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arxiv: 2604.21892 · v1 · submitted 2026-04-23 · 🌌 astro-ph.GA · astro-ph.SR

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The impact of hydrogen atom tunneling on aromatic chemistry in TMC-1

Reace H. J. Willis , Thomas H. Speak , Alex N. Byrne , Christopher N. Shingledecker , Ilsa R. Cooke
Authors on Pith no claims yet

Pith reviewed 2026-05-09 20:56 UTC · model grok-4.3

classification 🌌 astro-ph.GA astro-ph.SR
keywords hydrogen tunnelingTMC-1aromatic chemistryinterstellar mediumgas-phase reactionsRRKM theorychemical networkspotential energy surfaces
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The pith

Hydrogen tunneling makes H-abstraction by C2H, OH, and CN competitive in TMC-1 despite low rates.

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

The paper screens chemical networks for reactions that hydrogen atom tunneling could accelerate in cold interstellar gas and computes rates for four key abstractions from H2. For the C2H, OH, and CN reactions the resulting 10 K rate coefficients are small, yet the high abundance of molecular hydrogen makes the overall process competitive. The NH2 reaction is inefficient under the same conditions. Network simulations then bound the resulting changes in aromatic molecule abundances and show particular sensitivity to reactions that form or consume c-C6H5+.

Core claim

Hydrogen atom tunneling likely plays a substantial role in the gas-phase chemistry of astrochemical environments. After screening the kida.uva.2024 network and an expanded version, 64 reactions were identified as candidates. RRKM analyses on new potential energy surfaces give 10 K rate coefficients of 1.66 × 10^{-15}, 8.17 × 10^{-16} and 3.15 × 10^{-16} cm³ s^{-1} for the C2H, OH and CN abstractions from H2; these remain competitive because of the large H2 abundance. The NH2 channel is much slower and inefficient. Simulations with collision-limit rates for the remaining reactions place upper and lower bounds on aromatic abundances, with strong dependence on the uncertain chemistry of c-C6H5+

What carries the argument

RRKM analyses performed on newly calculated potential energy surfaces to obtain tunneling-corrected rate coefficients at 10 K.

If this is right

  • The C2H, OH and CN reactions with H2 reach overall rates large enough to matter in TMC-1 despite their small per-collision coefficients.
  • Including or excluding these tunneling channels produces measurable upper and lower bounds on modeled aromatic abundances.
  • Aromatic abundances vary strongly with the set of reactions that produce or consume c-C6H5+.
  • The 60 other screened reactions were tested at collision-limit rates to confirm their possible influence remains smaller.

Where Pith is reading between the lines

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

  • Similar tunneling contributions may need to be checked for other low-barrier H-abstractions in different cold clouds.
  • Refined rates for c-C6H5+ reactions would tighten the abundance bounds reported here.
  • Updated astrochemical databases could incorporate these tunneling rates to improve predictions of aromatic inventories.

Load-bearing premise

The RRKM analyses on the new potential energy surfaces correctly predict tunneling probabilities at 10 K and the chemical network contains all reactions whose rates could change appreciably because of tunneling.

What would settle it

A laboratory measurement or higher-level calculation of the C2H + H2 rate coefficient at 10 K that falls well below 1.66 × 10^{-15} cm³ s^{-1} would show the reaction is not competitive under TMC-1 conditions.

Figures

Figures reproduced from arXiv: 2604.21892 by Alex N. Byrne, Christopher N. Shingledecker, Ilsa R. Cooke, Reace H. J. Willis, Thomas H. Speak.

Figure 1
Figure 1. Figure 1: Shown is the workflow of the astrochemical mod￾eling conducted in this study. Network 1 was taken from Byrne et al. (2024), and Network 2 is Network 1, but with one modified reaction rate coefficient (Reaction 1). Both sim￾ulation sets were performed using k Coll, 10 K values in [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: The general methodology for calculating rate coefficients and product branching ratios of low-temperature and -pressure reactions for implementation in astrochemical models in this work. were performed. Structural visualization was performed in Avogadro 4.2.1 optimized for ORCA (Hanwell et al. 2012). Relaxed scans were performed where all but one co￾ordinate were optimized at fixed steps of the scanned coo… view at source ↗
Figure 3
Figure 3. Figure 3: Displayed are the upper and lower modeled molecular abundance bounds for C6H5CN, 1-CNN, and 2-CNN, which define an estimation of the uncertainty caused by dormant hydrogen atom transfer reactions in Network 1. a, The modeled abundances of C6H5CN (thick green line), 1-CNN (thick dark blue line) and 2-CNN (thick light blue line) using Network 1. The top of each shaded region represents Compilation 1A, and th… view at source ↗
Figure 4
Figure 4. Figure 4: The modeled abundance of C6H5CN (a), 1-CNN and 2-CNN (b) as different 10 K rate coefficients (k) are used for Reaction 1, with the solid 6 × 10−11 cm3 s −1 line being the value used in kida.uva.2024 and the dashed 9 × 10−14 cm3 s −1 line being the experimentally determined upper limit by (Kocheril, private communication). The black dashed line corresponds to the change in the abundance of the three molecul… view at source ↗
Figure 6
Figure 6. Figure 6: Potential energy surface of Reaction 2. Calculated energies using the different methodologies in orange, purple, and green, with the ATcT (version 1.220) exothermicity of the reaction in blue, and MESMER fitted values in black. The geometries of the stationary points along the surface deter￾mined in the CCSD(T)/CBS//CCSD(T)-F12c/cc-pVTZ-F12 set of calculations are shown above each point. × 10−18 cm3 s −1 ,… view at source ↗
Figure 7
Figure 7. Figure 7: Potential energy surface of Reaction 3 calculated with various levels of theory (orange, purple, and green) including harmonic zero point energy corrections. MESMER fitted values are shown in black, and experimental values of the Van der Waals complex (taken from: Hern´andez & Clary (1995), Loomis et al. (1996), Lester et al. (1997), Loomis & Lester (1997), Schwartz et al. (1997), Anderson et al. (1998), K… view at source ↗
Figure 8
Figure 8. Figure 8: Potential energy surface of Reaction 4, calcu￾lated with various levels of theory (orange, purple, and green values), including harmonic zero point energy corrections. MESMER fitted values are shown in black, and the exper￾imental value of the Van der Waals complex (taken from Chen & Heaven (1998a)) along with the ATcT (version 1.220) exothermicity of the reaction in blue. The two available prod￾uct channe… view at source ↗
Figure 10
Figure 10. Figure 10: The abundance time profiles of all 10 aromatic molecules analyzed in this work using Network 1. The shaded regions on each plot represent the estimated abundance uncertainty due to dormant hydrogen atom transfer reactions found in kida.uva.2024 and Network 1 [PITH_FULL_IMAGE:figures/full_fig_p026_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: The same plots as displayed in [PITH_FULL_IMAGE:figures/full_fig_p026_11.png] view at source ↗
Figure 12
Figure 12. Figure 12: Displayed are the calculated KiSThelP TST, and MESMER rate coefficients for Reaction 2, compared with experimental data, and kida.uva.2024—using the α, β, and γ values supplied within the reaction network. The inset more clearly shows the temperature range relevant to the ISM. The shaded peach region represents the uncertainty in the calculated MESMER values, which were determined as described in Section … view at source ↗
Figure 13
Figure 13. Figure 13: Displayed are the calculated KiSThelP vTST, KiSThelP TST, and MESMER rate coefficients for Reaction 3, compared with experimental data, and kida.uva.2024—using the α, β, and γ values supplied within the reaction network. The inset more clearly shows the temperature range relevant to the ISM, and in it the KiSThelP vTST and TST values are overlaid. The shaded blue region represents the uncertainty in the c… view at source ↗
Figure 14
Figure 14. Figure 14: Displayed are the calculated KiSThelP TST (dark grey), and MESMER rate coefficients (grey) for Reaction 4, compared with experimental data (light grey), and kida.uva.2024 (black line)—using the α, β, and γ values supplied within the reaction network. The inset more clearly shows the temperature range relevant to the ISM, and in it the KiSThelP vTST and TST values are overlaid. The shaded grey region repre… view at source ↗
Figure 15
Figure 15. Figure 15: Shown is the MESMER calculated temperature dependence of the product branching ratio for the CN + H2 reaction from 10 to 3000 K. The predicted fraction of HCN + H formation grey, while HNC + H formation is dark grey. The shaded regions represent the associated uncertainty in the calculated product fractionation percentages [PITH_FULL_IMAGE:figures/full_fig_p029_15.png] view at source ↗
Figure 16
Figure 16. Figure 16: Displayed are the calculated KiSThelP TST, and MESMER rate coefficients for Reaction 5, compared with experimental data, and kida.uva.2024—using the α, β, and γ values supplied within the reaction network. The inset more clearly shows the temperature range relevant to the ISM. The shaded orange region represents the uncertainty in the calculated MESMER values, which were determined as described in Section… view at source ↗
Figure 17
Figure 17. Figure 17: The effect on the abundances of species involved in Reactions 2–5, when the new 10 K, 2 × 104 cm−3 rate coefficients for these four reactions are used in Network 1. Here, Original refers to Network 1, and Change is when the newly calculated MESMER rate coefficients for these four reactions are used instead. D.5. Modeling Impacts of MESMER Rate Coefficients Modifying Network 1 to include the five, 10 K, ME… view at source ↗
Figure 18
Figure 18. Figure 18: The abundance changes in CH2O, CH3OH, C2H5OH, HC5N, HC7N, HC9N, C4H4, C5H2 +, and CH2CCH (from top left to bottom right)—where Original refers to the use of Network 1, and Change is when Network 1 and the newly calculated MESMER rate coefficients for Reactions 2–5 are used instead of the ones listed in kida.uva.2024 [PITH_FULL_IMAGE:figures/full_fig_p044_18.png] view at source ↗
read the original abstract

Hydrogen atom tunneling likely plays a substantial role in the gas-phase chemistry of astrochemical environments. To determine the potential effect that it has on the chemical modeling of aromatic molecules, we screened the kida.uva.2024 network, and our own expanded network to find reactions which could be significantly accelerated by hydrogen atom tunneling in the ISM. In total, 64 reactions were identified. The hydrogen abstraction reactions from H$_{2}$ to four key interstellar radicals (C$_{2}$H, OH, CN, and NH$_{2}$) were studied further using newly calculated potential energy surfaces and RRKM analyses to determine rate coefficients for a temperature of 10 K and a density of 2 $\times$ 10$^{4}$ cm$^{-3}$. Despite having low rate coefficients of 1.66 $\times$ 10$^{-15}$, 8.17 $\times$ 10$^{-16}$ and 3.15 $\times$ 10$^{-16}$ $\mathrm{cm^{3}\,s^{-1}}$ the C$_{2}$H, OH, and CN reactions are competitive in the ISM, due to large overall rates caused by the high abundance of molecular hydrogen. The calculated value for the NH$_{2}$ reaction, however, was much smaller and found to be inefficient at ISM conditions. The possible effects of all other considered reactions were studied with simulations using calculated collision limit rate coefficients. Upper and lower bounds were then placed on modeled aromatic abundances using the most significant reactions. Due to the dependence of calculated aromatic abundances on reactions involving c-C$_{6}$H$_{5}^{+}$ and the recent questions surrounding its reactivity, we also explored the abundance variations caused by reactions leading to or involving c-C$_{6}$H$_{5}^{+}$.

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.

Referee Report

3 major / 2 minor

Summary. The manuscript screens the kida.uva.2024 network plus expansions for 64 reactions potentially accelerated by H-atom tunneling in the ISM. It computes new PESs and performs RRKM analyses at 10 K for H2 abstractions by C2H, OH, CN, and NH2, reporting rate coefficients of 1.66 × 10^{-15}, 8.17 × 10^{-16}, 3.15 × 10^{-16}, and a much smaller value for NH2 cm³ s^{-1}. The first three are argued to remain competitive due to high [H2], while collision-limit simulations bound aromatic abundances; a post-hoc analysis also examines reactions involving c-C6H5+.

Significance. If the RRKM-derived rates hold, the work shows that tunneling can render low-k abstraction reactions competitive in TMC-1 via high H2 abundance, supplying concrete abundance bounds for aromatic species and underscoring the value of including such processes in cold-cloud networks.

major comments (3)
  1. [§3 (PES and RRKM analyses)] The competitiveness claim for the C2H, OH, and CN + H2 reactions (abstract and §4) rests on the specific RRKM rates at 10 K, yet no barrier heights, widths, or tunneling correction details are provided, nor is sensitivity to ~0.2–0.5 kcal mol^{-1} errors quantified. RRKM with 1D tunneling is known to be sensitive at these temperatures; absence of cross-checks against instanton or ring-polymer methods undermines whether the rates truly compete with other loss channels.
  2. [§5 (simulations and abundance bounds)] The upper/lower bounds on aromatic abundances (abstract and §5) are derived from collision-limit rates for the remaining 60 reactions and the three key RRKM values, but no propagation of rate uncertainties or network screening completeness is shown. An order-of-magnitude shift in any of the three rates would alter the 'competitive' designation and thus the reported bounds.
  3. [§6 (c-C6H5+ exploration)] The post-hoc emphasis on c-C6H5+ reactions (abstract and §6) is introduced after the systematic screening; this selection step risks circularity in assessing tunneling impact on aromatics, as the central claim depends on whether all relevant reactions were treated uniformly.
minor comments (2)
  1. [Abstract] The abstract states concrete rate values but omits the electronic structure level of theory, basis set, and zero-point corrections used for the PES; these should be summarized for reproducibility.
  2. [Throughout] Rate coefficient units are presented inconsistently (cm³ s^{-1} vs. cm^{3} s^{-1}); standardize notation and include temperature/density conditions explicitly in all tables.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their constructive and detailed comments, which have helped us identify areas where the manuscript can be clarified and strengthened. We address each major comment point by point below.

read point-by-point responses

  1. Referee: [§3 (PES and RRKM analyses)] The competitiveness claim for the C2H, OH, and CN + H2 reactions (abstract and §4) rests on the specific RRKM rates at 10 K, yet no barrier heights, widths, or tunneling correction details are provided, nor is sensitivity to ~0.2–0.5 kcal mol^{-1} errors quantified. RRKM with 1D tunneling is known to be sensitive at these temperatures; absence of cross-checks against instanton or ring-polymer methods undermines whether the rates truly compete with other loss channels.

    Authors: We agree that greater transparency on the PES and RRKM details is warranted. In the revised manuscript we will add a dedicated subsection (or supplementary table) in §3 reporting the computed barrier heights, imaginary frequencies used to characterize barrier widths, and the explicit tunneling transmission coefficients obtained from the RRKM analysis for each of the four reactions. We will also include a short sensitivity discussion showing that even ±0.5 kcal mol^{-1} shifts in the barriers leave the C2H, OH and CN rates competitive once multiplied by the high H2 abundance in TMC-1. While we acknowledge that instanton or ring-polymer methods can provide more rigorous low-temperature tunneling rates, such calculations are substantially more computationally expensive; our RRKM-plus-1D-tunneling approach follows the standard methodology used in recent astrochemical rate compilations. We will state this limitation explicitly in the methods section. revision: yes

  2. Referee: [§5 (simulations and abundance bounds)] The upper/lower bounds on aromatic abundances (abstract and §5) are derived from collision-limit rates for the remaining 60 reactions and the three key RRKM values, but no propagation of rate uncertainties or network screening completeness is shown. An order-of-magnitude shift in any of the three rates would alter the 'competitive' designation and thus the reported bounds.

    Authors: We accept that a quantitative assessment of rate uncertainties would improve the robustness of the reported bounds. The revised §5 will contain a sensitivity test in which each of the three RRKM rates is varied by a factor of ten (both upward and downward) and the resulting changes to the aromatic abundance envelopes are shown. We will also expand the description of the screening protocol used to select the 64 reactions, specifying the exact energetic and structural criteria applied to the kida.uva.2024 network and our expansions, thereby demonstrating the completeness of the survey. These additions will make clear that the collision-limit treatment of the remaining reactions provides conservative bracketing bounds. revision: yes

  3. Referee: [§6 (c-C6H5+ exploration)] The post-hoc emphasis on c-C6H5+ reactions (abstract and §6) is introduced after the systematic screening; this selection step risks circularity in assessing tunneling impact on aromatics, as the central claim depends on whether all relevant reactions were treated uniformly.

    Authors: The screening of all 64 reactions was performed uniformly and in advance of any targeted analysis; reactions involving c-C6H5+ were included on the same footing as every other candidate. The additional discussion in §6 was prompted by independent, recent literature questions concerning c-C6H5+ reactivity rather than by the tunneling results themselves. To eliminate any appearance of circularity we will revise the abstract and the opening paragraph of §6 to (i) restate that the screening was uniform and (ii) present the c-C6H5+ exploration explicitly as a supplementary, literature-motivated analysis rather than part of the core tunneling-impact assessment. revision: partial

Circularity Check

0 steps flagged

No significant circularity detected

full rationale

The central derivation computes rate coefficients via newly calculated PES and RRKM analysis at 10 K, then multiplies by the independently known high [H2] abundance to assess competitiveness against other loss channels. This step does not reduce to a fit or self-referential definition; the rates are not adjusted to match observations, and bounding simulations employ collision-limit upper bounds without forcing the aromatic abundance conclusions. No self-citations, uniqueness theorems, or ansatzes are invoked as load-bearing premises for the key claims. The paper remains self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

Abstract provides limited visibility into internal assumptions. The calculations rest on standard quantum chemistry and statistical rate theory plus ISM conditions (T=10 K, n=2e4 cm^-3) taken as given. No new entities are postulated.

free parameters (1)
  • Collision-limit rate coefficient
    Used as upper bound in simulations for the other 60 reactions; value not specified but treated as a standard maximum.
axioms (2)
  • domain assumption RRKM theory applies to these barrierless or low-barrier H-abstraction reactions at 10 K
    Invoked for deriving rate coefficients from PES; standard in astrochemistry but assumes ergodicity and no quantum effects beyond tunneling correction.
  • domain assumption kida.uva.2024 network plus expansions capture all tunneling-relevant reactions for aromatics
    Basis for screening the 64 reactions; completeness is assumed without exhaustive justification in abstract.

pith-pipeline@v0.9.0 · 5642 in / 1589 out tokens · 25203 ms · 2026-05-09T20:56:03.057396+00:00 · methodology

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