A computational study of the gas-phase pyruvic acid decomposition: Potential energy surfaces, temporal dependence, and rates

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.