Reactive Decarboxylation of Atmospheric Acids: A Theoretical Study

Carboxylic acids are ubiquitous in the atmosphere in both the gas and particle phase. One of their major degradation pathways is known to be their reaction with OH radicals. To enhance our understanding of carboxylic acid reactivity, we investigated the gas phase reactions of OH radicals with a series of these acids by performing quantum chemistry calculations at the ωB97X-V/def2-TZVPPD, ωB97M-V/def2-TZVPPD, CCSD(T)/6-311+G(2df,2p), and CCSD(T)/aug-cc-pVTZ level. Our calculations are mainly focused on the reactive decarboxylation pathway energetics that occur following the OH-driven abstraction of the carboxylic hydrogen of formic, acetic, malonic, fumaric, maleic, trifluoroacetic, and trichloro-acetic acids. All the reactions are exoergic, and in all cases, we find that a hydrogen-bonded complex with a 6-membered ring structure is formed between the acid group and the OH, followed by reactive decarboxylation with small or nonexistent barriers to both abstraction and decarboxylation steps. Finally, in the case of trifluoroacetic acid decarboxylation, we calculate the energetics of several pathways that may lead to stable oxidation product formation, and we compare our theoretical work to other experimental and theoretical studies.