Phenylium Ion Strikes Back: Key Aromatic Ring Builder in Space Restored

In the frigid darkness between the stars, chemistry proceeds by rules that often defy laboratory intuition. One of the deepest puzzles in astrochemistry concerns how the first aromatic ring — the hexagonal carbon ring at the heart of benzene and of every polycyclic aromatic hydrocarbon — manages to form in the interstellar medium. Now, a team of French molecular scientists has reignited a debate over the identity of the crucial ion that drives this process, presenting fresh experimental and theoretical evidence that the phenylium cation, C6H5+, is far more reactive under space-like conditions than a recent high-profile study had claimed. The finding, published in Nature Astronomy, restores phenylium to its place as a central player in the bottom-up growth of aromatic molecules in space.

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