2023A&A...676L...5C

We report the first laboratory and interstellar detection of the α-cyano vinyl radical (H₂CCCN). This species was produced in the laboratory by an electric discharge of a gas mixture of vinyl cyanide, CH₂CHCN, and Ne. Its rotational spectrum was characterized using a Balle-Flygare narrowband-type Fourier-transform microwave spectrometer operating in the frequency region of 8-40 GHz. The observed spectrum shows a complex structure due to tunneling splittings between two torsional sublevels of the ground vibronic state, 0⁺ and 0^–, derived from a large-amplitude inversion motion. In addition, the presence of two equivalent hydrogen nuclei makes it necessary to discern between ortho- and para-H₂CCCN. A least-squares analysis reproduces the observed transition frequencies with a standard deviation of ca. 3 kHz. Using the laboratory predictions, this radical was detected in the cold dark cloud TMC-1 using the Yebes 40 m telescope and the QUIJOTE¹ line survey. The 4_0, 4_–30, 3_ and 5_0, 5_–40, 4_ rotational transitions, composed of several hyperfine components, were observed in the 31.0-50.4 GHz range. Adopting a rotational temperature of 6 K, we derived a column density of (1.4±0.2)×10¹¹ cm^–2 and (1.1±0.2)×10¹¹ cm^–2 for ortho-H₂CCCN and para-H₂CCCN, respectively. The reaction of C + CH₃CN emerges as the most likely route to H₂CCCN in TMC-1, and possibly that of N + CH₂CCH as well.