featured their work on the cover of its
September 3 issue.
Thirst for Details
Environmental scientists want to know how contaminants move through watery
environments below ground, and atmospheric scientists want to know how small
particles flutter through water vapor in the sky. To get at the basics, they
study a simpler interaction: water and ions, small atoms or molecules that have
a slight electrical charge and exist everywhere in nature.
For example, when common table salt -- sodium chloride -- dissolves in water,
the negatively charged chloride ions (Cl–) and the positively charged
sodium ions (Na+) each interact separately with the water molecules.
Previous work with chloride ions and water has yielded conflicting results
about how a water molecule (which is shaped like a boomerang) and a chloride
ion (shaped like a ball) face each other. Other groups study barbell-shaped
cyanide ions because many molecules found naturally in the environment contain
cyanide. The chemical interactions of water and either chloride or cyanide are
influenced by the charge and the shape of the molecules, as well as the temperature
in which they find themselves.
But directly observing temperature's role in how water and cyanide ions interact
has been difficult. So, the team developed a unique instrument that allowed
them to precisely control the temperature down to almost absolute zero, or the
temperature at which everything freezes. The team used "temperature-controlled
photoelectron spectroscopy" in EMSL, the DOE's Environmental Molecular
Sciences Laboratory on the PNNL campus, to determine how tightly one cyanide
ion and one to three water molecules interact at the very low temperature of
-438 F (12 Kelvin) and again at ambient temperature of 80 F (equivalent to 300
Kelvin).
Unexplained Energy
The team measured the molecules' "electron binding energy" at low
and high temperatures. This energy is an indication of how tightly the molecules
hold onto their electrons -- the tighter the hold, the stronger the bonds that
will form between molecules. The team found that ones at low temperature exhibited
higher electron binding energy than the ones at high temperatures, as they had
expected. However, the difference between the two scenarios was greater than
the team expected.
To explore the unexpected difference in energy, the researchers ran computer
simulations on the Chinook supercomputer in EMSL. This also let them determine
how the boomerang-shaped water and barbell-shaped cyanide faced each other.
First they estimated how much energy the molecules used to take different configurations.
Then they compared the computer-based estimates to the data they collected in
their unique instrument at different temperatures.
The team found that the molecules behaved differently at cold and warm temperatures.
At lower temperatures, the boomerang-shaped water held still while the cyanide
teetered at the end of one of water's two arms. There, the cyanide flipped,
sometimes pointing its carbon (C) atom towards the water's arm, and sometimes
pointing its nitrogen (N). At the coldest temperature tested, -438 F, the molecules
froze, with cyanide pointing its nitrogen end at the water.
Hot to Trot
At ambient temperatures, however, the barbell-shaped cyanide held steady while
the water molecule rocked and flipped around the cyanide. Although the researchers
were surprised at how much the water moved, the many positions water could take
explained why they saw less electron binding energy than they expected at room
temperature: A wiggly water means that the bond between molecules isn't that
tight.
"Water can interact with cyanide's carbon or nitrogen and rock back and
forth on one atom," said Wang. He added that the detail they get with this
instrument is impressive. "Scientists have known for years that atoms move
around when temperature rises. Now they can determine the most probable position
that the molecule is in at different temperatures."
The results also explain the conflicting results with chloride ions and water,
the researchers said, because of the importance of temperature on that interaction
as well.
The researchers plan to follow up with studies that include many water molecules
and ions at once, as well as with more complex ions than cyanide.
Posted september 2nd, 2009
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