How Certain Molecules Speed Up Hydrogen Transfer in Air Pollution Reactions

Jim Crocker
30th January, 2024

How Certain Molecules Speed Up Hydrogen Transfer in Air Pollution Reactions

Aldehyde general structure

Atmospheric aldehydes – chemicals released from sources like vehicle exhaust and plant life – play a significant role in air quality and climate. They contribute to the formation of tiny particles in the air, known as aerosols, which affect human health, visibility, and cloud formation. Understanding how these aldehydes react in the atmosphere is therefore crucial. A key aspect of these reactions is how quickly they occur, which is determined by ‘energy barriers’ – essentially, the amount of energy needed to kickstart the reaction. Researchers at Tomsk State University have recently investigated these energy barriers in detail[1], aiming to clarify the factors that control how aldehydes transform in the atmosphere. The study focused on acetaldehyde and glyoxal, two common atmospheric aldehydes, and their reactions with ammonia in water droplets – a common occurrence in clouds and fog. The researchers used complex computer simulations, specifically ‘quantum chemical calculations’, to model these reactions at a molecular level. These calculations predict the energy required for each step of a reaction, allowing scientists to understand which steps are slow (high energy barrier) and which are fast (low energy barrier). A central finding was that the size of the molecular structure formed during the reaction – the ‘transition state’ – significantly impacts the energy barrier. Transition states are fleeting, unstable arrangements of atoms that occur as reactants transform into products. The simulations showed that as the transition state grew from a 4-membered ring to a 6-membered, and then to an 8-membered ring structure, the energy barrier consistently decreased. This means that larger transition states facilitate faster reactions. Specifically, 8-membered transition states were found to be the most efficient. The study also examined the role of ammonia itself, acting as a catalyst – a substance that speeds up a reaction without being consumed. While the size of the transition state was the dominant factor, the researchers found that the specific properties of ammonia (its acidity or basicity) also played a role, though a secondary one. This influence varied depending on the specific reaction step. Interestingly, one particular reaction step – ‘intramolecular hydrogen transfer’ – behaved differently. This involves the movement of a hydrogen atom within a molecule. Unlike the other reactions, the energy barrier didn’t consistently decrease with increasing transition state size. Instead, a significant drop in the barrier was only observed when a 7-membered transition state formed. Smaller structures (5-membered) were actually less efficient than the simplest 3-membered structures. These findings build upon previous research into how aldehydes form atmospheric particles. For example, studies have shown that aldehydes don’t directly contribute to new particle formation, but their reaction products – formed through processes like hydration and aldol condensation – can stabilize sulfuric acid, a key component of these particles[2]. The current study helps explain how these reactions occur, focusing on the underlying energy barriers. Furthermore, the importance of water in these atmospheric processes is highlighted by earlier work[3], which demonstrated that imidazoles, compounds formed from aldehyde reactions, are more abundant in humid environments like fog. The research from Tomsk State University provides a mechanistic understanding of why these reactions are accelerated under high humidity – the presence of water facilitates the formation of the larger, more efficient transition states. Additionally, research has shown that formic acid can also catalyze the hydrolysis of formaldehyde, lowering the energy required for this reaction[4][5], and this new study provides a framework for understanding how catalysts influence energy barriers in similar aldehyde reactions. By identifying the key factors controlling energy barriers in aldehyde reactions, this research provides a more detailed understanding of atmospheric chemistry and could ultimately improve models used to predict air quality and climate change.

EnvironmentBiochemEcology

References

Main Study

1) Nature or number of species in a transition state: the key role of catalytically active molecules in hydrogen transfer stages in atmospheric aldehyde reactions.

Published 30th January, 2024

https://doi.org/10.1039/d3cp04500e

Related Studies

2) A density functional theory study of aldehydes and their atmospheric products participating in nucleation.

https://doi.org/10.1039/c7cp06226e

3) Insights into high concentrations of particle-bound imidazoles in the background atmosphere of southern China: Potential sources and influencing factors.

https://doi.org/10.1016/j.scitotenv.2021.150804

4) Gas phase hydrolysis of formaldehyde to form methanediol: impact of formic acid catalysis.

https://doi.org/10.1021/jp4008043

5) Theoretical studies on gas-phase reactions of sulfuric acid catalyzed hydrolysis of formaldehyde and formaldehyde with sulfuric acid and H2SO4···H2O complex.

https://doi.org/10.1021/jp312844z


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