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

This study investigates the impact of hydrogen atom tunneling on aromatic chemistry in the TMC-1 interstellar cloud by identifying 64 potentially accelerated reactions, determining that while most are inefficient, hydrogen abstraction from H₂ by C₂H, OH, and CN radicals remains competitive due to high molecular hydrogen abundance, and further analyzing how these processes and c-C₆H₅⁺ reactivity influence modeled aromatic abundances.

Original authors: Reace H. J. Willis, Thomas H. Speak, Alex N. Byrne, Christopher N. Shingledecker, Ilsa R. Cooke

Published 2026-04-24

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CC BY 4.0

Original authors: Reace H. J. Willis, Thomas H. Speak, Alex N. Byrne, Christopher N. Shingledecker, Ilsa R. Cooke

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, freezing cold kitchen. In this kitchen, the "chefs" are tiny atoms and molecules, and they are trying to cook up complex recipes to create the building blocks of life, like aromatic rings (which are like molecular wheels or honeycombs).

The problem? It's so cold in this cosmic kitchen (about -263°C) that the ingredients are too sluggish to mix. Usually, to get two molecules to react, they need a little "push" or energy to jump over a hurdle. In this freezing environment, they don't have enough energy to jump, so the cooking stops.

The Secret Ingredient: Quantum Tunneling
This paper is about a magical trick called Quantum Tunneling. Imagine you are trying to walk through a solid brick wall. Normally, you can't. But in the quantum world, particles are like ghosts; sometimes, instead of jumping over the wall, they simply "tunnel" through it.

The authors of this paper asked: "How many of these ghostly shortcuts are we missing in our recipes for the universe?" They found that 64 different reactions in space might be using this tunneling trick to happen much faster than we thought.

The Detective Work

The team acted like chemical detectives. They took a massive list of known space reactions (a database called KIDA) and looked for ones where a Hydrogen atom (the smallest, lightest ingredient) is being swapped. Because Hydrogen is so light, it's the best at "ghosting" through walls.

They found that many reactions we thought were too slow to matter in the cold dark clouds of space might actually be happening at a decent pace because of this tunneling.

The "Big Four" Experiments

To prove their theory, they didn't just guess; they did deep computer simulations (like running a super-advanced video game of chemistry) on four specific reactions involving Hydrogen gas ( $H_2$ ) and four famous space radicals (C2H, OH, CN, and NH2).

Think of these four reactions as the "Star Players" of the team. They calculated exactly how fast these reactions happen at 10 Kelvin (the temperature of deep space).

The Result: Even though the speed was still slow by Earth standards, it was billions of times faster than what previous models predicted.
The Catch: Because there is so much Hydrogen gas in space (it's the most common ingredient), even a "slow" reaction becomes a busy highway when you have that many cars.

Why This Matters for Aromatics

The main goal was to see how this affects aromatic molecules (like benzene or naphthalene). These are the "wheels" that might eventually turn into the complex rings needed for life.

The paper found that if we ignore quantum tunneling, our models of space chemistry are like a map with missing roads.

The "Ghost" Effect: When they added the tunneling speeds to their models, the amount of these aromatic molecules changed drastically. In some cases, the models predicted thousands of times more of these molecules than before.
The Uncertainty: It turns out our current models are very shaky. Depending on which "tunneling roads" we include, the amount of these molecules could be way too low or way too high.

The Plot Twist: The "Phenylium" Debate

There was also a subplot about a specific molecule called phenylium ( $c-C_6H_5^+$ ). Recent studies suggested this molecule might not be as good at making benzene as we thought. The authors tested this too.

If phenylium is a bad chef, the whole recipe for making space aromatics falls apart.
Their models showed that if we remove the old, inefficient ways of making these molecules, the abundance of aromatics drops significantly.

The Takeaway

This paper is a wake-up call for astrochemists. It says: "Stop assuming the cold stops everything!"

Because of quantum tunneling, the universe is actually a much busier, more active kitchen than we thought. Even in the freezing dark, atoms are finding ghostly shortcuts to build complex molecules. If we want to understand where the ingredients for life come from, we have to stop looking at the "brick walls" and start looking for the "ghost tunnels."

In short: The universe is colder than we thought, but thanks to quantum magic, the chemistry is hotter and more active than our old maps suggested.

1. Problem Statement

Astrochemical models frequently fail to reproduce the observed abundances of complex organic molecules, particularly aromatic species like cyanonaphthalenes (1- and 2-CNN) and naphthalene, in the interstellar medium (ISM). A significant source of uncertainty lies in the kinetic rate coefficients of reactions at low temperatures (10 K).

The Gap: Many reactions involving hydrogen atom transfer possess energy barriers. In standard models, these are often assumed to be negligible at ISM temperatures or are extrapolated from high-temperature data, leading to rate coefficients that are orders of magnitude too low.
The Mechanism: Quantum mechanical tunneling allows hydrogen atoms to penetrate energy barriers even at low temperatures, potentially accelerating these reactions by many orders of magnitude.
The Specific Challenge: The formation pathways of interstellar aromatics are highly sensitive to reaction rates. Recent debates regarding the reactivity of the phenylium cation ( $c\text{-C}_6\text{H}_5^+$ ) and the efficiency of specific formation routes (e.g., involving $H_2$ ) have introduced further uncertainty. The full impact of tunneling on the entire network of aromatic chemistry has not been systematically quantified.

2. Methodology

The authors employed a multi-stage approach combining reaction network screening, high-level quantum chemistry, and astrochemical modeling.

A. Reaction Screening

Networks Analyzed: The authors screened the kida.uva.2024 network and their own expanded network (Network 1, based on kida.uva.2014 with additional aromatic routes).
Criteria: They identified neutral-neutral, gas-phase reactions involving single hydrogen atom transfer that are exothermic and possess a potential energy barrier (indicated by a modified Arrhenius $\gamma$ value between 0 and $10^4$ K).
Outcome: 64 reactions were identified as candidates for significant tunneling acceleration.

B. Astrochemical Simulations (Upper/Lower Bounds)

Tool: The NAUTILUS code was used to simulate a standard dark molecular cloud (TMC-1 conditions: $T=10$ K, $n_{H_2} = 2 \times 10^4$ cm $^{-3}$ ).
Strategy: Two sets of simulations were run:
1. Set 1 (Network 1): Used standard kida.uva.2024 rates.
2. Set 2 (Network 2): Modified the rate coefficient for the reaction $c\text{-C}_6\text{H}_5^+ + H_2 \rightarrow C_6H_7^+ + h\nu$ based on recent experimental upper limits ( $9 \times 10^{-14}$ cm $^3$ s $^{-1}$ vs. the standard $6 \times 10^{-11}$ ).
Bounding: For the 64 candidate reactions, the authors replaced the standard low-temperature rates with the collision limit rate coefficients ( $k_{Coll, 10K}$ ) calculated via Classical Capture Theory (CCT). This established the theoretical upper bound of the impact of tunneling.
Compilation: Reactions were compiled into sets that either increased or decreased aromatic abundances to determine the maximum possible uncertainty range.

C. High-Level Kinetic Calculations (Ab Initio & MESMER)

Four critical reactions involving $H_2$ abstraction by radicals ( $C_2H, OH, CN, NH_2$ ) were studied in depth to obtain accurate low-temperature rates:

Potential Energy Surfaces (PES): Calculated using CCSD(T)-F12 methods with extrapolation to the Complete Basis Set (CBS) limit.
Rate Coefficients:
- KiSThelP: Used for Transition State Theory (TST) and variational TST (vTST) calculations.
- MESMER: A Master Equation Solver used to calculate pressure- and temperature-dependent rates, accounting for Van der Waals complexes and quantum tunneling via an Eckart barrier.
- Fitting: MESMER parameters were fitted to existing experimental data to validate the theoretical models before extrapolating to 10 K.

3. Key Contributions

Systematic Identification: Identified 64 reactions in standard astrochemical networks that are likely accelerated by hydrogen tunneling, a number significantly higher than previously recognized for aromatic chemistry.
Quantification of Uncertainty: Demonstrated that ignoring tunneling introduces uncertainties in modeled aromatic abundances of >9 orders of magnitude for certain species (e.g., 1-CNN) at early chemical ages.
Refined Kinetics: Provided new, rigorously calculated rate coefficients for four key $H_2$ abstraction reactions at 10 K, replacing estimates that were often $10^{40}$ – $10^{170}$ times too slow.
Network Sensitivity: Investigated the sensitivity of aromatic formation to the reactivity of $c\text{-C}_6\text{H}_5^+$ and the removal of inefficient low-temperature pathways proposed by recent literature (Loison et al., 2025).

4. Key Results

A. Impact of Tunneling on Aromatic Abundances

Upper Bounds: When using collision-limit rates (assuming maximum tunneling efficiency), the abundances of key aromatics increased dramatically.
- 1-CNN and 2-CNN: Abundances could be $10^5$ to $10^6$ times higher than standard model predictions at chemical ages of $\sim 10^4$ – $10^5$ years.
- Benzene ( $C_6H_6$ ): Showed increases of up to 100–200 times.
Dominant Reactions: Reactions involving the saturation of carbon chains ( $C_x + H_2$ ) and the formation of methane/acetylene/ethylene were the primary drivers of these increases. Specifically, reaction #36 ( $H_2 + C_4 \rightarrow H + C_4H$ ) caused the most significant single increase (~3200 $\times$ for 1-CNN in Network 2).
Competitiveness: Despite calculated rate coefficients being small (e.g., $10^{-15}$ to $10^{-16}$ cm $^3$ s $^{-1}$ ), the reactions are competitive in the ISM due to the high abundance of molecular hydrogen ( $H_2$ ).

B. Detailed Kinetic Results (Reactions 2–5)

The study calculated specific 10 K rate coefficients ( $k$ ) for four reactions, comparing them to standard KIDA values:

$C_2H + H_2 \rightarrow C_2H_2 + H$ :
- $k_{10K} \approx 1.66 \times 10^{-15}$ cm $^3$ s $^{-1}$ .
- ~53 orders of magnitude faster than the KIDA value ( $3.97 \times 10^{-68}$ ).
$OH + H_2 \rightarrow H_2O + H$ :
- $k_{10K} \approx 8.17 \times 10^{-16}$ cm $^3$ s $^{-1}$ .
- ~42 orders of magnitude faster than KIDA.
$CN + H_2 \rightarrow HCN/HNC + H$ :
- Total $k_{10K} \approx 3.15 \times 10^{-16}$ cm $^3$ s $^{-1}$ .
- ~36 orders of magnitude faster than KIDA.
- Note: Tunneling opens a minor channel for HNC formation (~4% branching), which was previously unaccounted for.
$NH_2 + H_2 \rightarrow NH_3 + H$ :
- $k_{10K} \approx 5.47 \times 10^{-29}$ cm $^3$ s $^{-1}$ .
- While still ~144 orders of magnitude faster than KIDA, the absolute rate is too low to be competitive in the ISM due to a wide, high barrier.

C. Sensitivity to $c\text{-C}_6\text{H}_5^+$ Reactivity

Reducing the rate of $c\text{-C}_6\text{H}_5^+ + H_2$ (based on Kocheril et al., 2025) increased $c\text{-C}_6\text{H}_5^+$ abundance by 10–100 $\times$ but decreased all other aromatic abundances by 3–10 $\times$ .
Removing reactions proposed as inefficient by Loison et al. (2025) (e.g., $C_2H_2 + C_4H_3^+$ ) further decreased aromatic abundances by 4–20 $\times$ .

5. Significance

Resolving Discrepancies: The study suggests that the long-standing under-prediction of aromatic molecules in astrochemical models may be partially resolved by including hydrogen tunneling. The "dormant" reactions identified here, if active, could bridge the gap between model predictions and observations in TMC-1.
Methodological Shift: It highlights the failure of standard sensitivity analyses (which vary rates by small factors) to detect critical reactions where the true uncertainty spans many orders of magnitude.
Future Directions: The authors argue that future chemical networks must prioritize calculating accurate low-temperature rates for $H_2$ abstraction reactions, particularly those involving abundant radicals and $H_2$ . The provided rate coefficients and the list of 64 candidate reactions serve as a roadmap for refining interstellar chemistry models.
Broader Implication: While focused on aromatics, the findings imply that tunneling effects likely influence the abundances of many other molecular classes in the ISM, necessitating a re-evaluation of current astrochemical databases.