Lactic acid photochemistry following excitation of S0 to S1 at 220 to 250 nm

Abstract

AbstractLactic acid, a small α‐hydroxy acid, is a multifunctional molecule that is prevalent on modern Earth and found in abiotic environments. Recently, there has been interest in the photochemistry of carboxylic acids in general. Specifically, the photochemistry of lactic acid, due to its prevalence and functionality, has garnered attention by the biomass valorization and abiotic chemistry communities. However, aqueous lactic acid photochemistry studies are limited, and gas‐phase lactic acid photolysis had not been performed. This work combines theory and experiments to explore the gas‐phase and aqueous phase photochemistry of lactic acid following excitation of S0 to S1 at 220–250 nm. We find that lactic acid primarily photodecomposes via decarboxylation in both phases. In the gas phase, secondary chemistry leads to mainly CO2 and CO, while in the aqueous phase, subsequent radical chemistry leads to a variety of products with one to four carbons. Isotopic substitutions, including the use of 13C tagged lactic acid and using D2O as a solvent, are used to infer mechanistic pathways for the major photolysis products. Computation shows that individual conformers may contribute to the overall photochemistry to a different degree than their relative abundance would suggest. The identified products and proposed mechanisms shown here serve to illustrate the photochemistry of lactic acid in the presence of high‐energy ultraviolet radiation. This knowledge may aid process and catalyst design for biomass valorization/organic synthesis and may provide insight into abiotic chemistry in environments exposed to high energy ultraviolet radiation.