Mechanistic Insights into the Photodecarboxylation of the Pyruvate Anion in an Aqueous Environment

Pyruvic acid (PA) and its conjugate base, the pyruvate anion (PA^-), are ubiquitous in the atmosphere. The loss of a proton endows PA^- with photochemical behavior distinct from that of neutral PA; however, the anion remains far less investigated. Herein, we employed the high-level multireference method combined with quantum mechanics/molecular mechanics (QM/MM) calculations to investigate the photochemistry and photophysics of PA^- both in solution and in the gas phase. The results revealed that, for isolated PA^-, photodecarboxylation via the T₁-state represents a more favorable reaction pathway, whereas the aqueous environment alters the fate of the excited states and significantly suppresses the photoreactivity of PA^-. This suppression primarily arises from solvent-induced inhibition of intersystem crossing, together with effective stabilization of the T₁-state minimum by solvation, which increases the photodissociation barrier by more than 10 kcal/mol. Moreover, we demonstrate that the binding of only two water molecules is sufficient to stabilize the T₁ minimum and suppress the photoreactivity of PA^-, providing a new mechanistic explanation for recent experimental observations.