Methanimine As A Sink In The HCN And HNC Solid State Hydrogenation Network

We aim to provide a systematic and quantitative description of the hydrogenation network connecting HCN and HNC to methylamine on interstellar water ices, while identifying dominant pathways and bottlenecks.

To this end, we performed a comprehensive quantum-chemical investigation of H-addition, H-abstraction, reactions with H₂, and water-assisted H-transfer isomerization, covering intermediates linking HCN and HNC to CH₃NH₂. Calculations were carried out on amorphous solid water clusters of 14 molecules.

Using benchmarked density functional theory, we derived activation barriers, elucidated mechanisms, and determined the binding energy distribution of H₂CN and CNH₂, also assessing deuterium substitution effects. H-addition reactions generally involve activation barriers, except for radical species.

Considering both barrier heights and tunneling crossover temperatures, the most favorable sequence originates from HNC rather than HCN. The network evolves toward methanimine (H₂CNH), the central species, or the singlet carbene HC:NH₂, from which further hydrogenation leads to methylamine. Along these paths, several reactions are barrierless, while some H-abstraction processes compete with addition. Reactions involving H₂ are uncommon, as most are endoergic.

Deuterium substitution weakly affects classical barriers but significantly influences tunneling efficiencies. Our results support efficient formation of methanimine and methylamine from HNC on cold interstellar ices, with methanimine acting as a chemical sink, whereas HCN is less reactive and more likely to persist. These findings provide quantitative constraints for astrochemical models.

Schematic representation of the •CN chemical network on interstellar ice surfaces. The figure summarizes the reactions investigated in this work (the hydrogenation of HCN and HNC) together with related processes reported in previous studies (Enrique-Romero et al. 2022; Molpeceres et al. 2024; Enrique-Romero & Lamberts 2024; Enrique-Romero & Lamberts 2025b), shown as colored regions. Unless otherwise indicated, radical species have doublet spin multiplicity. Solid arrows denote barrier-less reactions, whereas dashed arrows indicate reactions with an activation barrier. Activation energies are given in kJ mol⁻¹ . Arrow colors indicate reaction types: blue for H addition and abstraction, red for reactions with H₂, gray for wHts, and black for other processes (e.g., isomerization or radical–radical reactions). Energies are reported on a per-reaction basis, assuming efficient thermalization on amorphous solid water, such that each activation barrier is referenced to the products of the preceding step. — astro-ph.GA

Joan Enrique-Romero, Thanja Lamberts

Comments: Accepted in A&A (Apr 2026)
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2604.09228 [astro-ph.GA](or arXiv:2604.09228v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2604.09228
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Submission history
From: Joan Enrique-Romero
[v1] Fri, 10 Apr 2026 11:36:56 UTC (5,340 KB)
https://arxiv.org/abs/2604.09228

Astrobiology, Astrochemistry,