Quantum tunnelling can let some reactions proceed even when cold conditions leave molecules with too little thermal energy to cross an activation barrier. But a striking low-temperature result for hydroxyl radicals and methanol was measured in the gas phase—not on ice—so it demonstrates a possible mechanism, not a general rate for reactions on icy dust grains.
Why chemistry can happen when it is cold
A reaction with an activation barrier usually needs reactants to gain enough energy to get over that barrier. Cooling reduces the energy available from molecular motion, which can make such reactions slower. Quantum mechanics offers another route: a particle can tunnel through a barrier rather than cross over its top using thermal energy alone.
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Tunnelling makes some low-temperature reactions plausible; it does not make every barriered reaction fast. Whether it matters depends on the particular reaction pathway and energy landscape. The 2021 review of astrochemical models identifies activation energy as one factor that reduces reaction rates, while models also account for different chemical environments.
What the 63 K hydroxyl–methanol experiment showed
Shannon and colleagues reported a particularly clear low-temperature example in a 2013 Nature Chemistry study. For the gas-phase reaction between hydroxyl radical (OH) and methanol, the measured rate coefficient at 63 K was almost two orders of magnitude larger than in previous measurements at about 200 K. The products included the methoxy radical.
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“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.”
The authors interpreted the result as tunnelling enabled by a hydrogen-bonded intermediate complex that lasts long enough for the reaction to proceed. They proposed that this mechanism might be widespread in low-temperature interstellar environments. That is a proposed implication of a gas-phase experiment, not a measurement of the same reaction on an ice surface.
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How that result relates to icy dust grains
Interstellar dust grains collect icy mantles, mainly water ice, with volatile molecules such as carbon monoxide (CO), ammonia (NH3), carbon dioxide (CO2), methane (CH4) and methanol (CH3OH). Molecules can accrete onto grains and react there, making surface chemistry important to astrochemistry. The composition and physical setting of the ice affect which pathways are available.
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Laboratory experiments use controlled ice analogues to investigate those pathways. Reviews describe routes that form formaldehyde, methanol, water and carbon dioxide, including reactions in which hydrogen atoms add to molecules on ice. These results support low-temperature surface synthesis, but they do not show that every proposed elementary reaction is equally efficient.
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CO hydrogenation and methanol
One important surface pathway involves successive hydrogen-atom additions to CO, leading to formaldehyde and methanol. A 2025 review describes surface hydrogenation of CO as the primary route by which methanol forms in the interstellar medium, and identifies methanol as its most abundant complex organic molecule. This is evidence about surface chemistry and its role in astrochemical pathways—not a surface-specific tunnelling rate for the OH + methanol reaction.
Other proposed ice reactions
Computational work has examined reactions between energetic gas-phase cations and icy mantles. Cluster calculations describe possible barrierless reactions and discuss C+ reacting with methanol or formic acid to produce possible organic precursors. These are calculation-led mechanistic proposals; the authors emphasize the need for experimental confirmation.
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What each kind of evidence can establish
Cold-surface chemistry is studied through complementary approaches. A measured rate coefficient, a laboratory ice product and a calculated reaction pathway answer different questions, so they should not be treated as interchangeable evidence.
| Setting or method | What it can establish | What it does not establish by itself |
|---|---|---|
| Gas-phase laboratory measurement | For OH + methanol, the rate-coefficient comparison at 63 K and about 200 K, and a result interpreted through a hydrogen-bonded complex (Shannon et al., 2013). | A reaction rate on an ice surface, or the rate of unrelated surface reactions. |
| Laboratory ice analogue | Whether controlled ice experiments produce molecules such as formaldehyde, methanol, water and carbon dioxide through tested pathways (2019 review). | That every proposed reaction step is efficient in interstellar conditions. |
| Astronomical observation | Constraints on which molecules exist in astronomical environments (2019 review). | A complete account of how each observed molecule formed. |
| Astrochemical model | How proposed pathways combine to describe chemistry in an interstellar environment; some gas-grain models distinguish surface and bulk-ice phases (2021 review). | Direct experimental confirmation of each reaction or its rate. |
| Quantum-chemical calculation | Possible molecular structures and reaction-energy profiles at atomic scale (2019 review). | Experimental confirmation that a proposed pathway occurs at a particular rate. |
What the evidence says about tunnelling on cold surfaces
The gas-phase OH + methanol result shows that a barrier does not necessarily prevent a reaction from becoming faster at low temperature; the authors’ explanation invokes tunnelling through a sufficiently long-lived intermediate. Separately, laboratory ice-analogue studies support the formation of several molecules through surface chemistry, and calculations propose additional pathways. Together, these lines of evidence make tunnelling and cold-surface reactions important to investigate, but they do not provide a representative measured tunnelling rate for a particular interstellar ice-surface reaction.
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