The Singlet–Triplet Gap of Pyruvic Acid

Understanding the gas-phase photochemistry of pyruvic acid is crucial to assessing its role and evolution in the atmosphere and relies on knowledge of the relative energy gaps between the atmospherically relevant excited electronic states of the molecule. However, accurate determination of these gaps, particularly between the lowest excited singlet and triplet states, has remained elusive due to the challenge of directly interrogating triplet states of the isolated molecule. In this work, we combine anion photoelectron spectroscopy and computational photochemistry to determine that the adiabatic S1-T1 energy gap of pyruvic acid is 0.29 ± 0.04 eV. This study provides a reference value for the singlet–triplet energy gap of pyruvic acid and validates an approach that combines theory and experiment to determine energy gaps for volatile organic compounds of atmospheric interest.