jp6b11916_si_liveslides.zip (6.16 MB)
Reactivity of Ketyl and Acetyl Radicals from Direct Solar Actinic Photolysis of Aqueous Pyruvic Acid
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posted on 2017-05-01, 11:19 authored by Alexis
J. Eugene, Marcelo I. GuzmanThe variable composition
of secondary organic aerosols (SOA) contributes
to the large uncertainty for predicting radiative forcing. A better
understanding of the reaction mechanisms leading to aerosol formation
such as for the photochemical reaction of aqueous pyruvic acid (PA)
at λ ≥ 305 nm can contribute to constrain these uncertainties.
Herein, the photochemistry of aqueous PA (5–300 mM) continuously
sparged with air is re-examined in the laboratory under comparable
irradiance at 38° N at noon on a summer day. Several analytical
methods are employed to monitor the time series of the reaction, including
(1) the derivatization of carbonyl (CO) functional groups
with 2,4-dinitrophenylhydrazine (DNPH), (2) the separation of photoproducts
by ultrahigh pressure liquid chromatography (UHPLC) and ion chromatography
(IC) coupled to mass spectrometry (MS), (3) high resolution MS, (4)
the assignment of 1H NMR and 13C gCOSY spectroscopic
features, and (5) quantitative 1H NMR. The primary photoproducts
are 2,3-dimethyltartaric acid and unstable 2-(1-carboxy-1-hydroxyethoxy)-2-methyl-3-oxobutanoic
acid, a polyfunctional β-ketocarboxylic acid with eight carbons
(C8) that quickly decarboxylates into 2-hydroxy-2-((3-oxobutan-2-yl)oxy)propanoic
acid. Kinetic isotope effect studies performed for the first time
for this system reveal the existence of tunneling during the initial
loss of PA. Thus, the KIEs support a mechanism initiated by photoinduced
proton coupled electron transfer (PCET). Measured reaction rates at
variable initial [PA]0 were used to calculate the sum of
the quantum yields for the products, which displays a hyperbolic dependence:
∑Φproduct = 1.99 [PA]0/(113.2 +
[PA]0). The fast photochemical loss of aqueous PA with
an estimated lifetime of 21.7 min is interpreted as a significant
atmospheric sink for this species. The complexity of these aqueous
phase pathways indicates that the solar photochemistry of an abundant
α-ketocarboxylic acid can activate chemical processes for SOA
formation.
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α- ketocarboxylic acidphotoinduced protonreaction mechanismsreaction ratesAqueous Pyruvicultrahigh pressurePCET21.7 minmass spectrometry1 H NMRPAresolution MSelectron transferDirect Solar Actinic Photolysistime seriesquantum yieldsKIEs supportchemical processesAcetyl Radicalsphase pathwaysion chromatographyKinetic isotope effect studiespyruvic acidC 8summer dayaerosol formationSOA formationDNPHpolyfunctional β- ketocarboxylic acidIC13 C gCOSY spectroscopic featuresUHPLC
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