Propargylimine in the laboratory and in space: millimetre-wave spectroscopy and its first detection in the ISM

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Bizzocchi, L., Prudenzano, D., Rivilla, V. M., Pietropolli-Charmet, A., Giuliano, B. M., Caselli, P., Martín-Pintado, J., Jiménez Serra, J., Martín, S., Requena-Torres, M. A., Rico-Villas, F., Zeng, S., Guillemin, J. C. (2020). Propargylimine in the laboratory and in space: millimetre-wave spectroscopy and its first detection in the ISM. Astronomy and Astrophysics, 640, DOI: 10.1051/0004-6361/202038083

Context. Small imines containing up to three carbon atoms are present in the interstellar medium (ISM). As alkynyl compounds are abundant in this medium, propargylimine (2-propyn-1-imine, HC C-CH=NH) thus represents a promising candidate for a new interstellar detection.

Aims. The goal of the present work is to perform a comprehensive laboratory investigation of the rotational spectrum of propargylimine in its ground vibrational state in order to obtain a highly precise set of rest frequencies and to search for it in space.

Methods. The rotational spectra of E and Z geometrical isomers of propargylimine have been recorded in the laboratory in the 83-500 GHz frequency interval. The measurements have been performed using a source-modulation millimetre-wave spectrometer equipped with a pyrolysis system for the production of unstable species. High-level ab initio calculations were performed to assist the analysis and to obtain reliable estimates for an extended set of spectroscopic quantities. We searched for propargylimine at 3 mm and 2 mm in the spectral survey of the quiescent giant molecular cloud G+0.693-0.027 located in the central molecular zone, close to the Galactic centre.

Results. About 1000 rotational transitions have been recorded for the E- and Z-propargylimine, in the laboratory. These new data have enabled the determination of a very accurate set of spectroscopic parameters including rotational, quartic, and sextic centrifugal distortion constants. The improved spectral data allowed us to perform a successful search for this new imine in the G+0.693-0.027 molecular cloud. Eighteen lines of Z-propargylimine were detected at level >2.5 sigma, resulting in a column-density estimate of N = (0.24 x 0.02) x 10(14) cm(-2). An upper limit was retrieved for the higher energy E isomer, which was not detected in the data. The fractional abundance (with respect to H-2) derived for Z-propargylimine is 1.8 x 10(-10). We discuss the possible formation routes by comparing the derived abundance with those measured in the source for possible chemical precursors.

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