Evidence for Phenylium Reactivity under Interstellar Relevant Conditions

Recent work by Kocheril \textit{et al.}\cite{kocheril2025} claimed that phenylium–the cyclic structure of the \ce{C6H5+} species–is unreactive toward key interstellar molecules such as molecular hydrogen (\ce{H2}) and acetylene (\ce{C2H2}). This finding challenges the previously proposed role of phenylium as a cornerstone in the formation of polycyclic aromatic hydrocarbons (PAHs) \cite{cherchneff1992,byrne2024}. The study focused on the reactivity of \ce{C6H5+}, formed via the radiative association between \ce{C4H3+} and \ce{C2H2}, believed to be a major pathway for phenylium formation in astrochemical model, e.g. \cite{byrne2024}. Here, we present new experimental and theoretical evidence that challenges this assumption. Our results demonstrate that phenylium does indeed react with \ce{C2H2} under astrophysically relevant conditions. Quantum chemical calculations support this finding by revealing a barrierless mechanism, indicating that the reaction is feasible even in cold interstellar environments. We believe this clarification is critically important, and that further investigations into the formation of the first aromatic ring in space–a process that remains a key bottleneck in our understanding of PAHs formation and growth–is essential.

💡 Research Summary

The authors set out to reassess the claim made by Kocheril et al. (2025) that the aromatic cation phenylium (C₆H₅⁺) is chemically inert toward key interstellar molecules such as H₂ and acetylene (C₂H₂). Using a combination of state‑of‑the‑art vacuum‑ultraviolet (VUV) photo‑ionization at near‑threshold energies (≈11 eV) of nitrobenzene, they generate a highly pure sample of the cyclic C₆H₅⁺ ion. This method avoids the production of higher‑energy isomers (e.g., linear l‑C₆H₅⁺) that have plagued earlier studies relying on high‑energy electron impact or collisional ionization. The mass‑selected C₆H₅⁺ ions are then introduced into a reaction cell containing acetylene at a pressure of 8.5 × 10⁻⁵ mbar, ensuring single‑collision conditions. By varying the photon energy they control the internal energy of the ion (up to ~3 eV above the appearance energy) and monitor product ions with a second quadrupole mass filter.

Two reaction channels are observed. At low internal energies the dominant pathway is a barrier‑free addition forming a C₈H₇⁺ adduct (m/z 103) with the release of a photon. As the internal energy rises above ~1.5 eV, a secondary channel involving H‑atom loss becomes significant, yielding C₈H₆⁺ (m/z 102). The measured reaction cross‑sections and derived rate constants (≈3–4 × 10⁻¹⁰ cm³ s⁻¹) are in excellent agreement with earlier flow‑tube studies (Knight et al., 1987) and with the authors’ own density‑functional theory (DFT) calculations.

The theoretical component employs the M06‑2X functional with the AVTZ basis set to map the singlet potential energy surface (PES) for the C₆H₅⁺ + C₂H₂ encounter. Geometry optimizations reveal that the approach of acetylene to the non‑hydrogenated carbon of phenylium proceeds without any activation barrier; the entrance channel is essentially a long‑range electrostatic attraction. Transition states and intermediates (designated 1–3 in the manuscript) lie at or below the energy of the separated reactants after zero‑point energy correction, confirming a barrierless, exothermic reaction pathway. The formation of a bicyclic adduct preserves aromaticity, and subsequent isomerizations can lead to a fused pentalene‑type ring (product iii), which is only mildly endothermic, while the other pathways are clearly exothermic.

A key point of contention with Kocheril et al. is the pressure regime. Their experiments were conducted at ultra‑low pressures (~10⁻⁸ mbar), where the nascent adduct cannot be stabilized by collisions and therefore rapidly dissociates, giving the impression of “no reaction”. In contrast, the present work’s pressure is three orders of magnitude higher, allowing collisional stabilization of the adduct and its detection. The authors argue convincingly that the difference lies in product stabilization rather than intrinsic reactivity.

The paper also revisits the proposed formation route of phenylium via the radiative association of C₄H₃⁺ with C₂H₂. Citing previous ab‑initio molecular dynamics (AIMD) studies (Peverati et al., 2016), they note that this pathway predominantly yields a non‑cyclic isomer (l‑C₆H₅⁺) rather than the aromatic cation. Consequently, the “non‑reactivity” reported by Kocheril et al. likely stems from the generation of the wrong isomer under their experimental conditions.

To assess astrochemical implications, the authors incorporate their new kinetic data into the Nautilus three‑phase chemical model, updating the KIDA.UVA.2024 network with the barrierless C₆H₅⁺ + C₂H₂ and C₆H₅⁺ + H₂ reactions. Simulations of a cold dense cloud (parameters akin to TMC‑1) show that while the inclusion of the reactive phenylium channel modestly enhances the production of C₈H₇⁺ and downstream benzene (C₆H₆), the overall benzene abundance is not dramatically altered; removing the C₄H₃⁺ + C₂H₂ → C₆H₅⁺ route changes benzene levels by only about an order of magnitude. This suggests that phenylium is an important, but not exclusive, precursor to the first aromatic ring; other ion‑molecule pathways also contribute significantly.

In summary, the study provides compelling experimental evidence and supporting quantum‑chemical calculations that phenylium reacts efficiently with acetylene via a barrierless addition mechanism under interstellar‑relevant conditions. It demonstrates that the previously reported lack of reactivity is an artifact of experimental design—specifically, the use of high‑energy ionization methods that generate competing isomers and ultra‑low pressures that prevent adduct stabilization. The work re‑establishes phenylium as a viable intermediate in the formation of the first aromatic ring, urging astrochemical modelers to include its barrierless reactions with abundant species such as C₂H₂ and H₂, while also recognizing that multiple formation routes must be considered to reconcile observed aromatic abundances in molecular clouds. Future investigations should focus on systematic pressure and internal‑energy studies, spectroscopic identification of reaction intermediates, and extended modeling across a broader range of interstellar environments.