When G. Stephen Kocheril started as a research associate at Jila, his first project was to experimentally test a decades-old theoretical reaction pathway for interstellar benzene formation. He was surprised by the result: the reaction terminated before benzene was produced (Nat. Astron. 2025, DOI: 10.1038/s41550-025-02504-y). On Wednesday at the American Chemical Society Spring 2025 meeting, Kocheril explained how he came to this unexpected conclusion.
Researchers have long assumed that benzene is a precursor to interstellar polycyclic aromatic hydrocarbons (PAHs), molecules that scientists think act as reservoirs of cosmic carbon. Benzene formation is considered a rate-limiting step of PAH formation and is a cornerstone of some mathematical models that predict PAH concentration. In the late 1990s, scientists proposed that ion-molecule collisions could be one way benzene forms. Kocheril wanted to test the proposed mechanism experimentally.
“We've been able to prepare a laboratory environment where you can actually monitor a chemical reaction at the single-molecule level,” Kocheril told C&EN, and where conditions match those found in space. In space, PAHs form at pressures lower than one-trillionth the air pressure at sea level on Earth. Kocheril’s experiments are performed far closer to these pressures than previous experiments. His specialized system cools the ions to 1 K and is kept at about 10–10 Torr (roughly 10–8 Pa).
Speaking at a symposium organized by the ACS Division of Physical Chemistry, Kocheril described using a low-pressure coulomb crystal to achieve these extreme conditions. The crystal is a unique state of matter he created by using laser light to cool calcium ions confined within an ion trap. After the crystal formed, Kocheril added the reactants: a strong proton donor (N2H+) and neutral acetylene. The gaseous reactants permeated the center of the coulomb crystal and became cold and sluggish, colliding infrequently, about once per second. Then, he identified the product of each collision with time-of-flight mass spectrometry.
For the most part, Kocheril found that the experimental formation pathway matches the theoretical one proposed decades ago. Neutral acetylene is protonated by N2H+. After two more sequential reactions with acetylene, the expected phenylium ion, C6H5+, forms. But in the final step, when H2 is added to react with C6H5+, no reaction occurs. Benzene is never produced.
When it comes to building PAHs in space, “we always assumed it's no problem to get to that first ring. Once you have it, you can proceed onward,” said Brett McGuire, an astrochemist at the Massachusetts Institute of Technology who presided over the ACS meeting session at which the work was presented. “What this work is showing is that, at least for some of the major reactions, that assumption appears to be wrong.”
There are other possible pathways to interstellar benzene formation, writes Ralf Kaiser of the University of Hawaii at Manoa in an email. Kaiser was not involved with the work. In fact, in 2011, Kaiser and his colleagues proposed an alternative mechanism in which neutral ethynyl radicals react with neutral 1,3-butadiene. Based on Kocheril’s recent work, such “neutral – neutral reactions can clearly do a much better job in forming aromatics in deep space,” Kaiser adds.