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A long-sought ammonia-dimer solution

University of Delaware physicist Krzysztof Szalewicz and collaborators have resolved a long-standing debate about ammonia dimers — two joined ammonia molecules — that will be helpful for chemists, biologists and other scientists.
University of Delaware physicist Krzysztof Szalewicz and collaborators have resolved a long-standing debate about ammonia dimers — two joined ammonia molecules — that will be helpful for chemists, biologists and other scientists.

UD’s Krzysztof Szalewicz and collaborators determine ammonia molecules form hydrogen bonds with each other

It takes a lot of brain power to be a theoretical physicist. It also takes far more than brain power to be a theoretical physicist.

The calculating minds of University of Delaware physicist Krzysztof Szalewicz and his collaborators, for example, use more than 26 million hours annually on Department of Defense computers. They routinely use UD’s High Performance Computing clusters as well.

And that’s what it takes now to produce increasingly precise information to support new science and advanced applications.

Such muscular machines weren’t available 30 years ago, when an active debate was going on about the likelihood of ammonia dimers — two joined ammonia molecules — forming hydrogen bonds.

The debate was an important one. Ammonia is a molecule of significance on many fronts, including those on our planet and far beyond it. Understanding the properties of ammonia molecules and how they interact with other molecules has critical value for industry, pharmaceuticals, biology and production of environmentally sustainable fuels, for example.

Szalewicz, an expert in the study and calculation of intermolecular forces, and his collaborators found a reliable, highly accurate answer to the question. Their findings were published recently in Nature Communications. Aling Jing, a graduate student on Szalewicz’s team, was the lead author. Ad van der Avoird, a theoretical chemist from the Netherlands, was a third collaborator.

Their work resolves the debate and gives chemists and biologists and other scientists new confidence as they develop new experiments, materials and processes.

A bit of background on hydrogen bonds may be helpful to understand how Szalewicz’s new calculations shed light on this issue.

When two hydrogen atoms connect with one oxygen atom the bonds are strong and are called “covalent” bonds. These strong bonds form water molecules — H2O.

When two water molecules are near each other, a hydrogen atom from one molecule will form a bond with the oxygen atom of the other molecule. This is a  hydrogen bond, which is not as strong as the covalent bond intrinsic to the water molecule, but is still a powerful part of intermolecular dynamics.

The covalent bond is what holds the water molecule together. The hydrogen bond is what holds multiple water molecules together, making it possible to pour yourself a big glass of water.

The hydrogen bonds between water molecules are settled science.

Until 1985, the ammonia-hydrogen bond question was considered settled, too. An ammonia molecule (NH3) is made of one nitrogen atom connected to three hydrogen atoms by covalent bonds.

The debate about whether ammonia molecules could form hydrogen bonds with other ammonia molecules was reopened in 1985, when new experiments suggested that the ammonia dimers — pairs of ammonia molecules — are not hydrogen bound, in contrast to the predictions of previous theories.

More calculations, experiments and debate followed.

“Finally, people said ‘It is too hard. We cannot do anything more,’” Szalewicz said.

But as computing muscle became increasingly available, more accurate calculations were possible, providing increasingly precise pictures of the mechanisms in play.

Szalewicz and his collaborators now have produced a calculation of the potential energy surface of the ammonia dimer, which shows how the interaction energy of the molecules is related to their geometric shapes.

“What we have found now is that, yes, it was a hard problem,” Szalewicz said. “The answer is not completely ‘yes, period.’ We cannot say that.”

What they have shown, with highest confidence, is that ammonia dimers are quite flexible, not rigid, as the 1985 experiment concluded. This means that a broad range of intermolecular separations and orientations is covered during the intermolecular motions.

The published experimental configuration turned out to be an average between two hydrogen-bonded configurations. This is like a snapshot of intermolecular motion, which was assumed by the experimental group to be the most likely configuration, but actually is fairly rare.

By factoring in many more data points, Szalewicz and collaborators went far beyond single configurations to show that the hydrogen bonds were far more likely than not. That kind of precision makes a huge difference in how you incorporate ammonia molecules in various applications and has many other implications for chemistry.

Szalewicz compares it to taking an extended hike through a mountain range.

“You go up, up, up from a valley to a pass,” he said. “Then you go down to another valley. If the pass is high above the valley, it is a hard hike. The valley corresponds to the hydrogen-bonded configurations and with a high pass, getting from one valley to another is difficult. Thus, molecules stay mostly in the valleys and finding the dimer at the top of the pass is a very rare event.

“The ammonia-dimer valley surface is different from those of typical hydrogen-bonded molecules. Instead of two well-separated valleys, there is one very narrow one containing both hydrogen-bonded configurations, with almost no pass between them. Finding the dimer at the top of the pass is a fairly likely event. Therefore, it could be observed in experiments.”

This is why experimental physicists need theoretical physicists and also why theoretical physicists need experimental physicists.

“There is an old joke that is actually very true,” Szalewicz said. “When an experimentalist publishes a result, everyone believes it — except the experimentalist, who always knows they might have overlooked something. When a theorist publishes a result, nobody believes it — except the theorists.”

When they work together, as they must, great insights are likely.

Experiments also measure excitations of intermolecular motions. Szalewica and collaborators performed quantum-mechanical calculations of such excitations, obtaining excellent agreement with the experiment. This is a strong validation of the correctness of the surface developed in the calculations.

Using similar calculations with water, Szalewicz has previously published potential energy surfaces that help to explain properties of water that have not been previously explained. They now are used by industrial chemists who work on steam engines and need to know those properties at various temperatures.

The National Institute of Standards and Technology now recommends using these theoretical calculations, which have shown greater accuracy than experimental measurements.

The research was supported by a grant from the National Science Foundation.

About the researcher

Krzysztof Szalewicz is a professor of physics and astronomy at the University of Delaware. His research interests include intermolecular forces, atomic and molecular physics and quantum mechanics. He has made many contributions to understanding these dynamics, including development of the symmetry-adapted perturbation theory (SAPT) used to perform calculations of intermolecular interactions.

He earned his doctorate at the University of Warsaw, Poland. Before joining the faculty at UD in 1988, he was an associate research scientist at the University of Florida and an assistant professor of chemistry at the University of Warsaw.

He was elected to the International Academy of Quantum Molecular Science and is a fellow of the American Physical Society.