2022A&A...657A..24C

Context. The reaction between carbon atoms and vinyl cyanide, CH₂CHCN, is a formation route to interstellar 3-cyano propargyl radical, CH₂C₃N, a species that has recently been discovered in space. The 1-cyano propargyl radical (HC₃HCN), an isomer of CH₂C₃N, is predicted to be produced in the same reaction at least twice more efficiently than CH₂C₃N. Hence, HC₃HCN is a plausible candidate to be observed in space as well. Aims. We aim to generate the HC₃HCN radical in the gas phase in order to investigate its rotational spectrum. The derived spectroscopic parameters for this species will be used to obtain reliable frequency predictions to support its detection in space. Methods. The HC₃HCN radical was produced by an electric discharge, and its rotational spectrum was characterized using a Balle-Flygare narrowband-type Fourier-transform microwave spectrometer operating in the frequency region of 4-40 GHz. The spectral analysis was supported by high-level ab initio calculations. Results. A total of 193 hyperfine components that originated from 12 rotational transitions, a- and b-type, were measured for the HC₃HCN radical. The analysis allowed us to accurately determine 22 molecular constants, including rotational and centrifugal distortion constants as well as the fine and hyperfine constants. Transition frequency predictions were used to search for the HC₃HCN radical in TMC-1 using the QUIJOTE survey between 30 and 50 GHz. We do not detect HC₃HCN in TMC-1 and derive a 3σ upper limit to its column density of 6.0 x 10¹¹ cm^–2.