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"The dream of chemists is to control chemical reactions," says
coauthor Greg Schenter of the Pacific Northwest National Laboratory (PNNL)
in Richland, Wash. Adds coauthor Maciej Gutowski, formerly of PNNL and
now at the Heriot-Watt University in Edinburgh, UK, "We want the
reaction to happen when we want it to happen, and to go along a
certain chemical pathway."
"We may be able to use this to get hydrogen out of the solid state,
like in hydrogen storage materials," says Schenter. If so, that might
lead to economic, safe and practical hydrogen-fueled automobiles. The
fundamental result could help illuminate biological reactions as well,
such as when radiation damages DNA within cells, says coauthor Kit
Bowen of Johns Hopkins University in Baltimore, Md.
"Its value in my mind is that this reaction is a simple prototype.
There are some very complicated reactions that occur this way," Bowen
says. "It also shows that environmental effects are very important in
reactivity."
The reaction is so common in everyday life that students to
grandmothers are aware of it in some way. Many people know not to mix
window cleaner and toilet bowl cleaner: compounds in each like to
react, sometimes giving off dangerous fumes and leaving ammonium
chloride in their wake. But what many people don't know is that if you
take just one molecule each of the troublemakers, ammonia and hydrogen
chloride, the two just can't get their act together.
In water, the reaction between ammonia (NH3) and hydrogen
chloride (HCl) is a textbook example of acid-base chemistry most of us
learn in high school. By its chemical nature, the nitrogen in ammonia
prefers to be attached to four hydrogens rather than the mere three it
has, so it steals the hydrogen from hydrogen chloride.
The theft leaves chloride alone and negative. But the nitrogen
molecule (now called ammonium) has gained a positive charge from the
stolen hydrogen, and that attracts the chloride. The attraction is not
as strong as the so-called covalent bond between the nitrogen and its
fan base, but the ammonium and chloride form an ionic bond, one that
forms when opposites attract. To a chemist, this looks like NH4+Cl-.
But that's in a crowd - not so in private.
Previous research has shown that when one ammonia molecule exists in
isolation with one hydrogen chloride molecule, nothing happens. All
the necessary, classical components are there: positive hydrogens
(also called protons) and negative electrons, but still, nothing
happens. Researchers have long suspected additional electrons floating
around in the high-volume environment could somehow help the ammonia
and hydrogen chloride molecules to react. If so, an ammonium chloride
in nature would really look like [NH4+Cl-]-.
"Extra electrons are everywhere," says computational chemist Schenter.
"When you rub a balloon in your hair, you knock electrons off your
hair and the balloon's surface and you get static electricity. You
can't get away from them."
To test the idea, the experimentalists, led by physical chemist Bowen,
had to do the reaction in reverse. First, they created a molecule of
ammonium chloride adorned with an extra electron - [NH4+Cl-]-.
Using a beam of light, they then measured how easily different colors
of light knocked off that electron. Losing the electron leaves behind
an offkilter NH4+Cl-, which
immediately rearranges into a cozy pair, NH3 and HCl.
With computer programs developed to understand the nature of chemical
bonding and structure at the DOE's Environmental Molecular Sciences
Laboratory on the PNNL campus, the theory and modeling team took that
data and used it to gauge how closely the chloride's hydrogen was
sidling up to the ammonia's nitrogen when the extra electron is around.
The resulting picture showed how losing the surplus electron can cause
ammonia and hydrogen chloride to transform into ammonium chloride.
"It's like a switch," says Schenter. "In the presence of electrons, it
behaves one way. Without electrons, it behaves another way."
The researchers solved another riddle as well. Chemists have long
wondered about that interaction between that cozy pair, one molecule
of ammonia and one molecule of hydrogen chloride. The bond could be
either ionic in nature or more like a so-called hydrogen bond, weaker
than both ionic and covalent bonds but with characteristics of each.
Comparing the data in the absence and presence of electrons, the
theoretical team determined the types of arrangements the nitrogen,
hydrogens and chloride could be in. From these, they concluded the
molecules formed a hydrogen bond.
Understanding the reaction brings hope that chemistry will have a
clean future. "If you can control the reaction, you can operate in a
safe, environmentally friendly way," Gutowski says.
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