Letter to the Editor

Discovery of two new isomers of cyanoacenaphthylene (C₁₂H₇CN) in the Taurus molecular cloud 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

^⋆ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 29 October 2025
Accepted: 19 November 2025

Abstract

We report the discovery of the 3- and 4-cyano isomers of acenaphthylene (C₁₂H₈) using the QUIJOTE line survey of the Taurus molecular cloud 1 (TMC-1). The frequency range of the line survey in the Q band (31.2–50.3 GHz) was complemented with data from a new receiver installed at the Yebes 40m radio telescope that covers the K and K_a bands (18–32 GHz). The identification of 3-cyanoacenaphthylene was based on the detection of 135 individual lines that correspond to 194 rotational transitions. For 4-cyanoacenaphthylene, the lines are slightly weaker, but we identified 37 individual lines that correspond to 66 rotational transitions. As a first step for the identification, we compared the derived rotational constants with their expected theoretical values and obtained a match better than 0.2%. Finally, the new isomers were synthesized in our chemical laboratory and observed in our microwave spectroscopic facilities. The derived laboratory constants match those derived from TMC-1 data perfectly, which confirms our identifications. Lines in the K − K_a band from the previously detected isomers 1- and 5-cyanoacenaphthylene are also reported. The derived column density for 3-cyanoacenaphthylene is (7.0 ± 0.7) × 10¹¹ cm⁻², and for the 4-cyano isomer, we derive a column density of (5.0 ± 0.6) × 10¹¹ cm⁻². The four possible isomers of cyanoacenaphthylene are finally found in TMC-1. Despite their high relative energies, they exhibit similar abundances (within a factor of ∼2).