2022A&A...663A.132M

Context. Methanimine, CH₂NH, and ethanimine, CH₃CHNH, were both detected in the interstellar medium (ISM), the former in the 1970s and the latter nearly a decade ago. By analogy with nitriles, for which hydrogen cyanide, acetonitrile, and ethyl cyanide are the most abundant, n-propanimine was suggested as a candidate for detection in the ISM.
Aims. To date, no measurements of the rotational spectrum of 1-propanimine have been made. We present measurements and an analysis of its spectrum in the millimeter- and submillimeter-wave domain in order to provide an accurate prediction that is required to search for this molecule in the ISM.
Methods. The rotational spectrum of propanimine was investigated from 150 to 470 GHz. Watson's asymmetric top Hamiltonian in the I^r representation and S reduction was used for the analysis. The new prediction allowed us to search for 1-propanimine toward the high-mass star forming region Sagittarius B2(N), the dark molecular cloud TMC-1, as well as a sample of low-mass protostellar objects. These sources were observed with the Atacama Large Millimeter/submillimeter Array, the Green Bank Telescope, or the IRAM 30 m telescope.
Results. A total of 585 and 296 transitions of the E and Z-1-propanimine, respectively, were newly assigned and fitted in the laboratory spectra recorded up to 470 GHz. Quantum numbers up to J = 73 and K_a = 18 were reached for the E isomer and J = 56 and K_a = 17 for the Z isomer. We report the nondetection of 1-propanimine toward all astronomical sources investigated in this work. We find in particular that 1-propanimine is at least 20 times less abundant than methanimine in the envelope of the high-mass star forming region Sagittarius B2(N).
Conclusions. The accurate spectroscopic prediction of its spectrum provided in this work will allow astronomers to continue the search for 1-propanimine in the ISM.