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

¹ Center for Astrochemical Studies, Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany
e-mail: bizzocchi@mpe.mpg.de
² INAF–Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
³ Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca’ Foscari Venezia, Via Torino 155, 30172 Mestre, Italy
⁴ Centro de Astrobiología (CSIC–INTA), Ctra. de Torrejón a Ajalvir, km 4, Torrejón de Ardoz, 28850 Madrid, Spain
⁵ European Southern Observatory (ESO), Alonso de Córdoba 3107, Vitacura, 763-0355 Santiago, Chile
⁶ Joint ALMA Observatory (ESO), Alonso de Córdoba 3107, Vitacura, 763-0355 Santiago, Chile
⁷ University of Maryland, College Park, ND 20742-2421, USA
⁸ Department of Physics, Astronomy and Geosciences, Towson University, Towson, MD 21252, USA
⁹ Star and Planet Formation Laboratory, Cluster for Pioneering Research, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
¹⁰ Univ. Rennes, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, ISCR – UMR6226, 35000 Rennes, France

Received: 3 April 2020
Accepted: 12 June 2020

Abstract

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 >2.5σ, resulting in a column-density estimate of N = (0.24 ± 0.02) × 10¹⁴ 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 H₂) derived for Z-propargylimine is 1.8 × 10⁻¹⁰. We discuss the possible formation routes by comparing the derived abundance with those measured in the source for possible chemical precursors.