Building formamide and N-substituted formamides from isocyanates on hydrogenated water ices

Abstract

Context. Many complex organic molecules (COMs) observed in the interstellar medium (ISM) are probably not formed in the gas phase. A large consensus has developed that it could be related to the icy surfaces in this environment. Aims. We investigate the process of building N-substituted formamides in the ISM by successive additions of atomic hydrogen to isocyanates. The key point is to see whether the pre-adsorption of the atomic hydrogen on the ice surface is a driving vector as it is for the formation of CH3OH from CO. Methods. We use quantum numerical simulations, namely density functional theory (DFT) and post Hartree–Fock (p-HF) methods derived from coupled-cluster implementations. Several chemical models are presented: the addition of H directly to the isocyanate in the gas phase, the addition of H to the isocyanate pre-adsorbed on ices, the addition of the isocyanate to the hydrogen pre-adsorbed on ices. These ices are successively simulated by a few water molecules up to full bi-layers of them. Results. The formation of formamide (NH2CHO) from the isocyanic acid (HNCO) is taken as a case study. Whatever the level of the calculation and the size of the water cluster supporting the adsorbed isocyanate, the addition of the incoming atomic hydrogen reveals no opportunity to eliminate the energy barrier found in the gas phase. By contrast, the formation of H2NCHO, as well as CH3NHCHO or C2H5NHCHO, is possible without any barrier on the same ice surfaces, with the express condition that the H atom to be added is already attached to the ice, prior to the attack by the isocyanate species. Conclusions. There is a way for the N-substituted formamides to be easily built by two successive hydrogenations on ices starting from the isocyanates HNCO, CH3NCO, and C2H5NCO. Some of those species are already detected; if not, they appear as strong candidates worth considering for future observation campaigns. Moreover, this suggests that other hydrogenation processes neglected to date, could be considered when similar pre-conditions are satisfied.