The primary near-UV photochemistry of aqueous pyruvic acid

Abstract

The near-UV photochemistry of aqueous pyruvic acid is potentially important in atmospheric and climate chemistry, yet the understanding of its primary reaction pathways and pH dependence remain incomplete. Here we combine sub-picosecond UV pump -IR probe transient absorption spectroscopy with density functional theory calculations to resolve the initial photodynamics of pyruvic acid following excitation at 340 nm. Protonated pyruvic acid undergoes decarboxylation in less than 0.8 ps, forming carbon dioxide and methyl hydroxy carbene. In contrast, excited pyruvate relaxes back to the ground state within 50 ps with no detectable photolysis. The sharply different behavior of the acid and its conjugate base provides a mechanistic explanation for the puzzling pH dependence of pyruvic acid photolysis.