Methanimine as a sink in the HCN and HNC solid state hydrogenation network

Aims. We aim to provide a systematic and quantitative description of the hydrogenation network connecting HCN and HNC to methylamine on interstellar water ices, and to identify the dominant reaction pathways and potential bottlenecks. Methods. We performed a comprehensive quantum-chemical investigation of H-addition, H-abstraction, reactions with H 2 , and water-assisted H-transfer isomerization involving all intermediates connecting HCN and HNC up to CH 3 NH 2 , considering amorphous solid water molecular clusters composed of 14 water molecules. We employed benchmarked density functional theory to derive activation barriers, elucidate the reaction mechanisms, and determine the binding energy distribution of H 2 CN and CNH 2 . We also discuss the effect of deuterium substitutions on selected points of the network. Results. H-addition reactions are generally mediated by activation energy barriers, except for radical reactants. Considering both barriers and the associated tunneling crossover temperatures, the most favorable hydrogenation sequence starts from HNC rather than from HCN. The network initially evolves toward either methanimine (H 2 CNH; which emerges as the central species) or the singlet open-shell carbene 1 [HCNH 2 ]. Subsequent hydrogenation steps lead to methylamine (CH 3 NH 2 ). Along these reaction paths, several processes are barrier-less (e.g., the hydrogenation of molecular radicals), while some H-abstraction reactions can compete with addition channels. Reactions involving H 2 as a reactant are rare, as most of these channels are endoergic. Deuterium substitution has only a minor impact on classical activation energies but significantly affects the transition-state imaginary frequencies and, therefore, the expected tunneling efficiencies. Conclusions. Our results support the idea that methanimine and methylamine can efficiently form from HNC on cold interstellar ices, with methanimine acting as a chemical sink within the network. HCN, on the other hand, is less reactive and hence has a higher chance of being preserved. The reaction network presented here provides quantitative constraints for astrochemical models of nitrogen-bearing organic chemistry in star-forming regions.