Water-Catalyzed Reactions in Atmospheric Chemistry

Water vapour and microscopic clusters of water molecules play a pivotal role in modulating gas‐phase processes that govern air quality, pollutant lifetimes and secondary aerosol formation. By forming hydrogen‐bonded adducts with trace gases or radical species, water can act as a proton shuttle or stabilise transition states, thereby reducing activation barriers for key reactions. Such water-catalysed pathways include proton-coupled electron transfer, nucleophilic substitution and radical recombination. In the troposphere, where relative humidity varies widely, single water molecules or small water clusters may dramatically alter the kinetics of hydroxyl-driven oxidation, the removal of anthropogenic greenhouse gases and the formation of organic imines. The net effect of water assistance is highly context-dependent: in some cases it accelerates pollutant degradation, whilst in others it can hinder reaction progress by stabilising reactant complexes. Understanding these processes at the molecular level is essential for accurate modelling of atmospheric chemistry and for devising strategies to mitigate climate-relevant emissions.

Research from Nature Portfolio

Computational investigations of methylenimine hydration have mapped the potential energy surface for CH2NH + H2O and CH2NH + 2H2O reactions. These studies reveal that although a weakly bound CH2NH⋯H2O complex forms readily, its atmospheric lifetime is too short for significant generation of formaldehyde and ammonia under typical conditions.

Variational transition state analyses of the OH + CH3OH reaction with a single water molecule demonstrate that water assistance shifts the transition state geometry and lowers the intrinsic barrier. However, under tropospheric humidity the overall catalytic effect remains negligible, owing to the low concentration of water-bound complexes relative to the free reaction pathway.

High-level quantum chemical and kinetic modelling of thiophene oxidation by hydroperoxyl radicals shows that introduction of one water molecule leads to ring-like addition complexes and decreases relative energies of all addition channels. Rice–Ramsperger–Kassel–Marcus simulations indicate that water-induced stabilisation of initial HO2–thiophene complexes competes with transition state interactions to modify the rates of aromatic oxidation under humid conditions.

Water-Catalyzed Reactions in Atmospheric Chemistry publication trend

The graph below shows the total number of articles in water-catalyzed reactions in atmospheric chemistry across all publications each year (not limited to Nature Index journals).

Technical terms

Hydrogen bond: A noncovalent interaction between a hydrogen atom bound to an electronegative donor and an acceptor atom, stabilising intermediate complexes.

Transition state: The highest energy configuration along a reaction coordinate that must be surmounted for reactants to convert into products.

Potential energy surface: A multidimensional representation of system energy as a function of nuclear coordinates, used to identify minima and transition states.

Radical: A reactive species possessing an unpaired electron, often initiating chain reactions in the atmosphere.

Activation barrier: The energy difference between reactants and the transition state, determining the intrinsic rate of a chemical reaction.

References

1. Computational studies on the gas phase reaction of methylenimine (CH2NH) with water molecules. Scientific Reports (2020).
2. Effect of ammonia and water molecule on OH + CH3OH reaction under tropospheric condition. Scientific Reports (2021).
3. Impact of a single water molecule on the atmospheric oxidation of thiophene by hydroperoxyl radical. Scientific Reports (2022).
4. Unveiling the Influence of Water Molecules for NF3 Removal by the Reaction of NF3 with OH: A DFT Study. Molecules (2024).
5. Can a single ammonia and water molecule enhance the formation of methanimine under tropospheric conditions?: kinetics of •CH2NH2 + O2 (+NH3/H2O). Frontiers in Chemistry (2023).