Discovery of linear propadienone

Study of the chemistry of linear and cyclic H₂C₃O and H₂C₃S in TMC-1

¹ Observatorio Astronómico Nacional (OAN), Alfonso XII, 3, 28014 Madrid, Spain
² Institut des Sciences Moléculaires ISM, CNRS UMR 5255, Univ. Bordeaux, 351 Cours de la Libération, 33400 Talence, France
³ Instituto de Física Fundamental, CSIC, Calle Serrano 123, 28006 Madrid, Spain
⁴ Observatorio de Yebes, IGN, Cerro de la Palera s/n, 19141 Yebes, Guadalajara, Spain

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 22 September 2025
Accepted: 17 November 2025

Abstract

We report the first detection in space of propadienone, the linear isomer (l-H₂C₃O) of cyclopropenone (c-H₂C₃O). We also report the first detection of the isotopologues c-H₂¹³ CCCO and c-HDCCCO of c-H₂C₃O. The astronomical observations are part of QUIJOTE, a line survey of TMC-1 in the frequency range 31.0–50.3 GHz, complemented with data between 71.6–116.0 GHz, and carried out with Yebes-40 m and IRAM-30 m telescopes, respectively. We obtain a total column density of 3.7 × 10¹⁰ cm⁻² for l-H₂C₃O at an excitation temperature of 4.8 K. We find that the isomer is about eight times less abundant than the cyclic one. We also report a detailed line-byline study of cyclopropenethione (c-H₂C₃S) to compare the abundance of the O and S isomers. We find that cyclic O-isomers are more abundant than cyclic S-isomers; however, the opposite trend is found for the most stable linear isomers, with l-H₂C₃S being more than one order of magnitude more abundant than l-H₂C₃O. A comprehensive theoretical chemical analysis shows that the abundances of the H₂C₃O and H₂C₃S isomers are controlled by different formation pathways. In particular, while l-H₂C₃O is potentially produced by dissociative electron recombination reactions, ion-neutral chemistry is more effective at producing l-H₂C₃S and c-H₂C₃S.