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.