Photoinduced oligomerization of aqueous pyruvic acid

The 320 nm-band photodecarboxylation of aqueous pyruvic acid (PA), a representative of the α-oxocarboxylic acids widely found in the atmospheric aerosol, yields 2,3-dimethyltartaric (A) and 2-(3-oxobutan-2-yloxy)- 2-hydroxypropanoic (B) acids, rather than 3-hydroxy-2-oxobutanone as previously reported. A and B are identified by liquid chromatography with UV and ESI-MS detection, complemented by collisionally induced dissociation and ²H and ¹³C isotope labeling experiments. The multifunctional ether B gives rise to characteristic δ ∼ 80 ppm ¹³C NMR resonances. Product quantum yields are proportional to [PA](a + [PA]) ^-1 in the range [PA] = 5-100 mM. CO ₂(g) release rates are halved, while A and B are suppressed by the addition of > 1.5 mM TEMPO. A and B are only partially quenched in air-saturated solutions. These observations are shown to be consistent with an oligomerization process initiated by a bimolecular reaction between ³PA* and PA producing ketyl, CH ₃C(OH)C(O)OH, and acetyl, CH ₃C(O)·, radicals, rather than by the unimolecular decomposition of ³PA* into 1-hydroxyethylidene, ³HO(CH ₃)C: (+CO ₂), or [CH ₃C(O) · + ·C(O)OH] pairs. A arises from the dimerization of ketyl radicals, while B ensues the facile decarboxylation of the C ₈β- ketoacid formed by association of acetyl radicals with the ketyl radical adduct of PA. Since the radical precursors to A and B are scavenged by O ₂ with a low probability per encounter (k _sc ∼ 1 × 10 ⁶ M ^-1 s ^-1), PA is able to accrete into multifunctional polar species in aerated aqueous media under solar illumination.