Shedding light on the atmospheric photochemistry of pyruvic acid with theory and experiment

Pyruvic acid (PA) is an atmospherically relevant organic compound that belongs to the family of keto acids, molecules that contain both a carbonyl and a carboxylic moiety and whose reactivity is suggested to contribute to the formation of secondary organic aerosols (SOAs) in the atmosphere [1]. The photochemistry of PA has received a great deal of attention due to it being its primary atmospheric sink, with stark differences observed when PA is in the gas phase or in an aqueous environment. In the gas phase, PA photochemistry is primarily driven by singlet states [2], although experimental evidence suggests that triplet states may still contribute to the formation of specific photoproducts [3]. Conversely, triplet pathways appear to dominate the photochemical reactivity of the molecule in aqueous phase [4], giving rise to different photoproducts. Despite the advances in the understanding of the photochemistry of PA, the underlying chemical mechanisms governing the photochemistry of the system remain unclear.

Recently, we have focused on studying the light-induced reactivity of PA using computational methods, in direct collaboration with experimental spectroscopists, to rationalise the phase-dependency of the photochemistry of this molecule. The computational approaches employed include static explorations of the ground and excited states potential energy surfaces of PA, which involve the determination of critical points and connected pathways between them, conformational analyses to establish the most relevant conformers of the molecule in gas and aqueous phases, and the obtention of absorption properties and vibrationally resolved photoelectron spectra.

In this contribution, we will discuss the main computational analyses carried out in a recent study in which we combined anion photoelectron spectroscopy and computational photochemistry to accurately determine the energy gap between the lowest singlet and triplet excited states of the molecule in the gas phase, a key quantity for understanding the photochemistry of the molecule that has remained elusive until now [5]. Furthermore, we will comment on the effect of aqueous solvation on the excited-state properties of the system. The results presented here will contribute to having a better understanding of why the light-induced reactivity of PA changes significantly from the gas phase to an aqueous environment and, ultimately, will help to assess the role and fate of this molecule in the atmosphere.

References:

[1] Rapf, R. J.; Perkins, R. J.; Carpenter, B. K.; Vaida, V. J. Phys. Chem. A 2017, 121 (22), 4272–4282.

[2] Hutton, L.; Curchod, B. F. E. ChemPhotoChem. 2022, 6 (11), e202200151.

[3] Sauer, L. J.; Davis, H. F. J. Phys. Chem. Lett. 2025, 16 (15), 3721– 3726.

[4] Griffith, E. C.; Carpenter, B. K.; Shoemaker, R. K.; Vaida, V. Proc. Natl. Acad. Sci. U. S. A. 2013, 110 (29), 11714– 11719.

[5] Burrow, E. M.; Carmona-García, J.; Clarke, C. J.; Curchod, B. F. E.; Verlet, J. R. R. J. Am. Chem. Soc. 2025, 147 (41), 36987-36991.