Letter to the Editor

Discovery of the cyclic C₅H radical in TMC-1^⋆

¹ Grupo de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain
e-mail: carlos.cabezas@csic.ese-mail: jose.cernicharo@csic.es
² Department of Applied Chemistry, Science Building II, National Yang Ming Chiao Tung University, 1001 Ta-Hsueh Rd., Hsinchu, 300098 Taiwan
³ Observatorio Astronómico Nacional (IGN), C/ Alfonso XII, 3, 28014 Madrid, Spain
⁴ Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain

Received: 20 May 2022
Accepted: 31 May 2022

Abstract

Cyclic C₅H (c-C₅H), the radical formed by substituting an ethynyl group CCH for the hydrogen atom in the c-C₃H radical, has been detected for the first time in the space. The c-C₅H species is an isomer of the well-known linear radical l-C₅H and is ∼6 kcal mol⁻¹ less stable. A total of 17 rotational transitions were detected for the c-C₅H species in TMC-1 within the 31.0–50.3 GHz range using the Yebes 40m radio telescope. We derive a column density of (9.0 ± 0.9) × 10¹⁰ cm⁻² for c-C₅H. Additionally, we observed 12 lines for l-C₅H and derive a column density for it of (1.3 ± 0.3) × 10¹² cm⁻², which results in an abundance ratio c-C₅H/l-C₅H of 0.069. This is in sharp contrast with the value found for the analogue system c-C₃H/l-C₃H, whose ratio is 5.5 in TMC-1. We discuss the formation of c-C₅H and conclude that this radical is probably formed in the reaction of atomic carbon with diacetylene.