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Termination of bottom-up interstellar aromatic ring formation at C6H5+

Nature Astronomy volume 9, pages 685–691 (2025)Cite this article

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Abstract

The aromatic molecule benzene is considered to be the essential building block for larger polycyclic aromatic hydrocarbons (PAHs) in space. Despite the importance of benzene in the formation of PAHs, the formation mechanisms of interstellar benzene are not well understood. A single ion–molecule reaction sequence is considered when modelling the formation of benzene in the interstellar medium, beginning with the protonation of acetylene. Although this process has been used to model the initial steps in the formation of PAHs, it has not been experimentally measured. To explore this reaction mechanism, we have carried out an experimental study of sequential ion–molecule reactions beginning with protonation of acetylene under single-collision conditions. Surprisingly, we found that the reaction sequence does not result in benzene but, instead, terminates at C6H5+, which is unreactive towards either acetylene or hydrogen. This result disproves the previously proposed mechanism for interstellar benzene formation, thus critically altering our understanding of interstellar PAH formation.

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Fig. 1: Proposed synthesis mechanism for benzene in ion–molecule reactions.
Fig. 2: Schematic of the experimental apparatus.
Fig. 3: Ion-reactant depletion and product growth for the sequential chain reactions starting with N2H+ + C2H2.
Fig. 4: Schematic diagram of the full series of sequential reactions.

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All data are available in the text or in the Supplementary Information. Source data are provided with this paper.

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Acknowledgements

We thank L.-S. Wang and M.-A. Martin-Drummel for helpful discussions during the preparation of this manuscript. This work was supported by the National Science Foundation (Grant Nos. PHY-2317149 and CHE-1900294) and the Air Force Office of Scientific Research (Grant No. FA9550-20-1-0323).

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Authors and Affiliations

  1. JILA, National Institute of Science and Technology and University of Colorado, Boulder, CO, USA

    G. S. Kocheril, C. Zagorec-Marks & H. J. Lewandowski

  2. Department of Physics, University of Colorado, Boulder, CO, USA

    G. S. Kocheril, C. Zagorec-Marks & H. J. Lewandowski

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  1. G. S. Kocheril
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  2. C. Zagorec-Marks
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Data collection and analysis were carried out by G.S.K. and C.Z.-M. All authors contributed to interpreting the results and writing the manuscript.

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Correspondence to G. S. Kocheril.

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Kocheril, G.S., Zagorec-Marks, C. & Lewandowski, H.J. Termination of bottom-up interstellar aromatic ring formation at C6H5+. Nat Astron 9, 685–691 (2025). https://doi.org/10.1038/s41550-025-02504-y

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