Letter to the Editor

Discovery of the acetyl cation, CH₃CO⁺, in space and in the laboratory^⋆,⋆⋆

¹ Grupo de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain
e-mail: jose.cernicharo@csic.es
² Univ. Lille, CNRS, UMR 8523 – PhLAM – Physique des Lasers Atomes et Molécules, 59000 Lille, France
³ Department of Applied Chemistry, Science Building II, National Chiao Tung University, 1001 Ta-Hsueh Rd., Hsinchu 30010, Taiwan
⁴ CNRS, IPAG, Univ. Grenoble Alpes, 38000 Grenoble, France
⁵ Observatorio Astronómico Nacional (IGN), C/ Alfonso XII, 3, 28014 Madrid, Spain
⁶ Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain

Received: 7 December 2020
Accepted: 30 December 2020

Abstract

Using the Yebes 40 m and IRAM 30 m radiotelescopes, we detected two series of harmonically related lines in space that can be fitted to a symmetric rotor. The lines have been seen towards the cold dense cores TMC-1, L483, L1527, and L1544. High level of theory ab initio calculations indicate that the best possible candidate is the acetyl cation, CH₃CO⁺, which is the most stable product resulting from the protonation of ketene. We have produced this species in the laboratory and observed its rotational transitions J_u = 10 up to J_u = 27. Hence, we report the discovery of CH₃CO⁺ in space based on our observations, theoretical calculations, and laboratory experiments. The derived rotational and distortion constants allow us to predict the spectrum of CH₃CO⁺ with high accuracy up to 500 GHz. We derive an abundance ratio N(H₂CCO)/N(CH₃CO⁺) ∼ 44. The high abundance of the protonated form of H₂CCO is due to the high proton affinity of the neutral species. The other isomer, H₂CCOH⁺, is found to be 178.9 kJ mol⁻¹ above CH₃CO⁺. The observed intensity ratio between the K = 0 and K = 1 lines, ∼2.2, strongly suggests that the A and E symmetry states have suffered interconversion processes due to collisions with H and/or H₂, or during their formation through the reaction of H₃⁺ with H₂CCO.