411 research outputs found
Direct observation of the gas-phase Criegee intermediate (CH2OO)
No full textCarbonyl oxide species play a key role in tropospheric oxidation of organic molecules and in low-temperature combustion processes. In the late 1940s, Criegee first postulated the participation of carbonyl oxides, now often called "Criegee intermediates," in ozonolysis of alkenes. However, despite decades of effort, no gas phase Criegee intermediate has before been observed. As a result, knowledge of gas phase carbonyl oxide reactions has heretofore been inferred by indirect means, with derived rate coefficients spanning orders of magnitude. We have directly detected the primary Criegee intermediate, formaldehyde oxide (CH2OO), in the chlorine-initiated gas-phase oxidation of dimethyl sulfoxide (DMSO). This work not only establishes that the Criegee intermediate is formed in DMSO oxidation also but opens the possibility for explicit kinetics studies on this critical atmospheric species. Copyright © 2008 American Chemical Society
Hydroxyacetone Production From C<sub>3</sub> Criegee Intermediates
No full textHydroxyacetone
(CH3C(O)CH2OH) is observed as a stable end product
from reactions of the (CH3)2COO Criegee intermediate,
acetone oxide, in a flow tube coupled with multiplexed photoionization
mass spectrometer detection. In the experiment, the isomers at m/z = 74 are distinguished by their different
photoionization spectra and reaction times. Hydroxyacetone is observed
as a persistent signal at longer reaction times at a higher photoionization
threshold of ca. 9.7 eV than Criegee intermediate and definitively identified by comparison
with the known photoionization spectrum. Complementary electronic structure
calculations reveal multiple possible reaction pathways for hydroxyacetone
formation, including unimolecular isomerization via hydrogen atom
transfer and −OH group migration as well as self-reaction of
Criegee intermediates. Varying the concentration of Criegee intermediates
suggests contributions from both unimolecular and self-reaction pathways
to hydroxyacetone. The hydroxyacetone end product can provide an effective,
stable marker for the production of transient Criegee intermediates
in future studies of alkene ozonolysis
Criegee intermediates and their impacts on the troposphere
No full textCriegee intermediates (CIs), carbonyl oxides formed in ozonolysis of alkenes, play key roles in the troposphere.</p
Direct Observation of the Gas-Phase Criegee Intermediate (CH<sub>2</sub>OO)
No full textCarbonyl oxide species play a key role in tropospheric oxidation of organic molecules and in low-temperature combustion processes. In the late 1940s, Criegee first postulated the participation of carbonyl oxides, now often called “Criegee intermediates,” in ozonolysis of alkenes. However, despite decades of effort, no gas phase Criegee intermediate has before been observed. As a result, knowledge of gas phase carbonyl oxide reactions has heretofore been inferred by indirect means, with derived rate coefficients spanning orders of magnitude. We have directly detected the primary Criegee intermediate, formaldehyde oxide (CH2OO), in the chlorine-initiated gas-phase oxidation of dimethyl sulfoxide (DMSO). This work not only establishes that the Criegee intermediate is formed in DMSO oxidation also but opens the possibility for explicit kinetics studies on this critical atmospheric species
Bimolecular Reaction of Methyl-Ethyl-Substituted Criegee Intermediate with SO<sub>2</sub>
No full textMethyl-ethyl-substituted Criegee
intermediate (MECI) is a four-carbon
carbonyl oxide that is formed in the ozonolysis of some asymmetric
alkenes. MECI is structurally similar to the isoprene-derived methyl
vinyl ketone oxide (MVK-oxide) but lacks resonance stabilization,
making it a promising candidate to help us unravel the effects of
size, structure, and resonance stabilization that influence the reactivity
of atmospherically important, highly functionalized Criegee intermediates.
We present experimental and theoretical results from the first bimolecular
study of MECI in its reaction with SO2, a reaction that
shows significant sensitivity to the Criegee intermediate structure.
Using multiplexed photoionization mass spectrometry, we obtain a rate
coefficient of (1.3 ± 0.3) × 10–10 cm3 s–1 (95% confidence limits, 298 K, 10 Torr)
and demonstrate the formation of SO3 under our experimental
conditions. Through high-level theory, we explore the effect of Criegee
intermediate structure on the minimum energy pathways for their reactions
with SO2 and obtain modified Arrhenius fits to our predictions
for the reaction of both syn and anti conformers of MECI with SO2 (ksyn = 4.42 × 1011 T–7.80exp(−1401/T)
cm3 s–1 and kanti = 1.26 × 1011 T–7.55exp(−1397/T)
cm3 s–1). Our experimental and theoretical
rate coefficients (which are in reasonable agreement at 298 K) show
that the reaction of MECI with SO2 is significantly faster
than MVK-oxide + SO2, demonstrating the substantial effect
of resonance stabilization on Criegee intermediate reactivity
Direct observation of OH formation from stabilised Criegee intermediates
No full textThe syn-CH3CHOO Criegee intermediate formed from the ozonolysis of propene and (E)-2-butene was detected via unimolecular decomposition and subsequent detection of OH radicals by a LIF-FAGE instrument. An observed time dependent OH concentration profile was analysed using a detailed model focusing on the speciated chemistry of Criegee intermediates based on the recent literature. The absolute OH concentration was found to depend on the steady state concentration of syn-CH3CHOO at the injection point while the time dependence of the OH concentration profile was influenced by the sum of the rates of unimolecular decomposition of syn-CH3CHOO and wall loss. By varying the most relevant parameters influencing the SCI chemistry in the model and based on the temporal OH concentration profile, the unimolecular decomposition rate k (293 K) of syn-CH3CHOO was shown to lie within the range 3-30 s(-1), where a value of 20 +/- 10 s(-1) yields the best agreement with the CI chemistry literature
Observational evidence for Criegee intermediate oligomerization reactions relevant to aerosol formation in the troposphere
No full textCriegee intermediates are reactive intermediates that are implicated in transforming the composition of Earth’s troposphere and in the formation of secondary organic aerosol, impacting Earth’s radiation balance, air quality and human health. Yet, direct identification of their signatures in the field remains elusive. Here, from particulate and gas-phase mass-spectrometric measurements in the Amazon rainforest, we identify sequences of masses consistent with the expected signatures of oligomerization of the CH2OO Criegee intermediate, a process implicated in ozonolysis-driven aerosol formation. We assess the potential contributions of oligomerization through laboratory ozonolysis experiments, direct kinetic studies of Criegee intermediate reactions, and high-level theoretical calculations. Global atmospheric models built on these kinetics results indicate that Criegee intermediate chemistry may play a larger role in altering the composition of Earth’s troposphere than is captured in current atmospheric models, especially in areas of high humidity. However, the models still capture only a relatively small fraction of the observed signatures, suggesting considerable underestimates of Criegee intermediate concentrations and reactivity and/or the dominance of other, presently uncharacterized, oxidation mechanisms. Resolving the remaining uncertainties in emission inventories and the effects of atmospheric water vapour on key chemical reactions will be required to definitively assess the role of Criegee intermediate oligomerization reactions
Reaction of Perfluorooctanoic Acid with Criegee Intermediates and Implications for the Atmospheric Fate of Perfluorocarboxylic Acids
No full textThe reaction of perfluorooctanoic acid with the smallest carbonyl oxide Criegee intermediate, CH2OO, has been measured and is very rapid, with a rate coefficient of (4.9 ± 0.8) × 10-10 cm3 s-1, similar to that for reactions of Criegee intermediates with other organic acids. Evidence is shown for the formation of hydroperoxymethyl perfluorooctanoate as a product. With such a large rate coefficient, reaction with Criegee intermediates can be a substantial contributor to atmospheric removal of perfluorocarboxylic acids. However, the atmospheric fates of the ester product largely regenerate the initial acid reactant. Wet deposition regenerates the perfluorocarboxylic acid via condensed-phase hydrolysis. Gas-phase reaction with OH is expected principally to result in formation of the acid anhydride, which also hydrolyzes to regenerate the acid, although a minor channel could lead to destruction of the perfluorinated backbone.</p
Regional and global impacts of Criegee intermediates on atmospheric sulphuric acid concentrations and first steps of aerosol formation
No full textCarbonyl oxides (“Criegee intermediates”), formed in the ozonolysis of alkenes, are key species in tropospheric oxidation of organic molecules and their decomposition provides a non-photolytic source of OH in the atmosphere (Johnson and Marston, Chem. Soc. Rev., 2008, 37, 699, Harrison et al., Sci. Total Environ., 2006, 360, 5, Gäb et al., Nature, 1985, 316, 535,). Recently it was shown that small Criegee intermediates, C.I.’s, react far more rapidly with SO2 than typically represented in tropospheric models, (Welz, Science, 2012, 335, 204,) which suggested that carbonyl oxides could have a substantial influence on the atmospheric oxidation of SO2. Oxidation of SO2 is the main atmospheric source of sulphuric acid (H2SO4), which is a critical contributor to aerosol formation, although questions remain about the fundamental nucleation mechanism (Sipilä et al., Science, 2010, 327, 1243, Metzger et al., Proc. Natl. Acad. Sci. U.S.A., 2010 107, 6646, Kirkby et al., Nature, 2011, 476, 429,). Non-absorbing atmospheric aerosols, by scattering incoming solar radiation and acting as cloud condensation nuclei, have a cooling effect on climate (Intergovernmental Panel on Climate Change (IPCC), Climate Change 2007: The Physical Science Basis, Cambridge University Press, 2007). Here we explore the effect of the Criegees on atmospheric chemistry, and demonstrate that ozonolysis of alkenes via the reaction of Criegee intermediates potentially has a large impact on atmospheric sulphuric acid concentrations and consequently the first steps in aerosol production. Reactions of Criegee intermediates with SO2 will compete with and in places dominate over the reaction of OH with SO2 (the only other known gas-phase source of H2SO4) in many areas of the Earth's surface. In the case that the products of Criegee intermediate reactions predominantly result in H2SO4 formation, modelled particle nucleation rates can be substantially increased by the improved experimentally obtained estimates of the rate coefficients of Criegee intermediate reactions. Using both regional and global scale modelling, we show that this enhancement is likely to be highly variable spatially with local hot-spots in e.g. urban outflows. This conclusion is however contingent on a number of remaining uncertainties in Criegee intermediate chemistry.<br/
EXPLORATIONS OF INFRARED SPECTRA OF CRIEGEE INTERMEDIATES AND THEIR REACTIONS
No full textCriegee intermediates, carbonyl oxides produced in ozonolysis of unsaturated hydrocarbons, play important roles in atmospheric chemistry. A new production scheme using photolysis of RCI + O facilitated the production and direct detection of Criegee intermediates with various spectral techniques and has stimulated rapidly expanding research.\footnote {Y.-P. ~Lee, \textit{J. Chem. Phys.} \underline{\textbf{143}}, 020901 (2015).}\footnote {D. L. ~Osborn, C. A. ~Taatjes, \textit{Int. Rev. Phys. Chem.} \underline{\textbf{34}}, 309 (2015).} Our understanding of important atmospheric reactions involving Criegee intermediates is becoming clarified because of the direct probing of Criegee intermediates in kinetic experiments. The infrared spectra of CHOO,\footnote { Y. T. ~Su, Y.-H. ~Huang, H. A. ~Witek, Y.-P. ~Lee, \textit{Science} \underline{\textbf{340}}, 174 (2013).}\footnote {Y.-H. ~Huang, J. ~Li, H. ~Guo, Y.-P. ~Lee, \textit{J. Chem. Phys.} \underline{\textbf{142}}, 214301 (2015).} CHCHOO,\footnote {H.-Y. ~Lin, Y.-H. ~Huang, X. ~Wang, J. M. ~Bowman, Y. ~Nishimura, H. A. ~Witek, Y.-P. ~Lee, \textit{Nature Comm.} \underline{\textbf{6}}, 7012 (2015).} and (CH3)COO\footnote {Y.-Y. ~Wang, C.-Y. ~Chung, Y.-P. ~Lee, \textit{J. Chem. Phys.} \underline{\textbf{145}}, 154303 (2016).} have been recorded with a step-scan FTIR with resolution 0.25 to 1 cm; rotational contours with unresolved rotational lines were reported. On employing a quantum cascade laser coupled with a Herriot cell, we recorded spectra of the O-O stretching bands of CHOO and CHCHOO in the region 880-932 cm at resolution 0.002 cm. In addition to improved rotational parameters, perturbation was observed at high-\textit{J} levels of \textit{K} = 3, 6, and 11 of CHOO. Distinct lines of \textit{syn}- and \textit{anti}-CHCHOO were also observed. Kinetic investigations based on this new experimental scheme will be presented. Taking advantage of the wide spectral coverage of an FTIR, we investigated the mechanism of the reactions of CHOO with SO, HNO, HCl, and HCOOH. For example, in the reaction of CHOO + HCOOH, eight observed bands are assigned to hydroperoxymethyl formate HPMF (P5). In the later reaction period, three bands are assigned to an isomer HPMF (P6) and three bands to the final product, \textit{anti}-FAN. According to our kinetic analysis, only P5, not P6, decomposes to form FAN
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