Role of hydrogen bonding in bulk aqueous phase decomposition, complexation, and covalent hydration of pyruvic acid

Pyruvic acid (PA) is a model for amphiphilic oxygenated organic compounds, and together with its hydrogen-bonded (H-bonded) water complexes, their presence can alter atmospheric aerosol formation. However, the fundamental understanding of PA reaction mechanisms in different environments is still being debated. Here, the role of H-bonding on PA's degradation, complexation, and covalent hydration in bulk aqueous phase is investigated theoretically. Using CCSD(T)-F12/aug-cc-pVDZ-F12 on B2PLYP-D3BJ structures with solvation model based on density, we revealed the stabilization by intramolecular H-bonding of an intermediate, PA hydrogen-transferred tautomer, altered the PA degradation mechanisms compared to gas phase. We also found that the intramolecular H-bonding in the most stable gas phase conformer (Tc) is weakened due to bulk solvation, leading to slower acetaldehyde production rate. Natural bond orbital analysis characterized the primary intermolecular H-bond in PA-water complexes as electron donation of an O-water lone pair (p) to the sigma* orbital of the OH group of PA. Stronger H-bonding is correlated to p to sigma* interaction, wider OH-O angles, and larger differences in the H-bond lengths between phases. The less charge difference between phases on H-bonded atoms also indicates aggressive competition of H-bonding with solvation. Water's cooperative behavior was observed by lowering the water-complexed 2,2-dihydroxypropanoic acid (DHPA-H2O) barrier from PA-water complexes compared to DHPA in both phases, stabilizing the transition state and product with intermolecular H-bonding. PA is vital in atmospheric keto-acid chemistry; thus, changes in PA reaction mechanisms in different environments due to H-bond behavior will affect aerosol formation.

Automated summary

The role of hydrogen bonding in the degradation, complexation, and covalent hydration of pyruvic acid in aqueous phase was investigated. The study revealed that intramolecular hydrogen bonding alters the degradation mechanisms of pyruvic acid compared to gas phase. The hydrogen bonding between pyruvic acid and water was characterized, and it was found that stronger hydrogen bonding is correlated with wider OH-O angles and larger differences in the hydrogen bond lengths between phases. The study also highlighted the importance of pyruvic acid in atmospheric keto-acid chemistry.