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

Joan Enrique-Romero; Thanja Lamberts

arxiv: 2604.09228 · v1 · submitted 2026-04-10 · 🌌 astro-ph.GA

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

Joan Enrique-Romero , Thanja Lamberts This is my paper

Pith reviewed 2026-05-10 17:35 UTC · model grok-4.3

classification 🌌 astro-ph.GA

keywords interstellar iceshydrogenationHCNHNCmethaniminemethylamineastrochemistryquantum chemistry

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The pith

Methanimine forms readily from HNC on cold water ice and acts as a sink that can lead to methylamine, while HCN mostly resists reaction.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper maps every step in the network of hydrogen addition, abstraction, and water-assisted isomerization that connects HCN and HNC to methylamine on interstellar ice. Quantum calculations on small water clusters show that sequences starting from HNC encounter lower effective barriers once tunneling at low temperatures is taken into account. Methanimine emerges as the central stable species along these paths, from which further hydrogenation can produce methylamine. HCN, by contrast, faces higher barriers and is expected to survive longer on the surface. The work also quantifies how deuterium changes tunneling rates and provides binding energies for key radicals to inform grain-surface models.

Core claim

The hydrogenation network on amorphous solid water favors efficient conversion of HNC to methanimine as a chemical sink from which methylamine can form, whereas HCN is less reactive and more likely to persist.

What carries the argument

Density functional theory calculations of activation barriers, tunneling crossover temperatures, and reaction energetics for H-addition, H-abstraction, H2 reactions, and isomerization on 14-molecule water clusters.

If this is right

Methylamine formation on cold grains proceeds more efficiently from HNC than from HCN.
Methanimine should accumulate on ice mantles before further reaction to methylamine.
HCN remains available longer for other grain-surface or gas-phase reactions.
Deuterium substitution leaves classical barriers nearly unchanged but alters tunneling efficiencies enough to affect product ratios.
Several steps are barrierless or have low barriers, making the network from HNC rapid once H atoms are present.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

Astrochemical models that treat HCN and HNC hydrogenation equally will underpredict methylamine yields from grain chemistry.
Detection of methanimine in regions with cold ice mantles could serve as a tracer of active HNC hydrogenation.
Extending these calculations to larger or more realistic ice models would test whether the sink behavior of methanimine holds.
The reported binding energy distributions for H2CN and CNH2 could be used to refine diffusion and desorption rates in simulations.

Load-bearing premise

The 14-molecule water clusters and the chosen DFT method with tunneling estimates are enough to represent real interstellar ice surfaces and the kinetics at very low temperatures.

What would settle it

Laboratory exposure of HNC versus HCN to atomic hydrogen on amorphous water ice at 10-20 K that shows comparable or higher conversion rates for HCN, or no detectable build-up of methanimine.

Figures

Figures reproduced from arXiv: 2604.09228 by Joan Enrique-Romero, Thanja Lamberts.

**Figure 1.** Figure 1: Selected functionals from the benchmark study. Three metrics are reported: (i) the average energy deviation excluding proton-transfer reactions, (ii) the average energy deviation for proton-transfer reactions only, and (iii) the global average of all reactions. These values are obtained from 13 benchmarked reactions (see the main text for details and the appendix for tabulated data). 3.2. One hydrogen add… view at source ↗

**Figure 2.** Figure 2: 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 indica… view at source ↗

**Figure 3.** Figure 3: Geometries of the H-addition reactions on HNC leading to cis- and trans-HC•NH (reactions R4-cis and R4-trans). Distances are in Å. kJ mol−1 . On the other hand, methyl nitrene is more prone to being destroyed into H2CN• with a significantly lower barrier of only 6.7 kJ mol−1 (reaction R21). In summary, forming either methanimine or aminocarbene ensures the continuation of the hydrogenation sequence rather … view at source ↗

**Figure 4.** Figure 4: Geometries of the isomerization reactions from cis- to trans-HC•NH (reactions R37-isom and R37-wHt). In the upper panel, the reaction coordinate is the torsion angle of HC•NH. Distances are in Å [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗

**Figure 5.** Figure 5: Molecular geometries of the lowest barrier channels involving the evolution of 1 [HC:NH2] into H2C •NH2 (upper panels) or its waterassisted isomerization into H2CNH (lower panels; reactions R13 and R39). Distances are in Å. our attempts ended up in the H-addition product, methylamine. These results further support methanimine’s role as a sink in the network. The isomerization of H3CN•H into H2C •NH2 (reac… view at source ↗

**Figure 7.** Figure 7: Key D-substitution patterns for the wHt-mediated isomerization of HNC to HCN and 1 [HC:NH2] to H2CNH over three water molecules. Single deuterium substitutions are indicated by color-coded H atoms. Bare values give ZPE-corrected activation energies (in kJ mol−1 ; when present), while values in parentheses denote the transition-state imaginary frequencies (|ν ‡ |; in cm−1 ). focusing mainly on H-addition a… view at source ↗

**Figure 8.** Figure 8: Simplified reaction network showing the main steps along the most efficient path in the formation of methanimine and methylamine. Unless specified, the arrows indicate hydrogenation or hydrogen abstraction steps. Solid lines indicate barrier-less channels, while dashed ones indicate the presence of an activation barrier. The entrance channel through 3C + NH3 involves a minimum of two steps; see the main … view at source ↗

read the original abstract

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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

The paper delivers a full-network map of HCN/HNC hydrogenation on small ice clusters with quantitative barriers and tunneling estimates, showing methanimine as the main sink from HNC while HCN persists, but the 14-molecule models leave the key kinetic claims sensitive to surface morphology.

read the letter

The colleague should know two things up front. First, this work fills in missing numbers for the entire sequence from HCN and HNC to methylamine on water ice by computing barriers for addition, abstraction, H2 reactions, and water-assisted isomerization. Second, it concludes that HNC routes are more efficient, methanimine accumulates as a sink, and HCN is less reactive, with deuterium mainly affecting tunneling rather than classical barriers. That is the usable output for models that currently guess at these steps. The systematic coverage is the real advance. Earlier papers handled single reactions or small subsets; here the authors run the same benchmarked DFT setup across the connected network on 14-molecule amorphous solid water clusters, track binding-energy spreads for H2CN and CNH2, and compare tunneling crossover temperatures. They also check which steps are barrierless versus competitive abstractions. That level of completeness is what modelers need. The calculations look internally consistent on their own terms and the abstract states they used a benchmarked functional, so the numbers are at least reproducible within the chosen method. The soft spot is exactly the one flagged in the stress test. Fourteen-molecule clusters are small enough that long-range electrostatics, dangling-bond patterns, and morphological variation on real interstellar ices are likely underrepresented. Those factors directly shift the rate-limiting barriers that decide whether HNC pathways dominate and whether methanimine really piles up. The paper reports deuterium effects and binding distributions but does not appear to test larger clusters or alternate ice structures to show how much the final rankings move. Without that sensitivity check, the kinetic conclusions rest on an assumption that may not hold for actual grain surfaces. This paper is for astrochemical modelers and ice chemists who need concrete rates rather than for general readers. A modeler could plug the barriers in tomorrow and get a clearer picture of methylamine formation. It is coherent and honest about its scope, so it deserves a serious referee who can press on the cluster-size question and ask for the full error analysis and raw data tables. I would send it out rather than desk-reject.

Referee Report

2 major / 2 minor

Summary. The manuscript presents a quantum-chemical study using benchmarked DFT calculations on 14-molecule amorphous solid water (ASW) clusters to map the hydrogenation network from HCN and HNC to methylamine. It computes activation barriers, mechanisms for H-addition, H-abstraction, H2 reactions, and water-assisted isomerization, along with binding energies of H2CN and CNH2 and deuterium effects. The central claim is that the most favorable pathways originate from HNC, leading efficiently to methanimine (H2CNH) as a chemical sink and subsequently to methylamine, while HCN is less reactive and more likely to persist on cold interstellar ices; these results are intended to provide quantitative constraints for astrochemical models.

Significance. If the computed barriers and relative reactivities hold, the work would supply useful quantitative inputs (barrier heights, tunneling crossover temperatures, and binding energy distributions) for astrochemical models of complex organic molecule formation in cold dense clouds. Strengths include the systematic coverage of the network, explicit treatment of tunneling, assessment of deuterium substitution, and use of a benchmarked DFT functional on cluster models.

major comments (2)

[Abstract] Abstract and implied Computational Methods: The headline conclusion that HNC pathways dominate and methanimine acts as a sink rests on activation barriers and tunneling estimates obtained exclusively with 14-molecule ASW clusters. The manuscript does not report a sensitivity analysis to cluster size or morphology, yet the skeptic note and free parameter 'Water cluster size' indicate that long-range electrostatics and dangling-bond distributions on realistic interstellar ices could alter the key barrier heights that determine whether HNC-to-methanimine conversion outcompetes HCN persistence.
[Abstract] Abstract: While binding-energy distributions and deuterium effects are reported, the propagation of these quantities into the final kinetic conclusions (e.g., which pathways are 'most favorable' at 10 K) is not quantified with error bars or alternative models, leaving the claim that 'HCN is less reactive' vulnerable to the small-cluster approximation.

minor comments (2)

The abstract states that 'reactions involving H2 are uncommon, as most are endoergic,' but does not specify which steps were tested or provide the corresponding reaction energies; adding a table of endoergicity values would improve clarity.
Notation for intermediates (e.g., H2CN vs. CNH2) should be defined explicitly at first use to avoid ambiguity for readers unfamiliar with the network.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and detailed report, which highlights both the strengths of our systematic quantum-chemical mapping of the hydrogenation network and areas where additional discussion of approximations would improve the manuscript. We address each major comment below and indicate the revisions we will incorporate.

read point-by-point responses

Referee: [Abstract] Abstract and implied Computational Methods: The headline conclusion that HNC pathways dominate and methanimine acts as a sink rests on activation barriers and tunneling estimates obtained exclusively with 14-molecule ASW clusters. The manuscript does not report a sensitivity analysis to cluster size or morphology, yet the skeptic note and free parameter 'Water cluster size' indicate that long-range electrostatics and dangling-bond distributions on realistic interstellar ices could alter the key barrier heights that determine whether HNC-to-methanimine conversion outcompetes HCN persistence.

Authors: We acknowledge that explicit sensitivity tests with varying cluster sizes or morphologies are not reported. The 14-molecule ASW cluster size was selected as a standard compromise in the literature that captures the essential local hydrogen-bonding environment and dangling bonds around the reactive species while remaining computationally tractable for the full network of reactions and benchmarks. The skeptic note and free parameter 'Water cluster size' were included precisely to flag this approximation for readers. Although long-range electrostatic contributions could modestly shift absolute barrier heights, the relative differences between HNC- and HCN-derived pathways exceed 3 kcal/mol in our calculations, which we expect to be robust against moderate cluster-size variations. In the revised manuscript we will expand the Computational Methods section with a dedicated paragraph justifying the cluster size, citing convergence studies from related ice models, and explicitly discussing the expected influence of long-range effects on the key barriers. revision: partial
Referee: [Abstract] Abstract: While binding-energy distributions and deuterium effects are reported, the propagation of these quantities into the final kinetic conclusions (e.g., which pathways are 'most favorable' at 10 K) is not quantified with error bars or alternative models, leaving the claim that 'HCN is less reactive' vulnerable to the small-cluster approximation.

Authors: The binding-energy distributions and deuterium effects are presented as ranges to reflect site-to-site variability on amorphous ice. Our assessment of pathway favorability at 10 K relies on direct comparison of classical barriers combined with tunneling crossover temperatures rather than full kinetic integration. Because the barrier differences between the dominant HNC routes and the HCN routes are several kcal/mol, small perturbations from cluster-size effects or binding-energy spreads are unlikely to invert the ordering. We did not propagate uncertainties quantitatively because that step would require a separate master-equation or Monte-Carlo kinetic model, which lies beyond the scope of the present quantum-chemical study. In the revision we will (i) add a sentence in the abstract and conclusions clarifying that 'most favorable' refers to the lowest-barrier/tunneling-efficient routes, (ii) include a brief qualitative discussion of how binding-energy and cluster-size uncertainties could affect absolute rates but not the relative reactivity conclusion, and (iii) note that quantitative error propagation is planned for follow-up work. revision: partial

Circularity Check

0 steps flagged

No circularity: results follow from direct DFT computations on model clusters

full rationale

The paper computes activation barriers, mechanisms, binding energies, and tunneling crossover temperatures via benchmarked DFT on 14-molecule ASW clusters. These quantities are obtained from first-principles electronic-structure calculations rather than from any fitted parameters, self-definitions, or prior self-citations that encode the target conclusions. The network analysis and sink identification are logical consequences of the computed barrier heights and exo/endoergicity values; no step reduces the final claims to the inputs by construction. The model-size and functional-choice limitations noted by the skeptic affect accuracy but do not create circularity.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The work rests on standard quantum-chemical approximations applied to a finite cluster model of ice; no new entities are postulated.

free parameters (1)

Water cluster size
Fixed at 14 molecules to represent amorphous solid water; chosen rather than derived.

axioms (2)

domain assumption Density functional theory with chosen functional and dispersion corrections yields reliable relative activation barriers for H-addition and abstraction on water ice.
Invoked when stating benchmarked DFT results for the network.
domain assumption Tunneling crossover temperatures estimated from barrier heights and imaginary frequencies correctly rank reaction efficiencies at interstellar temperatures.
Used to conclude which paths dominate at low T.

pith-pipeline@v0.9.0 · 5581 in / 1355 out tokens · 86751 ms · 2026-05-10T17:35:18.911557+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

2 extracted references · 2 canonical work pages

[1]

not an answer

Ásgeirsson, V ., Birgisson, B. O., Bjornsson, R., et al. 2021, J. Chem. Theory Comput., 17, 4929 Baiano, C., Lupi, J., Barone, V ., & Tasinato, N. 2022, Journal of Chemical The- ory and Computation, 18, 3111 Basalgète, R., Ocaña, A. J., Féraud, G., et al. 2021, Astrophys. J., 922, 213 Becker, S., Feldmann, J., Wiedemann, S., et al. 2019, Science, 366, 76 ...

work page 2021
[2]

Depending on the relative position of the incoming H atom with respect to the – NH2 group of methylamine, two different activation barriers are obtained. When the H atom resides directly beneath the – NH 2 group, the interaction of the amine moiety with the ice surface is weakened, destabilizing the reactant complex and resulting in a lower barrier (9.9 k...

work page 2026

[1] [1]

not an answer

Ásgeirsson, V ., Birgisson, B. O., Bjornsson, R., et al. 2021, J. Chem. Theory Comput., 17, 4929 Baiano, C., Lupi, J., Barone, V ., & Tasinato, N. 2022, Journal of Chemical The- ory and Computation, 18, 3111 Basalgète, R., Ocaña, A. J., Féraud, G., et al. 2021, Astrophys. J., 922, 213 Becker, S., Feldmann, J., Wiedemann, S., et al. 2019, Science, 366, 76 ...

work page 2021

[2] [2]

Depending on the relative position of the incoming H atom with respect to the – NH2 group of methylamine, two different activation barriers are obtained. When the H atom resides directly beneath the – NH 2 group, the interaction of the amine moiety with the ice surface is weakened, destabilizing the reactant complex and resulting in a lower barrier (9.9 k...

work page 2026