Theoretical Investigation of the Atmospheric Photochemistry of Glyoxylic Acid in the Gas Phase

The photochemistry of glyoxylic acid (HC(O)C(O)OH) is explored in the near UV in both the singlet (S₁/S₀) and triplet (T₁) manifolds using density functional theory (M06-2X/aug-cc-pVTZ) to reach an overall mechanistic picture of the atmospherically relevant photochemistry in the gas phase. The calculated energies and structures are also used in RRKM kinetics calculations to compare the relative reaction rates on each of these electronic states. The major photolysis pathways are two possible photodecarboxylation reactions: direct C-C bond cleavage (Norrish Type I reaction) and β-hydrogen transfer followed by CO₂ loss. These results indicate that from λ = 350-380 nm both photodecarboxylation pathways can occur following intersystem crossing to the T₁ surface. However, hydrogen transfer-decarboxylation initiated on S₁ becomes increasingly important at λ < 350 nm. At the lower energy UV wavelengths available in the atmosphere (λ = 380-400 nm), reactions can only occur in S₀ where concerted hydrogen transfer-decarboxylation is the dominant dissociation pathway with some minor contributions from CO loss/decarbonylation reactions.