Abstract
Pyruvic acid (PA) is a key intermediate in keto-acid chemistry and plays an integral part in atmospheric chemistry. However, there is still a lack of fundamental mechanistic understanding of the PA degradation processes. Here, we show the gas-phase PA degradation energetics, temporal dependence, and rates and compare with the hydration of PA and decomposition of methylglyoxal (MGY). The acetaldehyde production, via PA decarboxylation, was found to be dominant over acetic acid production. We confirmed the isomerization to enol and lactone forms and the roles of intermediates, methylhydroxycarbene (MHC)–CO2 and vinyl alcohol. We characterized additional pathways with their energy barrier represented in parentheses: the direct acetic acid conversion (54.21 kcal/mol), MHC–CO2 to acetaldehyde (30.82 kcal/mol), and MHC–CO2 to vinyl alcohol (23.80 kcal/mol). The calculated PA decomposition rates at 400 K–1000 K and 1 atm agree with the previous gas-phase experiments. The unsymmetrical Eckart tunneling is significant in 2,2-dihydroxypropionic acid (DHPA) and DHPA–H2O formation and MGY production, resulting in increased rates for DHPA formation. This implies a competition between PA decomposition and hydration in atmospheric conditions and a strong water concentration and temperature dependence.
Highlights
Pyruvic acid (PA) plays an integral part in atmospheric chemistry
We found that vinyl alcohol can tautomerize to acetaldehyde, so the gas-phase PA degradation generates only two sets of final products, acetaldehyde + CO2 and acetic acid + CO, consistent with experimental findings
We characterized additional pathways with their barrier represented in parentheses: the direct acetic acid conversion (54.21 kcal/mol), MHC–CO2 to acetaldehyde (30.82 kcal/mol), and MHC–CO2 to vinyl alcohol (23.80 kcal/mol)
Summary
Pyruvic acid (PA) plays an integral part in atmospheric chemistry. Its presence has been reported in gas phase, aerosols, and rainwater in the urban atmosphere, forested, and marine areas.. The oxidation of isoprene is one of the primary sources of PA.. Methylglyoxal (MGY) is one of the primary oxidation products of isoprene and other aromatic compounds.. Methylglyoxal (MGY) is one of the primary oxidation products of isoprene and other aromatic compounds.4–6 Another PA source in the atmosphere is the reaction of hydroxyl (OH) radical and hydrated MGY in the air–aqueous boundary, which primarily happens in clouds and aerosols. PA is a key intermediate in the keto-acid reactions and serves as a prototype for understanding other keto-acids. A fundamental mechanistic understanding of the PA degradation process is desirable
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