Laboratory and astronomical discovery of the cyanovinyl radical H<sub>2</sub>CCCN
C. Cabezas, Jian Tang, M. Agúndez, K. Seiki, Yoshihiro Sumiyoshi, Yasuhiro Ohshima, B. Tercero, N. Marcelino, R. Fuentetaja, P. de Vicente, Yasuki Endo, J. Cernicharo
Abstract
We report the first laboratory and interstellar detection of the α -cyano vinyl radical (H 2 CCCN). This species was produced in the laboratory by an electric discharge of a gas mixture of vinyl cyanide, CH 2 CHCN, 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-H 2 CCCN. 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 QUIJOTE 1 line survey. The 4 0, 4 -3 0, 3 and 5 0, 5 -4 0, 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)×10 11 cm −2 and (1.1±0.2)×10 11 cm −2 for ortho-H 2 CCCN and para-H 2 CCCN, respectively. The reaction of C + CH 3 CN emerges as the most likely route to H 2 CCCN in TMC-1, and possibly that of N + CH 2 CCH as well.