HR: 1340h
AN: A53C-0914    [Abstracts]
TI: Overtone-Induced Decarboxylation: A Potential Sink for Atmospheric Di-acids
AU: * Donaldson, J
EM: jdonalds@chem.utoronto.ca
AF: Department of Chemistry, University of Toronto, 80 St. George St., Toronto, Ont M5S 3H6 Canada
AU: Staikova, M
EM: mstaikov@chem.utoronto.ca
AF: Department of Chemistry, University of Toronto, 80 St. George St., Toronto, Ont M5S 3H6 Canada
AU: Oh, M
EM: moh@chem.utoronto.ca
AF: Department of Chemistry, University of Toronto, 80 St. George St., Toronto, Ont M5S 3H6 Canada
AU: Eliason, T
EM: Teresa.Eliason@colorado.edu
AF: Department of Chemistry and Biochemistry,, University of Colorado, Boulder, CO 80309-0215 United States
AU: Havey, D
EM: Daniel.Havey@colorado.edu
AF: Department of Chemistry and Biochemistry,, University of Colorado, Boulder, CO 80309-0215 United States
AU: Vaida, V
EM: vaida@colorado.edu
AF: Department of Chemistry and Biochemistry,, University of Colorado, Boulder, CO 80309-0215 United States
AB: Atmospheric photochemistry induced by solar excitation of vibrational overtone transitions has recently been demonstrated to be of importance in cleaving weak bonds (in HO2NO2) and inducing intramolecular rearrangement followed by reaction (in H2SO4). Here we propose another potentially important atmospheric process: the decarboxylation of organic diacids. To demonstrate this possibility, we have calculated (using the GAUSSIAN 98 quantum chemistry package) the decarboxylation pathways for malonic acid and its mono-hydrate and performed preliminary photolysis experiments. The barrier to the gas-phase decarboxylation was calculated to be in the range 26-28 kcal/mol at the B3LYP/6-311++G(3df,3pd) level of theory, in good agreement with previous results, suggesting that excitation of vOH ≥3 of either one of the OH stretching modes is sufficient to supply the energy needed for the decarboxylation. A low energy isomer of the malonic acid-water complex forms an 8-membered, multiply hydrogen bonded structure; decarboxylation of such complexes has a barrier of 20-22 kcal/mol, suggesting that complexes excited to vOH ≥ 2 possess sufficient energy to react. Photolysis of aqueous solutions of malonic acid was carried out using a filtered Xe lamp (to eliminate IR and UV light). A 3- to 4-fold increase in acetic acid production was observed in irradiated vs. non-irradiated solutions. We suggest that the overtone-induced decarboxylation of malonic acid and its water complex is competitive with wet deposition of the acid and with gas phase reaction with OH for removal of the acid.
DE: 0317 Chemical kinetic and photochemical properties
DE: 0322 Constituent sources and sinks
DE: 0365 Troposphere--composition and chemistry
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting