Laboratory and astronomical discovery of the cyanovinyl radical H2CCCN

Abstract

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 40, 4-30, 3 and 50, 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.