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Quantum tunnelling can let some reactions proceed even when molecules are too cold to cross an energy barrier using thermal energy alone. It helps explain why chemistry is possible in cold astronomical environments, but evidence must be read in context: the clearest low-temperature rate measurement discussed here is for a gas-phase reaction, while ice experiments and calculations address different settings and questions.
What quantum tunnelling changes in a cold reaction
A reaction may require its reactants to pass an energy barrier before products can form. As temperature falls, fewer collisions have enough thermal energy to go over that barrier, which tends to reduce reaction rates. Quantum mechanics allows a particle to tunnel through a barrier instead. The probability depends on the reaction pathway and barrier, so tunnelling can make a particular low-temperature reaction more plausible without making every barriered reaction fast.
In interstellar environments, dust grains collect icy mantles, made mainly of water ice with volatile molecules such as carbon monoxide (CO), ammonia (NH3), carbon dioxide (CO2), methane (CH4) and methanol (CH3OH). Molecules can accrete onto these grains and react in or on the ice. The surrounding ice and the available reaction pathways matter: a result for one reactant pair or physical setting cannot automatically be applied to another.
What the 63 K OH–methanol result actually shows
Shannon and colleagues reported a striking example in a gas-phase study of hydroxyl radicals (OH) reacting with methanol. Their measured rate coefficient at 63 K was almost two orders of magnitude larger than earlier measurements at about 200 K. The authors wrote: “Here we show that, despite the presence of a barrier, the rate coefficient for the reaction between the hydroxyl radical (OH) and methanol—one of the most abundant organic molecules in space—is almost two orders of magnitude larger at 63 K than previously measured at ∼200 K.”
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The proposed explanation was that OH and methanol form a hydrogen-bonded intermediate complex that lasts long enough for hydrogen tunnelling to help form products, including the methoxy radical. This is a measured gas-phase rate comparison interpreted through a proposed mechanism—not a rate measurement for molecules reacting on an icy grain. The authors suggested that the mechanism could be widespread in low-temperature interstellar environments; that suggestion should not be treated as proof that all cold surface reactions behave similarly.
What laboratory ice experiments establish
Laboratory ice-analogue studies investigate reactions in controlled ice samples intended to reproduce aspects of grain mantles. Reviews of this work describe pathways that form formaldehyde, methanol, water and carbon dioxide, including routes involving hydrogen-atom addition. They support the broader conclusion that chemistry can proceed in cold ices, but do not make every proposed elementary step equally efficient or establish one universal surface reaction rate.
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CO hydrogenation and methanol
One important sequence adds hydrogen atoms to CO on an icy surface, leading through successive reactions toward formaldehyde and methanol. A 2025 review identifies surface hydrogenation of CO as the primary formation route for methanol in the interstellar medium (ISM), where methanol is described as the most abundant complex organic molecule. This is a surface-chemistry account; it is distinct from the gas-phase OH–methanol measurement.
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Laboratory reviews also describe surface routes to water and carbon dioxide, as well as formaldehyde and methanol. These results concern specific ice-analogue conditions and reaction pathways. They should not be collapsed into a claim that every reaction in an ice mantle is efficient, or that tunnelling has been measured as the controlling mechanism for every product.
How to distinguish measurements, observations and proposals
Cold-surface chemistry is studied through several complementary methods. Each can support a different kind of claim, and none should be mistaken for another.
- Laboratory experiments test reactions in controlled ice analogues and can establish which products form under those conditions. They do not reproduce every feature of an astronomical grain.
- Astronomical observations constrain which molecules are present in space. By themselves, observations do not uniquely identify the reaction sequence that made them.
- Astrochemical models combine proposed reactions and physical conditions to estimate how abundances may evolve. Common gas–grain models distinguish surface chemistry from bulk-ice chemistry, so their phase structure affects how a pathway is represented.
- Quantum-chemical calculations can resolve molecular structures and reaction-energy profiles at atomic scale, helping to assess possible mechanisms. A calculated pathway or barrier is not itself an experimental measurement of a reaction rate.
For any claim about tunnelling, check the physical setting, the actual reactants and substrate, the reported temperature and conditions, and the quantity measured. A rate coefficient, a calculated barrier, a modeled efficiency and an observed astronomical abundance are not interchangeable evidence.
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What calculations suggest about cations on ice
Calculation-led work has examined reactions of energetic gas-phase cations with icy mantles. Cluster calculations describe some cation–ice reactions as potentially barrierless and discuss C+ reactions with methanol and formic acid that could yield organic precursors. These are mechanistic proposals based on calculations; the authors emphasize the need for experimental confirmation. They therefore add possible pathways to investigate, rather than a direct measurement of tunnelling rates on astronomical ice.
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Tunnelling provides a physically grounded way for selected reactions to proceed when thermal activation is strongly suppressed. The OH + methanol result demonstrates an unusually large low-temperature gas-phase rate coefficient and offers a proposed intermediate-complex explanation. Separately, laboratory ice analogues support formation pathways for several molecules, while models and calculations help connect controlled experiments to the more complex chemistry of interstellar grains.
The evidence described here does not establish a representative measured tunnelling rate for a specific cold ice-surface reaction. The gas-phase comparison at 63 K cannot supply that missing surface rate, and the existence of tunnelling does not show that every barriered reaction on ice is efficient.
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