Letter to the Editor

Discovery of two cyano derivatives of acenaphthylene (C₁₂H₈) in TMC-1 with the QUIJOTE line survey^⋆⋆

¹ Dept. de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain
² Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain
³ Observatorio Astronómico Nacional (OAN, IGN), C/ Alfonso XII, 3, 28014 Madrid, Spain
⁴ Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares (CiQUS) and Departamento de Química Orgánica, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain
⁵ Departamento de Química Física y Química Inorgánica, Facultad de Ciencias-I.U. CINQUIMA, Universidad de Valladolid, 47011 Valladolid, Spain
⁶ Department of Chemistry, University of Hawaii at Manoa, Honolulu, HI, 96822 USA
⁷ Department of Applied Chemistry, Science Building II, National Yang Ming Chiao Tung University, 1001 Ta-Hsueh Rd., Hsinchu, 300098 Taiwan

Received: 10 September 2024
Accepted: 18 September 2024

Abstract

We report the discovery in TMC-1 of two cyano derivatives of the PAH acenaphthylene (C₁₂H₈). We have found two series of lines with the QUIJOTE line survey that we assign to 1-C₁₂H₇CN and 5-C₁₂H₇CN. For the 1-isomer, we have detected and assigned 173 rotational transitions with J up to 46 and K_a up to 9, corresponding to 107 independent frequencies. For the 5-isomer, the identification is based on 56 individual lines, corresponding to 117 rotational transitions with J up to 40 and K_a up to 8. Identification of the carriers was achieved through a careful analysis of the derived rotational constants, which permit us to focus on molecules larger than naphthalene but smaller than anthracene and phenanthrene. Moreover, the derived rotational constants indicate that the species are planar; this allows us to discard derivatives of fluorene and acenaphthene, which are non-planar species. Quantum chemical calculations and subsequent chemical synthesis of these molecules, as well as the observation of their rotational transitions in the laboratory, unequivocally support our identifications. We also confirm, via a robust line-by-line detection, the previous claimed detection of 1- and 2-cyanonaphthalene, which were obtained through statistical stacking techniques. The column densities of 1- and 5-cyanoacenaphthylene are (9.5 ± 0.9) × 10¹¹ cm⁻², while those of 1- and 2-cyanonapthalene are (5.5 ± 0.5) × 10¹¹ cm⁻². Hence, it seems that acenaphthylene could be a factor of 1.7 more abundant than naphthalene. These results support a scenario in which PAHs grow in cold dark clouds based on fused five- and six-membered carbon rings.