At least we are looking closer at the nature of water bonding. What
will be welcome is when we can add the odd impurity and discover the
implied effect. That will revolutionize chemistry. We are certainly
close.
What continues to surprise is the complexities of water as theory.
Everything else appears simple.
It is all good.
Scientists confirm
tetrahedral model of the molecular structure of water
by Staff Writers
Mainz, Germany (SPX) Feb 15, 2013
Researchers at
Johannes Gutenberg University Mainz (JGU) have confirmed the original
model of the molecular structure of water and have thus made it
possible to resolve a long-standing scientific controversy about the
structure of liquid water.
The tetrahedral model
was first postulated nearly 100 years ago and it assumes that every
water molecule forms a so-called hydrogen bond with four adjacent
molecules. This concept was almost toppled in 2004 when an
international research group announced that it had experimentally
established that water molecules form bonds only with two other
molecules.
"The quality of
the results was excellent but they merely represent a snapshot of the
situation," explained Professor Dr. Thomas Kuhne. He has
demonstrated the fallacy of the 'double bonding' theory using
computer simulations based on new types of combinations of two
computational methods recently developed by his group.
Some very special and
unique features of water, such as its liquid aggregate state and high
boiling point, are attributable to the effect of the hydrogen bonds
between the water molecules.
The H bonds are formed
due to the different charges carried by the oxygen and hydrogen atoms
that make up water molecules and the resultant dipolar structure. The
traditional, generally accepted view was that water had a tetrahedral
structure at room temperature, so that on average each water molecule
would be linked with four adjacent molecules via two donor and two
acceptor bonds.
"In our
theoretical approach, the median result we observed over time was
always for quadruple bonding," said Kuhne. Thanks to the new
simulations, he and his colleague Dr. Rhustam Khaliullin have now
been able to confirm the old model and also supply an explanation for
why double bonding was observed in 2004. According to Kuhne, the
result was not indicative of double bonding "but of
instantaneous asymmetrical fluctuation" only.
There is thus
significant asymmetry in the four H bonds of the tetrahedral model
because of the different energy of the contacts. This asymmetry is
the result of temporary disruptions to the hydrogen bond network,
which take the form of extremely short term fluctuations occurring on
a timescale of 100 to 200 femtoseconds.
These fluctuations
mean that one of the two donor or acceptor bonds is temporarily much
stronger than the other. But these fluctuations precisely cancel each
other out so that, on average over time, the tetrahedral structure is
retained.
The results reported
in 2004 using x-ray absorption spectroscopy were obtained using water
molecules with high levels of momentary asymmetry, which is why
essentially only two strong hydrogen bonds were observed in an
otherwise tetrahedral structure.
"Our findings
have important implications as they help reconcile the symmetric and
asymmetric views on the structure of water," write the
scientists in an article published in Nature Communications. The
results may also be relevant to research into molecular and
biological systems in aqueous solutions and provide insight into
protein folding, for example.
The work of Thomas
Kuhne's group was undertaken within an interdisciplinary joint
project and was funded by the Research Unit Center for Computational
Sciences at Johannes Gutenberg University Mainz; T. D. Kuhne, R. Z.
Khaliullin, Electronic signature of the instantaneous asymmetry
in the first coordination shell of liquid water, Nature
Communications 4, 5 February 2013
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