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Five-membered ring compounds from the ortho-benzyne + methyl radical reaction under interstellar conditions

Nature Astronomy volume 7, pages 423–430 (2023)Cite this article

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Abstract

Reactive open-shell species, such as radicals and biradicals, are key intermediates in the formation of (poly)cyclic hydrocarbon species in a variety of interstellar environments, ranging from cold molecular clouds to the outflows of carbon-rich stars. In this work, we identify the products of the o-benzyne + methyl radical reaction isomer-selectively by photoion mass-selected threshold photoelectron spectroscopy. We assign the benzyl (\({\mathrm{C}}_7{\mathrm{{H}}_7}^\cdot\)) radical as the sole intermediate of the association reaction. Subsequent hydrogen-atom loss from benzyl yields the five-membered ring species fulvenallene (FA), 1-ethynylcyclopentadiene (1ECP) and 2-ethynylcyclopentadiene (2ECP), which have recently been detected in the cold molecular cloud TMC-1. We report a comprehensive C7H7 potential energy surface of the title reaction and show that the products form via direct barrierless addition followed by ring contraction and hydrogen elimination. A statistical model predicts 89% 1ECP, 8% FA and 3% 2ECP branching ratios at 0 K. Astrochemical simulations of TMC-1 incorporating this reaction result in the excellent reproduction of the abundance of a five-membered ring species, 1ECP, and provide strong evidence for the in situ ‘bottom-up’ formation of small cyclic species in cold cores. Last, we put the results in context of the recent detection of fulvenallene in TMC-1.

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Fig. 1: The main species discussed in this work.
Fig. 2: Mass spectra at 9.5 eV photon energy and ~1,100 K pyrolysis temperature.
Fig. 3: Mass-selected threshold photoelectron spectra recorded at a temperature of 1,089 K.
Fig. 4: Summary of the C7H7 PES of the methyl + o-benzyne reaction.
Fig. 5: Formation rates for the various product channels of the methyl + o-benzyne reaction.
Fig. 6: Results from astrochemical simulations.

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Data availability

The raw i2PEPICO datasets are available from the corresponding author upon reasonable request. Output files (.log files) from quantum chemical calculations are provided in Supplementary Data 1. Source data for the figures are provided with this paper.

Code availability

Quantum chemical calculations have been performed using Gaussian1663. Franck–Condon simulations have been performed using ezSpectrum33. Microcanonical rates have been calculated using the miniPEPICO programme64. Custom code used in this work as well as the chemical reaction network are available from C.N.S. (cshingledecker@benedictine.edu) upon request.

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Acknowledgements

J.B. acknowledges the Netherlands Organisation for Scientific Research (Nederlandse Organisatie voor Wetenschappelijk Onderzoek, NWO) for a Vidi grant (grant number 723.016.006). This work was carried out on the Dutch national e-infrastructure with the support of SURF Cooperative (EINF-997). This work was supported in part by NASA’s Solar System Exploration Research Virtual Institute (SSERVI): Institute for Modeling Plasma, Atmosphere, and Cosmic Dust (IMPACT). The i2PEPICO experiments were performed at the VUV beamline at the SLS. P.H. and A.B. gratefully acknowledge funding by the Swiss Federal Office of Energy (BFE Contract Number SI/501269-01). We also wish to thank P. Ascher for technical assistance.

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Authors and Affiliations

  1. Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA

    Jordy Bouwman

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

    Jordy Bouwman

  3. Institute for Modeling Plasma, Atmospheres and Cosmic Dust (IMPACT), NASA/SSERVI, Boulder, CO, USA

    Jordy Bouwman

  4. Laboratory for Astrophysics, Leiden Observatory, Leiden University, Leiden, the Netherlands

    Morgan N. McCabe

  5. Department of Physics and Astronomy, Benedictine College, Atchison, KS, USA

    Christopher N. Shingledecker, Joseph Wandishin & Virginia Jarvis

  6. Institute of Physical and Theoretical Chemistry, University of Würzburg, Würzburg, Germany

    Engelbert Reusch

  7. Laboratory for Synchrotron Radiation and Femtochemistry, Paul Scherrer Institute, Villigen, Switzerland

    Patrick Hemberger & Andras Bodi

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  1. Jordy Bouwman
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Contributions

J.B. wrote the manuscript with assistance from C.N.S., P.H. and A.B. The measurements were performed by J.B. and M.N.M with support from P.H. The experimental data were analysed by J.B. Potential energy surface calculations were performed by J.B. Ionization spectrum of the o-tolyl radical was simulated by P.H. The reaction entrance barrier of the reaction was characterized by A.B. The astrochemical reaction network was updated by J.W. and V.J. under supervision of C.N.S. Astrochemical simulations were performed by J.W. and V.J. under supervision of C.N.S. Rate coefficients were calculated by C.N.S., who also made the plots of the astrochemical simulation results. The ortho-benzyne precursor species, benzocyclobutenedione, was synthesised by E.R. The VUV beamline at the SLS where the measurements were conducted is managed by A.B.

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Correspondence to Jordy Bouwman.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–9, Tables 1 and 2 and supplementary discussion.

Supplementary Data 1

An archive file containing the Gaussian16 output (.log) files of the intermediates and transition states located on the potential energy surface.

Source data

Source Data Fig. 2

X–Y data of the mass spectra shown in Figure 2.

Source Data Fig. 3

X–Y data of the measured and simulated photoelectron spectra shown in Fig. 3.

Source Data Fig. 4

Energies of the intermediates and transition states located on the potential energy surface.

Source Data Fig. 5

Microcanonical rate data from our statistical model used to construct Fig. 5.

Source Data Fig. 6

Abundances of molecular species of interest as a function of time used to construct Fig. 6.

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Bouwman, J., McCabe, M.N., Shingledecker, C.N. et al. Five-membered ring compounds from the ortho-benzyne + methyl radical reaction under interstellar conditions. Nat Astron 7, 423–430 (2023). https://doi.org/10.1038/s41550-023-01893-2

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