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Interstellar detection of the highly polar five-membered ring cyanocyclopentadiene


Much like six-membered rings, five-membered rings are ubiquitous in organic chemistry, frequently serving as the building blocks for larger molecules, including many of biochemical importance. From a combination of laboratory rotational spectroscopy and a sensitive spectral line survey in the radio band toward the starless cloud core TMC-1, we report the astronomical detection of 1-cyano-1,3-cyclopentadiene (1-cyano-CPD, c-C5H5CN), a highly polar, cyano derivative of cyclopentadiene. The derived abundance of 1-cyano-CPD is far greater than predicted from astrochemical models that well reproduce the abundance of many carbon chains. This finding implies that either an important production mechanism or a large reservoir of aromatic material may need to be considered. The apparent absence of its closely related isomer, 2-cyano-1,3-cyclopentadiene, may arise from that isomer’s lower stability or may be indicative of a more selective pathway for formation of the 1-cyano isomer, perhaps one starting from acyclic precursors. The absence of N-heterocycles such as pyrrole and pyridine is discussed in light of the astronomical finding of 1-cyano-CPD.

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Fig. 1: Geometric structures of the two low-lying cyano-CPD isomers, along with their relative stabilities calculated theoretically at the G3//B3LYP level of theory.
Fig. 2: Velocity-stacked spectra of 1-cyano-CPD and 2-cyano-CPD and the impulse response function of the stacked spectra.
Fig. 3: Abundance predictions for 1-cyano-CPD and 2-cyano-CPD from two chemical models in comparison with those derived from observations of TMC-1.

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Data availability

The datasets analysed during the current study are available in the Green Bank Telescope archive ( A user manual for their reduction and analysis is available as well ( The complete, reduced survey data in the X band are available as supplementary information in ref. 15. The individual portions of reduced spectra used in the analysis of the individual species presented here are available in the Harvard Dataverse archive26.

Code availability

All the codes used in the MCMC fitting and stacking analysis presented in this paper are open source and publicly available at


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M.C.M. and K.L.K.L. acknowledge support from National Science Foundation (NSF) grant AST-1908576 and NASA grant 80NSSC18K0396. A.M.B. acknowledges support from the Smithsonian Institution as a Submillimeter Array Fellow. C.N.S. thanks the Alexander von Humboldt Stiftung/Foundation for their generous support, as well as V. Wakelam for use of the NAUTILUS v1.1 code. S.B.C. and M.A.C. were supported by the NASA Astrobiology Institute through the Goddard Center for Astrobiology. E.H. thanks the NSF for support through grant AST-1906489. C.X. is a Grote Reber Fellow, and support for this work was provided by the NSF through the Grote Reber Fellowship Program administered by Associated Universities, Inc./National Radio Astronomy Observatory and the Virginia Space Grant Consortium. Support for B.A.M. was provided by NASA through Hubble Fellowship grant HST-HF2-51396 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. The National Radio Astronomy Observatory is a facility of the NSF operated under cooperative agreement by Associated Universities, Inc. The Green Bank Observatory is a facility of the NSF operated under cooperative agreement by Associated Universities, Inc.

Author information

Authors and Affiliations



M.C.M. and K.L.K.L. performed the laboratory experiments and theoretical calculations and wrote the paper with the help of A.M.B. and C.N.S. A.M.B, B.A.M., A.J.R. and R.A.L. performed the astronomical observations and subsequent analysis. E.H. determined and/or estimated rate coefficients and is the originator of many of the chemical simulations. A.M.B. and C.N.S. contributed or undertook the astronomical modelling and simulations. E.R.W., M.A.C., S.B.C., S.K., C.X., B.A.M., A.J.R. and R.A.L. contributed to the design of the GOTHAM survey, and helped revise the manuscript.

Corresponding authors

Correspondence to Michael C. McCarthy or Brett A. McGuire.

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Extended data

Extended Data Fig. 1 Spectral data.

Total number of transitions of a given species within the range of the GOTHAM data, number of interfering lines, and total number included in MCMC fit.

Extended Data Fig. 2 Thermochemistry (0 K) for the reaction between CN radical and CPD.

The calculated has been performed at the G3//B3LYP level of theory, and energies in kJ/mol are given relative to the reactant asymptote. The reaction bifurcates as CN attacks CPD barrierlessly, forming a radical intermediate. Subsequent hydrogen atom loss yields 1-cyano- and 2-cyano-CPD.

Extended Data Fig. 3 Potential energy surface for the formation of CPD at 0 K.

Reaction energies are given relative to the product (CPD + H atom) asymptote. The red trace corresponds to reaction between propargyl radical (HCCCH2) and ethylene (H2C=CH2), and the blue trace represents gauche-butadiene (H2C(CH)2CH2) reacting with CH radical.

Supplementary information

Supplementary Information

Supplementary Tables 1–5, Figs. 1–6 and text.

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McCarthy, M.C., Lee, K.L.K., Loomis, R.A. et al. Interstellar detection of the highly polar five-membered ring cyanocyclopentadiene. Nat Astron 5, 176–180 (2021).

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