The controversy began in March 2025, when a group led by G. Kocheril reported measurements suggesting that cyclic C6H5+ is essentially inert toward the key molecules it would need to consume in order to build larger aromatic structures, including molecular hydrogen and acetylene, C2H2. If phenylium truly refused to react with acetylene, the ion could not serve as the gateway to naphthalene-like structures and the wider family of polycyclic aromatic hydrocarbons, or PAHs, that astronomers believe account for a substantial fraction of the carbon in the galaxy. The 2025 paper went so far as to argue that bottom-up interstellar aromatic ring formation effectively terminates at C6H5+, a conclusion that threatened to upend decades of astrochemical modelling.

That conclusion mattered because the formation of the first aromatic ring is widely regarded as the critical bottleneck in PAH growth. Once a single six-membered carbon ring exists, subsequent attachment of acetylene units can, in principle, build fused ring systems that grow into the large aromatic molecules whose infrared emission signatures pervade the Milky Way and beyond. Models of dark molecular clouds and ionized regions have long included phenylium-mediated pathways as a plausible route across this bottleneck. Removing that route would leave astrochemists scrambling to explain how the observed abundance of aromatic material could arise at all.

The new study, led by Jean-Christophe Loison of the Institute of Molecular Sciences at CNRS and the University of Bordeaux, together with Corentin Rossi and Ugo Jacovella of Université Paris-Saclay and colleagues at the Institute of Physical Chemistry and the SOLEIL synchrotron, set out to test the claimed unreactivity directly. Writing as a Matters Arising contribution in Nature Astronomy, the team combined laboratory measurements with high-level quantum chemical calculations and astrochemical modelling to re-examine how phenylium behaves when it encounters acetylene under conditions mimicking those in interstellar clouds, where temperatures hover near ten kelvin and densities are vanishingly low by terrestrial standards.

Their central result is striking: phenylium reacts efficiently with acetylene through a barrierless mechanism. In chemical kinetics, a barrierless reaction is one whose potential energy surface descends smoothly from reactants to products without any energetic hill that the colliding species must climb. Such reactions proceed at essentially every collision, governed only by how fast the two partners can find each other, which makes them extraordinarily effective in cold environments where molecules lack the thermal energy to overcome activation barriers. The team mapped the energy pathways along the reaction coordinate, showing that the association of C6H5+ with C2H2 leads to a stabilized intermediate that can proceed onward to products that incorporate the two-carbon unit into the growing carbon skeleton — precisely the chemistry needed to advance from one aromatic ring toward two.

A key technical strength of the work lies in how the phenylium ions were produced and characterized. The C6H5+ cation is a peculiar species: it possesses a low-lying triplet electronic state close in energy to its singlet ground state, and it can exist in both cyclic, ring-retaining forms and acyclic isomers with rearranged carbon skeletons. Earlier gas-phase studies, dating back to ion cyclotron resonance experiments in the 1970s and flow-tube measurements of acetylene ion chemistry in the 1980s, had already hinted at rich ion–molecule chemistry involving phenylium, but the identity and internal energy of the ions under study have often been uncertain. In the new experiments, the researchers exploited tunable synchrotron radiation to ionize a suitable precursor and measured the photon-energy dependence of C6H5+ production, allowing them to disentangle which isomers were being formed and to probe the reactivity of the cyclic cation specifically.

By monitoring how the phenylium signal decayed in the presence of acetylene, and by measuring the appearance of product ions as a function of photon energy, the team could extract rate behaviour that contradicts the earlier report of inertness. The measurements indicate that the cyclic cation, when prepared cleanly, participates in fast ion–molecule reactions with acetylene, consistent with a barrierless entrance channel predicted by the accompanying quantum chemical calculations. Loison performed the electronic structure computations that characterize the potential energy surface, identifying the intermediates and transition states that connect the initial association complex to the final products, while also running astrochemical models to assess what the revised rates mean for aromatic chemistry in interstellar clouds.

The theoretical picture also helps explain how the earlier discrepancy could have arisen. Phenylium ions are prone to isomerization and to fragmentation when produced by energetic ionization processes, and different production methods can yield mixtures of cyclic and acyclic C6H5+ with different internal energies. Acyclic isomers are known to display distinct reactivity patterns, and contamination of an ion population by unreactive or differently reactive isomers can mask the true behaviour of the cyclic cation. The photon-energy-resolved approach adopted by the French team provides a way to control for this, and their data suggest that earlier conclusions about phenylium unreactivity may have been confounded by such isomeric and energetic effects rather than reflecting an intrinsic property of the aromatic cation.

The astrophysical implications are immediate. In the cold gas of dark molecular clouds, where carbon-chain chemistry thrives and where observations have revealed surprising abundances of aromatic molecules, the availability of a fast, barrierless route from the first aromatic ring to larger structures changes the predicted chemistry substantially. Sensitivity analyses published in 2024 by Byrne and colleagues had already shown that models of aromatic chemistry in dark clouds are highly sensitive to the assumed gas-phase kinetics, meaning that a single rate coefficient can swing predicted abundances of benzene derivatives and PAH precursors by orders of magnitude. Reinstating efficient C6H5+ + C2H2 reactivity therefore reopens a pathway that recent models had been forced to close, and it strengthens the case that ion–molecule chemistry, rather than chemistry on the surfaces of dust grains alone, can seed the aromatic inventory of the interstellar medium.

The episode is also a reminder of how demanding laboratory astrochemistry can be. Ions as short-lived and isomer-sensitive as C6H5+ must be generated, transported and interrogated with exquisite control, and small differences in experimental design can produce apparently conflicting answers. The new work does not merely assert that the earlier measurements were wrong; it provides an independent, energy-resolved dataset, openly archived on Zenodo along with the data-reduction scripts, so that other groups can scrutinize and extend the analysis. Combined with the theoretical characterization of the reaction’s potential energy surface, the study offers a coherent physical picture: the phenylium cation, far from terminating bottom-up aromatic growth, is an efficient springboard from the first ring toward the polycyclic structures that glow across the galaxy. As infrared observatories continue to uncover aromatic signatures in ever more exotic environments, from dark clouds to protoplanetary disks, the rehabilitation of phenylium ensures that models of cosmic aromatic chemistry have a firmer — and faster — foundation on which to build.

Subject of Research: Laboratory and theoretical study of phenylium cation reactivity with acetylene under interstellar conditions

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

Article References: Loison, J.-C., Rossi, C., Solem, N., Thissen, R., Romanzin, C., Alcaraz, C., & Jacovella, U. (2026). Evidence for phenylium reactivity under interstellar-relevant conditions. Nature Astronomy. https://doi.org/10.1038/s41550-026-02973-9

Image Credits: AI Generated

DOI: 10.1038/s41550-026-02973-9

Keywords: phenylium, interstellar medium, polycyclic aromatic hydrocarbons, acetylene, ion–molecule reactions, laboratory astrophysics, aromatic ring formation, reaction kinetics, molecular clouds, astrochemistry, quantum chemistry, Nature Astronomy

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