The chemistry is a bit like traveling from Vienna to Venice. Your destination may be downhill, but you’ll need to cross the Alps to get there. We can think of the changes in energy that a molecule has to undergo as a landscape. Between initiation and termination of the reaction, the terrain can be very hilly, and if the molecules lack the energy to overcome the bumps, otherwise favorable reactions will not occur at all. In some , such reactions occur thanks to quantum tunneling. This allows particles to occasionally pass through energy barriers that they can never climb. I’m here.
Now, in a new study published in Nature, scientists have successfully discovered quantum tunneling, which classical physics would consider an impossible reaction between hydrogen molecules and deuterium ions. This is the first time that researchers have experimentally confirmed theoretical predictions about tunneling rates in reactions involving ions. “Theoretical quantum mechanics should be able to predict this [rate] Physicist Stephan Schlemmer of the University of Cologne in Germany, who was not involved in the study, said: “But no one knew if this was true.”
The idea that particles can simply appear The other side of the energy wall can be traced back to German physicist Friedrich Hund. In 1927, while studying how molecules interact with light, he discovered that tunneling was theoretically possible. According to quantum mechanics, particles are more like clouds of probability than solid spheres. These probability clouds, representing particle positions, extend to infinity.So, although highly improbable, it is theoretically possible that quantum particles could appear anywhere, Including opposite sides of the energy barrier that classical particles could never cross.
In 1928 Tunneling enjoyed perhaps the greatest of its first victories. We’ve neatly described nuclear alpha decay, a common type of radioactive decay in which an atomic nucleus spits out an “alpha particle” (a helium nucleus with two protons and two neutrons) that transforms into a smaller nucleus. process. This happens when his three atoms of a hydrogen molecule and a deuterium ion come close in line and instead he jumps over one hydrogen atom and bonds with deuterium. At low temperatures this reaction should not be possible, but it works by tunneling. Since then, scientists have used tunneling to explain the otherwise unexplainable, from semiconductors to the heart of stars.
But while the ideas behind quantum tunneling are now nearly a century old, combining theory and experiment to observe tunneling in chemical reactions has proven difficult.First, because quantum tunneling is so rare, the reactions that depend on it are usually ice-slow and difficult to observe in the laboratory. Scientists can only predict tunneling kinetics for very simple reactions.” [reactions among] You can do it with three atoms,” says Roland Wester, a molecular physicist at the University of Innsbruck in Austria, who co-authored the new study. “If you have four atoms, there are some groups that can handle it. Having five atoms basically means that no one in the world has the means to fully quantize .”
The reaction between hydrogen gas and deuterium ions is so simple that quantum mechanics alone can predict the reaction rate. That’s why Wester’s team decided to study this reaction. Researchers were actually able to compare theory and reality. In this reaction, a molecule of hydrogen gas collides with one of his deuterium ions to produce a heavy hydrogen molecule containing hydrogen ions and deuterium. However, when University of Florida theoretical physicist Viatcheslav Kokoouline and his colleagues calculated the numbers in his 2018 study, he found reaction rates hundreds of times lower than upper bound estimates previously measured by Wester’s team. I predicted.
“[The results] I didn’t agree to the experiment, so I didn’t want to publish it,” says Kokoouline. Worried that he had made a mistake, he and his colleagues repeated the calculations using three different theoretical methods and got the same results. It’s true that the calculations could have been wrong, but “I did my best to come up with this number. [could] provide,” says Isaac Yuen, a former Cochrane student who is now a theoretical physicist at Kansas State University.
The problem was that the reaction speed was very slow. The team in Innsbruck took about 15 years of troubleshooting and tuning to finally get it right. To do that, the researchers trapped deuterium ions in a cage in an electric field, flushed them with hydrogen gas, and cooled the whole thing down to a very cold 15 Kelvin. At that low temperature, hydrogen and deuterium lacked the energy to react without tunneling. After waiting about 15 minutes, the scientists found the reaction rate by measuring the number of hydrogen ions produced.
Fifteen minutes doesn’t seem like a lot of time, but for classical reactions scientists often say, “Make a measurement for 100 milliseconds and see that almost all the ions are converted to products.” Wester says. “After waiting 1,000 seconds, he converted less than 1% of the ions to products.”
Tunneling occurs only about once every 100 billion collisions between hydrogen and deuterium ions, which agrees very well with Cochrine and Yuen’s theoretical calculations. “It’s very surprising that the numbers match the experiments,” he says Yuen. “As a theorist, I feel it’s a big win.”
Such tunneling reactions between ions are thought to be important for the chemical synthesis of the interstellar soup of diffused ionized gas, which is the raw material for new star systems. The interstellar medium is so cold that the classical reaction is much slower, but tunneling is more likely. At low temperatures, particles pass through each other more slowly, making tunneling more likely.
The first capture of this small tunneling rate here on Earth shows that physicists are on the right track with quantum molecular theory. It also provides a benchmark for testing future theoretical efforts to unify chemistry and quantum mechanics. “[In] In the normal world of classical particles, reactions can be understood with some very simple concepts,” says Schlemmer. “But this tunnel is a whole other world, and measurements like this open this world up to us.”
