Super-cooled metallic ammonia gives clues about electron behavior

June 08, 2020

Buried deep within the varied properties of metals, there is a fundamental question up for grabs: What makes it a metal? In other words, which properties are inherent to a metal and which are incidental?

Scientists at USC Dornsife College of Letters, Arts and Sciences, collaborating with an international group of researchers, have helped further define what constitutes a metal. In the process, they open the door for organic chemists to improve the synthesis of useful compounds.

The study appears on the cover of the journal Science on June 5.

Going full metal

Intuition suggests that metals are dense, and while that bears true for some (think gold or lead), it fails to hold up for others. For example, lithium -- commonly used in batteries -- floats on water. Some metals are hard, such as titanium, yet others yield easily to pressure, including indium and aluminium. How about melting temperature? Platinum melts at more than 1,700 degrees Celsius (3,200 F), but mercury is a liquid well below zero.

Many other definitions of 'metal-hood' suffer similar contradictions, but British physicist and Nobel laureate Sir Nevill Mott provided one inherent property: "I've thought a lot about 'What is a metal?' and I think one can only answer the question at T = 0. There a metal conducts, and a non-metal doesn't."

Only metals are able to conduct electricity at absolute zero, the temperature at which even molecular motion all but stops. Conduction, unlike density or hardness, is an inherent property of metals.

Seeking to further understand the intrinsic properties of metals, the USC Dornsife scientists, led by Stephen Bradforth, professor of chemistry and divisional dean for natural sciences and mathematics, and their colleagues used a trick first noted by chemist Sir Humphry Davy in 1809. In essence, they made a metal from scratch.

The scientists cooled ammonia -- normally a gas at room temperature -- to minus 33 C to liquify it and then added, in separate experiments, the alkali metals lithium, sodium and potassium.

In these solutions, electrons from the alkali metal initially become trapped in the gaps between ammonia molecules. This creates what scientists call 'solvated electrons,' which are highly reactive but stabilized in the ammonia. These solutions have a characteristic blue color. Given enough solvated electrons (by adding about 1% alkali metal to the ammonia) the whole liquid turns bronze and, in essence, becomes a metal while remaining liquid.

Solvated electrons have proven to be extremely important to organic chemists. Through a reaction called the "Birch reduction," named after chemist Arthur Birch, they were key to synthesizing many important compounds and led to the manufacture of oral contraceptives in the 1950s.

Beaming in on electrons

The scientists next measured the amount of energy needed to bump the solvated electrons out of metallic ammonia using an X-ray beam. In a first-ever experiment, they forced different concentrations of the metallic ammonia through a microjet, which creates a stream about the width of a human hair that then passes through a hair-thin X-ray beam. Electrons interacting with the X-rays become dislodged.

The results show that, at low concentrations, solvated electrons are more easily dislodged from the solution by the interaction with the X-rays, giving a simple energy pattern. At higher concentrations, though, the energy pattern suddenly develops a sharp band edge, indicating the solution is behaving as a metal would.

Most important, however, the experiment presents a way for researchers to assess the behavior of solvated electrons in ammonia when other compounds are introduced to the mix. This opens a new window for chemists to synthesize important organic compounds, going well beyond Birch's experiments.

"This is the sort of things that goes in textbooks, or at least changes how textbooks are written," Bradforth said, noting the potentially historic importance of the work.

The work allows scientists to understand exactly how chemicals react with metallic ammonia, in essence giving the researchers a frame-by-frame view of what's happening in the test tube. Armed with this information, chemists can alter conditions to ensure they produce exactly the desired end product.
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Authors on the study include Bradforth, McMullen and Tillmann Buttersack of USC Dornsife; Pavel Jungwirth, Philip E. Mason, Tomas Martinek, Krystof Brezina, Martin Crhan and Axel Gomez of the Czech Academy of Sciences in the Czech Republic; Bernd Winter, H. Christian Schewe and Hebatallah Ali of the Fritz-Haber-Institut der Max-Planck-Gesellschaft in Germany; Ondrej Marsalek of Charles University in the Czech Republic; Dennis Hein, Garlef Wartner and Robert Seidel of Helmholtz-Zentrum Berlin für Materialien und Energie in Germany; and Stephan Thürmer of Kyoto University in Japan.

This study was supported by the European Regional Development Fund, project ChemBioDrug no. CZ.02.1.01/0.0/0.0/16_019/0000729 (Jungwirth); Grant Primus16/SCI/27/247019 (Marsalek); U.S. National Science Foundation CHE-1665532 (Bradforth and McMullen); Deutsche Forschungsgemeinschaft Emmy-Noether grant No. SE 2253/3-1 (Seidel); JSPS KAKENHI Grant No. JP18K14178 (Thürmer).

University of Southern California

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