Laboratory spectroscopy of allylimine and tentative detection towards the G+0.693-0.027 molecular cloud

Abstract

Context. Substituted methanimines and ethylenes have been identified in the interstellar medium. Therefore, allylimine (CH 2 CH CH NH) 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 allylimine in its ground vibrational state in order to obtain a highly precise set of rest frequencies to assist its search for astronomical sources. Methods. The rotational spectra of the two most stable trans-anti and trans-syn geometrical isomers of allylimine were recorded in the laboratory in the 84-300 GHz frequency interval. Measurements were 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 parameters. Guided by new laboratory data, allylimine was searched for in space using a sensitive spectral survey of the G+0.693-0.027 molecular cloud, located at the Galactic centre.Results. Almost 1000 rotational transitions have been recorded for trans-anti and trans-syn allylimine. These new data have enabled the determination of a very accurate set of spectroscopic parameters including rotational, quartic and sextic centrifugal distortion constants, as well as nuclear quadrupole coupling constants. The improved spectral data allowed us to report a tentative detection for both allylimine isomers in the G+0.693-0.027 molecular cloud, located at the Galactic centre.

First Glycine Isomer Detected in the Interstellar Medium: Glycolamide (NH2C(O)CH2OH)

“…The spectroscopic information of these transitions is presented in Table 2. To ensure that the observed spectral features are due to syn-glycolamide and properly evaluate possible line contamination by other molecules, we have considered the emission from the >130 molecules already identified toward G+0.693 (e.g., Requena-Torres et al 2006, 2008Rivilla et al 2018Rivilla et al , 2019Rivilla et al , 2020bRivilla et al , 2021aRivilla et al , 2021bRivilla et al , 2022aRivilla et al , 2022bRivilla et al , 2022cZeng et al 2018Zeng et al , 2021Zeng et al , 2023Bizzocchi et al 2020;Jiménez-Serra et al 2020Rodríguez-Almeida et al 2021aColzi et al 2022;Alberton et al 2023;San Andrés et al 2023). We note that the rest of the syn-glycolamide transitions that fall within the spectral coverage of the survey are consistent with the observed spectra, but they are heavily blended with lines from other molecular species or too faint to be detected.…”

Hunting for interstellar molecules: rotational spectra of reactive species

“…The list includes radicals and ions, but also closed-shell unsaturate systems such as imines and carbon chains. Illustrative examples are the HC 5 O radical, 6,7 the C 8 Hnegative ion 8,9 , the HC 9 N carbon chain 10 , the protonated HC 5 NH + carbon chain 11 , substituted imines like allylimine 12 , and the unstable enol form of glycolaldehyde, i.e. (Z)-1,2-ethenediol enol 13 .…”

“…As examples of closed-shell reactive species, the DC 7 N and ClC 3 N carbon chains have been studied, while as examples of open-shell systems, the CCS and CH 2 CN radicals are considered. The strategy relies on the interplay of experiment and theory, which is crucial because it often happens that, for this type of molecular species, no previous experimental data are available and the study starts from an accurate computational characterization 12,28 . In the following section (Methods), the strategy is introduced; then, the experimental and computational aspects are detailed.…”

Computational Protocol for the Identification of Candidates for Radioastronomical Detection and Its Application to the C3H3NO Family of Isomers

“…However, it has to be noted that a relevant number of discoveries have occurred since then and updated lists can be found in the Cologne Database for Molecular Spectroscopy [ 2 ] or on the Astrochymist website [ 3 ]. Despite the limited number of atoms and the severe constraints on chemical reactivity imposed by the ISM conditions (i.e., very low temperature and density, and ionizing radiation), interstellar molecules exhibit a large variety of functional groups, including nitriles [ 4 , 5 , 6 ], imines [ 7 , 8 , 9 , 10 , 11 , 12 , 13 ], amines [ 14 , 15 ], alcohols [ 16 , 17 , 18 ], amides [ 19 , 20 , 21 , 22 , 23 ], aldehydes [ 24 , 25 , 26 , 27 ], and many others. Indeed, several chemical linkages are possible within a specific chemical formula so that a generic isomeric family can be constituted by tens—if not hundreds—of structural isomers, each characterized by different functional groups.…”