Photoinduced oligomerization of aqueous pyruvic acid

The 320 nm-band photodecarboxylation of aqueous pyruvic acid (PA), a representative of the alpha-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 2 H and C-13 isotope labeling experiments. The multifunctional ether B gives rise to characteristic 6 similar to 80 ppm C-13 NMR resonances. Product quantum yields are proportional to [PA](a + [PA])(-1) in the range [PA] = 5 -100 mM. CO2(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 (3)PA* and PA producing ketyl, CH3C(OH)C(O)OH, and acetyl, CH3C(O)(.), radicals, rather than by the unimolecular decomposition of (3)PA* into I-hydroxyethylidene, (HO)-H-3(CH3)C: (+CO2), or [CH3C(O)(.) + C-.(O)OH] pairs. A arises from the dimerization of ketyl radicals, while B ensues the facile decarboxylation of the C-8 beta-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) similar to I x 10(6) M-1 s(-1)), PA is able to accrete into multifunctional polar species in aerated aqueous media under solar illumination.