Evidence for Phenylium Reactivity under Interstellar Relevant Conditions

Abstract

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.

Figures (4)

• Figure 1: a) Energy pathway for C2H2 approaching phenylium. b) Potential energy surface for the C6H5+ + C2H2 reaction. All calculations were performed using M06-2X/AVTZ level.
• Figure 2: Panel a displays the parent ion signal for the C6H5+ ion obtained from dissociation ionization of nitrobenzene. Panel b shows the reaction cross sections (left axis) and rate constants (right axis) with C2H2 as a function of photon energy.
• Figure 3: Schematic diagram illustrating key pathways for the formation of aromatic species, with neutral reactions shown in black and ionic reactions in red or green (for Dissociative Re-combination, DR). The thickness of each arrow is proportional to the integrated total production rate, while radiative association reactions are depicted with dashed lines. Radicals are shown within square boxes and close-shell species in circles. Species in bold are those detected in TMC1. The DRs (which are by far the largest fluxes for the reaction of ions that do not react with H$_2$) that do not lead to aromatics are not shown to simplify the figure.
• Figure 4: Gas-grain astrochemical model results for the gas-phase formation of C6H6, C6H4, C6H5CN, and C6H5C2H. Solid lines represent results from the standard network excluding the gas-phase C4H3+ + C2H2 reaction leading to phenylium, while dashed lines correspond to the same network with this reaction included. The horizontal rectangles indicate observed abundances of C6H4, C6H5CN, and C6H5C2H in TMC-1 (Cernicharo2021b, mcguire2018a, loru2023), with an estimated uncertainty of $\sqrt{3}$