2025ApJ...994..217M

The astronomical detection of CN-functionalized ring molecules in TMC-1 has opened a new and unexplored area in space-based aromatic organic chemistry. The rotational spectrum of 1-cyano-cyclopentene (1-CNCPE) has been investigated using a combination of low- and high-resolution microwave spectroscopy techniques under jet-cooled and room temperature conditions. A Stark-modulated low-resolution spectrum revealed a clear a-type R-branch band structure, indicative of a single dominant rotamer with prolate character. High-resolution spectra recorded with broadband (CP-FTMW) and narrowband (MB-FTMW) techniques exhibited a doubling of transitions, attributed to tunneling between two ring-puckering vibrational substates, denoted 0⁺ and 0^–. In addition, the millimeter-wave spectrum in the 329-394 GHz region has also been analyzed. Due to strong coupling between these levels, the spectral analysis required a Hamiltonian including Coriolis interaction terms within the reduced axis system formalism. This approach enabled the successful assignment and fitting of over 1244 rotational transitions, including hyperfine structure from the ¹⁴N nucleus and weak b- and c-type lines. The final spectroscopic parameters provide a reliable basis for astrophysical searches, and partition functions are reported at standard temperatures. We have searched for this species toward TMC-1 using the data of the QUIJOTE line survey. Only a 3σ upper limit to the column density of 3 × 10¹⁰ cm^–2 has been obtained. Using the available spectroscopic data, we also searched for the two axial and equatorial conformers of 3-cyanocyclopentene, establishing an upper limit similar to that of 1-CNCPE.