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.
Highlights
Among over 200 molecules detected in the interstellar medium (ISM) and circumstellar shells, approximately 70 have six or more atoms and contain carbon
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
The spectral measurements were performed using the frequency modulation (FM) technique: the carrier signal from the cm-wave synthesiser is sine-wave modulated at 50 kHz, and the detector output is demodulated at twice this frequency (2 f ) by a lock-in amplifier
Summary
Among over 200 molecules detected in the interstellar medium (ISM) and circumstellar shells, approximately 70 have six or more atoms and contain carbon. These compounds are called complex organic molecules (COMs, Herbst & van Dishoeck 2009), and have received increasing attention in the last decade in an effort to unveil how chemical complexity builds up, from simple species to the much larger molecular structures required to establish biochemical. Amino acids are prime targets for astrobiological studies, and their extraterrestrial formation has been a highly debated topic. Numerous compounds of this class have been found in carbonaceous chondrites The simplest amino acid, has not been detected in the ISM to date (Snyder et al 2005), it was found in the coma of the 67P/Churyumov– Gerasimenko comet through in situ mass spectrometry
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