Letter to the Editor

Discovery of interstellar 3-cyano propargyl radical, CH₂CCCN^⋆

¹ Grupo 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 Observatorio Astronómico Nacional (IGN), C/ Alfonso XII, 3, 28014 Madrid, Spain
³ Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain

Received: 4 September 2021
Accepted: 6 October 2021

Abstract

We report the first detection in interstellar space of the 3-cyano propargyl radical (CH₂C₃N). This species was observed in the cold dark cloud TMC-1 using the Yebes 40m telescope. A total of seven rotational transitions for both ortho- and para-CH₂C₃N species were observed in the 31.0–50.4 GHz range. We derive a total column density of (1.6 ± 0.4) × 10¹¹ cm⁻² and an ortho/para ratio of 2.4 ± 1.2, which implies an abundance ratio CH₂C₃N/CH₃C₃N ∼ 0.1, in sharp contrast with the smaller analogues, in which case CH₂CN/CH₃CN ∼ 3. This indicates that the chemistry of the cyanides CH₂C₃N and CH₃C₃N behaves differently to that of the smaller analogues CH₂CN and CH₃CN. According to our chemical model calculations, the radical CH₂C₃N is mostly formed through the neutral-neutral reactions C + CH₂CHCN, C₂ + CH₃CN, and CN + CH₂CCH together with the dissociative recombination of the CH₃C₃NH⁺ ion with electrons. The neutral-neutral reaction N + C₄H₃ could also lead to CH₂C₃N, although its role is highly uncertain. The identified radical CH₂C₃N could play a role in the synthesis of large organic N-bearing molecules, such as benzonitrile (c–C₆H₅CN) or nitrogen heterocycles.