Abstract
Context. Beyond NH3, only one primary alkylamine, CH3NH2, has been identified in the interstellar medium and the reason why is still not understood: its formation could occur in the gas phase or in icy environments. Aims. To consider any possible difference between the formation of primary and secondary amines, we studied the hydrogenation processes of CH3CN and CH3NC, which would lead to the simple primary CH3CH2NH2 and secondary CH3NHCH3 amines, respectively. Methods. Experimentally, the hydrogenation of CH3CN and CH3NC was carried out under ultra-high vacuum, using two beamlines to inject the nitrile/isonitrile and H onto substrate surfaces of gold or water ice. The reactions were monitored using infrared spectroscopy and the products were followed by mass spectrometry. Theoretically, the energetics of the hydrogenation paths were determined using the M06-2X functional after benchmarking against post Hartree–Fock procedures. Meanwhile, a survey of the high-mass star forming region W51/e2 has been performed. Results. Following co-deposition of CH3CN and H, we show that these species do not react together between 10 and 60 K. For CH3NC we found that the hydrogenation process works all the way through the CH3NHCH3 end product; we also identified the CH3NCH2 intermediate together with side products, CH4 and HCN, showing that the isonitrile backbone is breaking. These results are consistent with the calculations of a high barrier on the first hydrogenation step for CH3CN and a lower barrier for CH3NC. Conclusions. The formation of CH3CH2NH2 by hydrogenation of CH3CN appears rather unlikely in both the gas phase and ice environment whereas that of CH3NHCH3 is a clear possibility. The limiting factor appears to be the efficiency of the tunneling effect through the first activation barrier on the reaction paths. More surveys are required for further insight into the search for amines.
Highlights
More than 200 species have been detected so far in the interstellar medium (ISM) and circumstellar shells including complex molecules (COM) from 6 to 13 atoms (McGuire 2018)1
To consider any possible difference between the formation of primary and secondary amines, we studied the hydrogenation processes of CH3CN and CH3NC, which would lead to the simple primary CH3CH2NH2 and secondary CH3NHCH3 amines, respectively
For CH3NC we found that the hydrogenation process works all the way through the CH3NHCH3 end product; we identified the CH3NCH2 intermediate together with side products, CH4 and HCN, showing that the isonitrile backbone is breaking
Summary
More than 200 species have been detected so far in the interstellar medium (ISM) and circumstellar shells including complex molecules (COM) from 6 to 13 atoms (McGuire 2018). Mencos & Krim (2016) reported that CH3CN may react with ground state N atoms to form CH3NC and CH2CNH in the solid phase at temperatures ranging from 7 to 11 K These laboratory results support the large consensus that gas-phase chemistry alone cannot account for the diversity of complex organic molecules observed in the ISM and that solid-gas reactions, as well as chemical processes involving adsorbed or/and embedded partner, have to be taken into account. 3, we present the theoretical approach, starting by the benchmarking done to determine the more appropriate level of theory for this study This is followed by the results of the calculations of enthalpies and activation barriers of the successive steps of the hydrogenation processes in the gas phase and the ice. Section 4 is a first report of the tentative search for molecules relevant to this study in the W51/e2 region, using the IRAM 30 m telescope. The theoretical and experimental results are compared and discussed with an emphasis on the possible implications on the abundances of amines in the ISM as shown by the results of the observational campaign
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