The dispute began when a comment by J.-C. Loison and colleagues, published alongside the reply, presented results from an ion–molecule reaction experiment in which phenylium was generated by photodissociation of a precursor molecule. In that experiment, the resulting C6H5+ ions were found to react readily with acetylene, C2H2. According to the comment, this directly contradicts the Colorado group’s earlier publication, which reported that phenylium produced through sequential bottom-up ion–molecule reactions with acetylene was unreactive toward both acetylene and molecular hydrogen. The comment’s authors also calculated, using density functional theory, a barrierless pathway leading from phenylium and acetylene to a C8H7+ adduct, reinforcing their claim that the reaction should occur. They further argued that because their measurements operate in the single-collision regime, their conditions are highly relevant to the cold, collision-poor environment of the interstellar medium.

The stakes of this debate are considerable. Polycyclic aromatic hydrocarbons are thought to account for a substantial fraction of the carbon in the universe, and understanding how the first aromatic ring forms and grows from simple molecules such as acetylene is a central problem in astrochemistry. If phenylium reacts efficiently with acetylene, ring growth can continue, building larger hydrocarbons that eventually become the seeds of interstellar aromatic clouds. If it does not, the bottom-up assembly of aromatic rings stalls at the six-carbon stage, and astrochemical models must be revised accordingly. Which experiment is right changes how scientists simulate the chemistry of dark molecular clouds and circumstellar envelopes.

In their reply, the Colorado researchers make a striking concession that reframes the entire controversy: they are not surprised by the data presented by Loison and colleagues. Decades of similar experiments, they note, have consistently demonstrated that a reactive form of C6H5+ is produced through methods like photodissociation and dissociative ionization. That behavior stands in sharp contrast to observations of an unreactive species formed through ion–molecule chemistry, a pattern documented in the literature going back to the 1980s. The discrepancy between reactive and unreactive outcomes has appeared repeatedly across different laboratories, techniques and generations of instrumentation, making it one of the longest-running puzzles in gas-phase ion chemistry.

Historically, the debate over these divergent results has centered on molecular structure. When two experiments produce ions with the same chemical formula but different reactivity, the simplest explanation is that the experiments are producing different structural isomers. In this case, the acyclic form of C6H5+ would be expected to react with acetylene, while the ring-shaped phenylium cation might resist the reaction. Loison and colleagues adopted precisely this line of reasoning, suggesting that the simplest explanation for the discrepancy is that the Colorado experiments have produced a higher-energy, acyclic C6H5+ isomer instead of the true phenylium cation. Under this interpretation, the unreactivity reported by the Colorado team would be an artifact of making the wrong molecule.

The reply firmly rejects that interpretation. Kocheril, Zagorec-Marks and Lewandowski state their position plainly: they believe the structure of the C6H5+ produced in their experiments is the same as that produced by Loison and colleagues. What differs, they argue, is the internal energy of the ions. Two ionic species can share an identical atomic connectivity while carrying very different amounts of vibrational and electronic excitation, and that internal energy can govern whether a barrierless association reaction proceeds on the timescales probed by an experiment. An ion born with substantial internal energy can find reaction pathways that a cold, relaxed counterpart cannot access, even if both species are structurally indistinguishable.

This energy-based explanation carries particular weight because of how the ions are made. In the Colorado experiments, phenylium is built from the bottom up, assembled through a sequence of ion–molecule reactions with acetylene at low temperature. Such gentle assembly tends to leave ions in comparatively low internal energy states. In the commenting authors’ experiments, the ion is produced through photodissociation of a precursor, a process that can deposit significant energy into the fragment. The reply includes a schematic representation of the energetics of C6H5+ produced through dissociative ionization, underscoring the authors’ argument that the production method itself dictates the internal energy content and therefore the observed reactivity. The same ion, they contend, can behave differently depending on how hot it is when it meets a collision partner.

The reply also addresses the claim that single-collision experiments are the most faithful proxies for interstellar conditions. While the single-collision regime does mimic the extreme isolation of the interstellar medium, where particles collide rarely, the Colorado authors point out that decades of experimental history show reactive C6H5+ arising from precisely the kind of photodissociation-based production used in the comment. If that production route systematically yields an internally excited ion, then reactivity measured under those conditions may reflect the excess energy of the ion rather than the intrinsic low-temperature chemical behavior of relaxed phenylium in space. The unreactive behavior observed for ions assembled through ion–molecule reactions, by contrast, may better represent the quiescent chemistry of cold molecular clouds. Resolving which regime matters most for astrochemical models will require disentangling internal energy from structural identity, a task that demands careful spectroscopic characterization of the ions involved.

The authors point to a growing body of independent evidence relevant to this question, including recent photoelectron spectroscopy work that directly observed the fundamental arylium species and measured its singlet–triplet gap, as well as theoretical studies of the isomers of C6H5+ and their formation pathways in the interstellar medium. For readers following the debate, the exchange is a vivid reminder that in laboratory astrophysics, how you make a molecule can matter as much as what molecule you make. The Colorado team refers readers to their original publication for a full account of why they believe they have produced the phenylium isomer, and the exchange is likely to prompt follow-up experiments designed to measure the internal energy distributions of C6H5+ ions directly. Until then, the fate of aromatic ring growth in the coldest corners of the galaxy hangs on a subtle question of molecular temperature.

Subject of Research: Laboratory study of the internal-energy-dependent reactivity of the phenylium cation C6H5+ and its role in interstellar aromatic ring formation

Article Title: Reply to: Evidence for phenylium reactivity under interstellar-relevant conditions

Article References: Kocheril, G. S., Zagorec-Marks, C., & Lewandowski, H. J. (2026). Reply to: Evidence for phenylium reactivity under interstellar-relevant conditions. Nature Astronomy. https://doi.org/10.1038/s41550-026-02972-w

Image Credits: AI Generated

DOI: 10.1038/s41550-026-02972-w

Keywords: phenylium, astrochemistry, interstellar medium, ion–molecule reactions, aromatic ring formation, acetylene, C6H5+, laboratory astrophysics, internal energy, Nature Astronomy, isomers, molecular clouds