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Physical Sciences
Research Highlights

June 2007

Ammonia Molecules Straighten Up

New results shed light on origin of hydrogen bonding in ammonia

Ammonia molecules
At Pacific Northwest National Laboratory and the University of Southern California, scientists determined that ammonia clusters are held together by classical linear hydrogen bonds. Enlarged View

Results: While the bouncing and jostling behavior of collections of water molecules has been understood for some time now, scientists have just established that ammonia, thought to behave differently, has the same characteristics with water in the bonding of adjacent molecules.

Since the mid-1980s, scientists have argued whether or not an ammonia molecule forms a linear hydrogen bond with another ammonia molecule or it is involved in a more complex arrangement. Researchers at Pacific Northwest National Laboratory and the University of Southern California have determined that ammonia clusters are held together by classical linear hydrogen bonds. Ammonia is composed of a nitrogen atom with three smaller hydrogen atoms bonded to it. An ammonia cluster is a number of ammonia molecules held together by hydrogen bonds, a kind of interaction that is weaker than conventional chemical bonds.

As PNNL's Sotiris Xantheas explained, "Unlike water, which requires consideration beyond pairs of molecules, ammonia's interactions are dominated by mainly the pairs."

While ammonia gas and liquid behavior is dominated by the interactions between pairs of molecules, the main building block for solid ammonia is a trimer, three molecules arranged in an acute triangle. In contrast, in liquid water or ice, three adjacent water molecules form obtuse or more open triangles.

Why it matters: Hydrogen bonds and the networks they form play an important role in understanding and controlling processes from refining crude oil to metabolizing proteins. Understanding the basic properties of different archetypal hydrogen bonds, such as the ones in water and ammonia clusters, could lead to critical insights in catalysis, fate and transport of contaminants and biological function.

Methods: Using highly refined experimental methods and a solid understanding of chemical theory, the research team performed an elegant series of experiments. At the University of Southern California, researchers placed two to four molecules of ammonia inside a droplet of helium kept at near absolute zero or -459°F. The researchers used the boiling of the helium atoms from the droplet caused by the absorption of laser light to obtain detailed spectra.

At PNNL, researchers calculated the structures and fully anharmonic vibrational spectra for the ammonia clusters of various sizes that were probed in the experimental studies. These calculations aided in the assignment of the experimental spectra to specific arrangements between the ammonia molecules in a specific cluster.

After careful analysis, the research team determined that an ammonia molecule forms a nearly linear hydrogen bond with another ammonia molecule and that additional hydrogen bonds with another ammonia molecules do not perturb this pattern. Based on the rigor of the experiments and the computed detailed spectral data, the researchers believe that cyclic trimer is a main building block of solid ammonia.

What's next: With this joint research completed, the scientists plan to apply their techniques and experience to address other challenges in the area of hydrogen bonded clusters.

Acknowledgments: DOE's Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences and the National Science Foundation funded this work.

Reference: MN Slipchenko, BG Sartakov, and AF Vilesov, SS Xantheas, "Study of NH Stretching Vibrations in Small Ammonia Clusters by Infrared Spectroscopy in He Droplets and ab Initio Calculations", Roger E. Miller Memorial Issue, Journal of Physical Chemistry Part A, vol. 111, pp. 7460-7471 (2007).

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