Formation of amines: Hydrogenation of nitrile and isonitrile as selective routes in the interstellar medium

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.