Reactive Decarboxylation of Atmospheric Acids: A Theoretical Study.

Area of Science:

• Atmospheric Chemistry
• Physical Chemistry
• Computational Chemistry

Background:

• Carboxylic acids are prevalent in the atmosphere, existing in both gas and particle phases.
• Their degradation is significantly influenced by reactions with hydroxyl (OH) radicals.

Purpose of the Study:

• To investigate the gas-phase reaction mechanisms of OH radicals with various carboxylic acids.
• To determine the energetics of the decarboxylation pathway following OH radical addition.

Main Methods:

• Quantum chemistry calculations were performed using high-level theoretical methods.
• Specific methods include ωB97X-V/def2-TZVPPD, ωB97M-V/def2-TZVPPD, CCSD(T)/6-311+G(2df,2p), and CCSD(T)/aug-cc-pVTZ.
• Focus on calculating reaction energetics for decarboxylation of formic, acetic, malonic, fumaric, maleic, trifluoroacetic, and trichloro-acetic acids.

Main Results:

• All investigated OH radical reactions with carboxylic acids are exoergic.
• A hydrogen-bonded complex with a 6-membered ring structure forms, followed by decarboxylation with minimal energy barriers.
• Energetics for trifluoroacetic acid decarboxylation pathways leading to oxidation products were calculated.

Conclusions:

• The study elucidates the low-barrier decarboxylation mechanism of atmospheric carboxylic acids reacting with OH radicals.
• Findings provide critical data for atmospheric models and understanding the fate of these compounds.
• Theoretical results are compared with existing experimental and theoretical data for validation.