Interstellar chemistry makes use of quantum shortcut

Molecules floating in the dark, cold vacuum of interstellar space can exploit quantum mechanics to react and produce more complex chemicals, a new study suggests. Researchers explain the reactions using a quirky property of quantum physics, which may be a key cog in the cosmic assembly line that churns out intricate organic molecules, including those necessary for life.

Astronomers have long known that stars manufacture chemical elements, but it’s only recently that researchers have discovered complex organic molecules floating around in clouds of gas and dust in space (SN 1/30/10, p. 26). The formation of these chemicals, which include alcohols, sugars and even an ingredient found in tar, is hard to explain because molecules in space should very rarely collide.

Last year astronomers discovered a molecule called methoxy, or CH₃O, in a gas cloud. It forms when hydroxyl (OH) and methanol (CH₃OH)react. Yet that reaction requires more energy than is available in space, where temperatures hover just above absolute zero.

While not specifically pursuing this mysterious reaction, Dwayne Heard and his team at the University of Leeds in England were exploring the reactivity of hydroxyl with other molecules, including methanol. In the course of the work, the researchers placed the two reactants together in a cryogenic vessel. To their surprise, they found that the reaction was about 50 times faster at -210 degrees Celsius than at room temperature, even though the chilled molecules had far less energy to work with.

In an upcoming Nature Chemistry, Heard’s team explains its finding with a phenomenon called quantum tunneling. Ordinarily, a chemical reaction occurs only if the reacting molecules have enough energy to overcome a threshold known as the energy barrier, which is like a hill. But a peculiar consequence of quantum mechanics is that molecules can occasionally bypass that hill without the requisite energy. “A particle can go right through the bottom of the mountain, almost as if the mountain weren’t there,” says Eric Herbst, an astrochemist at the University of Virginia in Charlottesville.

Heard and his colleagues found that the chances for quantum tunneling improve at low temperatures because slow-moving hydroxyl and methanol molecules are more likely to stick together rather than bounce off each other when they collide. This temporary bond provides more opportunity for tunneling through the energy barrier, hastening the reaction. Heard estimates that about 1 in 10 hydroxyl-methanol collisions in space produce methoxy; without quantum tunneling, that would drop to about 1 in 10 million.

Other interstellar molecules may owe their existence to quantum mechanics, says Stephen Klippenstein, a theoretical chemist at Argonne National Laboratory in Illinois. “People will definitely find other reactions like this,” he says. “This will not be a unique case.”