2026A&A...705L...7C

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).