Letter to the Editor

Laboratory and astronomical discovery of the cyanovinyl radical H₂CCCN^⋆,⋆⋆

¹ Departamento de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain
e-mail: carlos.cabezas@csic.es; jose.cernicharo@csic.es
² Institute of Global Human Resource Development and Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama, 700-8531 Japan
e-mail: jtang@okayama-u.ac.jp
³ Department of Basic Science, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo, 153-8902 Japan
⁴ Division of Pure and Applied Science, Graduate School of Science and Technology, Gunma University, 4-2 Aramaki, Maebashi, Gunma, 371-8510 Japan
⁵ Department of Chemistry, School of Science, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro, Tokyo, 152-8550 Japan
⁶ Observatorio Astronómico Nacional (IGN), C/ Alfonso XII 3, 28014 Madrid, Spain
⁷ Observatorio de Yebes (IGN), Cerro de la Palera s/n, 19141 Yebes, Guadalajara, Spain
⁸ Department of Applied Chemistry, Science Building II, National Yang Ming Chiao Tung University, 1001 Ta-Hsueh Rd., Hsinchu, 300098 Taiwan

Received: 6 July 2023
Accepted: 18 July 2023

Abstract

We report the first laboratory and interstellar detection of the α-cyano vinyl radical (H₂CCCN). This species was produced in the laboratory by an electric discharge of a gas mixture of vinyl cyanide, CH₂CHCN, and Ne. Its rotational spectrum was characterized using a Balle-Flygare narrowband-type Fourier-transform microwave spectrometer operating in the frequency region of 8–40 GHz. The observed spectrum shows a complex structure due to tunneling splittings between two torsional sublevels of the ground vibronic state, 0⁺ and 0⁻, derived from a large-amplitude inversion motion. In addition, the presence of two equivalent hydrogen nuclei makes it necessary to discern between ortho- and para-H₂CCCN. A least-squares analysis reproduces the observed transition frequencies with a standard deviation of ca. 3 kHz. Using the laboratory predictions, this radical was detected in the cold dark cloud TMC-1 using the Yebes 40 m telescope and the QUIJOTE¹ line survey. The 4_0, 4-3_0, 3 and 5_0, 5-4_0, 4 rotational transitions, composed of several hyperfine components, were observed in the 31.0–50.4 GHz range. Adopting a rotational temperature of 6 K, we derived a column density of (1.4±0.2)×10¹¹ cm⁻² and (1.1±0.2)×10¹¹ cm⁻² for ortho-H₂CCCN and para-H₂CCCN, respectively. The reaction of C + CH₃CN emerges as the most likely route to H₂CCCN in TMC-1, and possibly that of N + CH₂CCH as well.