Photochemical behaviors of pyruvic acid in multiple phases have been extensively studied, while those of its conjugate base, the pyruvate anion (CH3COCOO−, PA−) are less understood and remain contradictory in gaseous versus aqueous phases. We present here a joint experimental and theoretical study combining cryogenic, wavelength-resolved negative ion photo electron spectroscopy (NIPES) and high-level quantum chemical computations to investigate PA− actinic photochemistry and its dependence on microsolvation in the gas phase. As the first step, PA−●nH2O (n = 0−5) clusters were generated and characterized, with their low-lying isomers identified. Second, NIPES conducted at multiple wavelengths across the PA− actinic regime revealed the PA− photochemistry extremely sensitive to its hydration extent. While bare PA− anions exhibit active photoinduced dissociations that generate the acetyl (CH3CO−), methide (CH3−) anions, their corresponding radicals, and slow electrons, one single attached water molecule results in significant suppression with a subsequent second water being able to completely block all dissociation pathways, effectively annihilating PA− photochemical reactivities. Finally, the underlying dissociation mechanisms of PA−●nH2O (n = 0−2) clusters are proposed involving nπ* excitation, dehydration, decarboxylation, and further CO loss. The insight unraveled here provides a much-needed critical link to connect the seemingly conflicting PA− actinic photochemistry between the gas and condensed phases.

Annihilating actinic photochemistry of the pyruvate anion by one and two water molecules

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