Photochemistry of the pyruvate anion produces CO2, CO, CH3–, CH3, and a low energy electron

Abstract

The photochemistry of pyruvic acid has attracted much scientific interest because it is believed to play critical roles in atmospheric chemistry. However, under most atmospherically relevant conditions, pyruvic acid deprotonates to form its conjugate base, the photochemistry of which is essentially unknown. Here, we present a detailed study of the photochemistry of the isolated pyruvate anion and uncover that it is extremely rich. Using photoelectron imaging and computational chemistry, we show that photoexcitation by UVA light leads to the formation of CO2, CO, and CH3−. The observation of the unusual methide anion formation and its subsequent decomposition into methyl radical and a free electron may hold important consequences for atmospheric chemistry. From a mechanistic perspective, the initial decarboxylation of pyruvate necessarily differs from that in pyruvic acid, due to the missing proton in the anion.

Annihilating Actinic Photochemistry of the Pyruvate Anion by One and Two Water Molecules

“…The perpendicular structure of PA − was also reported in a recent study by Clarke et al 22 The first H 2 O molecule is found to bind to the carboxylate group with the PA − anion still in a nearly perpendicular conformation, while the other conformers in which H 2 O is bridging between the carboxylate and the αcarbonyl groups are at least 1.06 kcal/mol higher in energy (Figure S1). The binding of H 2 O to the carboxylate instead of the α-carbonyl group also agrees with the natural population analysis (NPA) charge distribution, which shows slightly more negative charge on the carboxylate than the carbonyl per O atom (−0.740 vs −0.604, Figure S2a).…”

“…21 All these aqueous studies lead to a conclusion that the photochemical reactivity of PA − being significantly less than that of pyruvic acid. However, a different story was told in terms of the gaseous PA − in a recent investigation by Clarke et al 22 Based on their photoelectron spectroscopic study, upon UV radiation, PA − not only decarboxylated but also further lost a CO molecule since the methide anion (CH 3 − ) was detected. The high photochemical reactivity of gaseous PA − is unexpected, standing contrast to its aqueous behavior, thus bringing new puzzles to this field.…”

“…It has been proven that negative ion photoelectron spectroscopy (NIPES) is a powerful spectroscopic tool to determine the structures and charge chromophores of microsolvated anions with specific number of bound water molecules ranging from individual to bulklike. 28−30 Following an excitation upon resonant radiation with UV−Vis incident photons, subsequent processes, e.g., photodissociation and proton and electron transfers, could be directly probed using NIPES, 22,31 as a parallel to those adopting tandem-mass based UV−Vis absorption action spectroscopy. 32−34 Moreover, the related reaction energies or barriers could be estimated based on the appearance/ disappearance of resultant photofragments as a function of the incident photon energy.…”

Thermal Electron Attachment to Pyruvic Acid, Thermal Detachment from the Parent Anion, and the Electron Affinity of Pyruvic Acid

“…17,18 Recently, the ultraviolet photochemistry of the biologically-relevant pyruvate anion has been reported. 19 It was recently reported that PA can accept an excess electron to form the thermodynamically stable PA − anion: Zawadzki et al observed PA − from low-energy electron attachment to gas-phase PA, as well as a number of anion fragments. 20 The same research group later investigated the fragmentation pathways of electron attachment to pyruvic acid clusters.…”

“…Its charge-neutral conjugate, pyruvic acid (PA; CH 3 COCOOH), warrants consideration as a reactive species in acidic environments and the gas phase, and there have been numerous studies on its photochemistry, − atmospheric degradation with OH, − and chemistry involving the air–water interface. , In biologically relevant environments near neutral pH, PA can easily be reduced to pyruvate by losing its carboxylic proton. PA has also found use as a medical therapeutic. , Recently, the ultraviolet photochemistry of the biologically-relevant pyruvate anion has been reported …”