Photoinduced Oligomerization of Aqueous Pyruvic Acid
- 11 February 2006
- journal article
- research article
- Published by American Chemical Society (ACS) in The Journal of Physical Chemistry A
- Vol. 110 (10), 3619-3626
- https://doi.org/10.1021/jp056097z
Abstract
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. 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 3PA* and PA producing ketyl, CH3Ċ(OH)C(O)OH, and acetyl, CH3C(O)·, radicals, rather than by the unimolecular decomposition of 3PA* into 1-hydroxyethylidene, 3HO(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 C8β-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 O2 with a low probability per encounter (ksc ∼ 1 × 106 M-1 s-1), PA is able to accrete into multifunctional polar species in aerated aqueous media under solar illumination.Keywords
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