HR: 0800h
AN: A21C-0872    [Abstracts]
TI: Carbon Dioxide Emissions From Illuminated Frozen Pyruvic Acid Solutions Above Minus 140 Celsius
AU: * Guzman, M I
EM: mig@caltech.edu
AF: California Institute of Technology, Environmental Science and Technology, 1200 E. California Blvd 138-78, Pasadena, CA 91125 United States
AU: Colussi, A J
EM: ajcoluss@caltech.edu
AF: California Institute of Technology, Environmental Science and Technology, 1200 E. California Blvd 138-78, Pasadena, CA 91125 United States
AU: Hoffmann, M R
EM: mrh@caltech.edu
AF: California Institute of Technology, Environmental Science and Technology, 1200 E. California Blvd 138-78, Pasadena, CA 91125 United States
AB: Photodecarboxylation rates of deareated aqueous pyruvic acid solutions drop by a factor of two in the frozen state at 253 K relative to the fluid at 293 K. TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl), in contrast with experiments in water, has no effect on photodecarboxylation rates in ice. Preirradiated frozen pyruvic acid (PA) solutions still release carbon dioxide (CD)in the dark at rates that increase with temperature (Ea = 22 kJ/mol, below 268 K), but which extrapolate into measurable rates in fluid solutions, at variance with observations. We verified the absence of post-illumination CD emissions in the concerted photodecomposition of benzoylformic acid in frozen solutions. We infer that PA rapidly photodecarboxylates into intermediates that slowly yield further CD via a thermally activated process. Prompt CD is apparently formed even at cryogenic temperatures, although is only able to sublimate above -140 Celsius. The detection of distant triplet radical pairs in ice below 190 K via electron paramagnetic resonance spectrometry indicates that the primary process in the photodecarboxylation of pyruvic acid involves photoinduced charge transfer between the carbonyl groups of H-bonded PA dimers into triplet [PA^- PA⁺], followed by ultrafast deprotonation/decarboxylation of the radical cation PA⁺. The radical anion PA^- readily protonates into an alkoxyl radical to initiate polymerizations.
DE: 0317 Chemical kinetic and photochemical properties
DE: 0736 Snow (1827, 1863)
DE: 4932 Ice cores (0724)
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005