Direct observation of the gas-phase Criegee intermediate (CH2OO)
ABSTRACT Carbonyl 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.
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ABSTRACT: Autoignition chemistry is central to predictive modeling of many advanced engine designs that combine high efficiency and low inherent pollutant emissions. This chemistry, and especially its pressure dependence, is poorly known for fuels derived from heavy petroleum and for biofuels, both of which are becoming increasingly prominent in the nation's fuel stream. We have investigated the pressure dependence of key ignition reactions for a series of molecules representative of non-traditional and alternative fuels. These investigations combined experimental characterization of hydroxyl radical production in well-controlled photolytically initiated oxidation and a hybrid modeling strategy that linked detailed quantum chemistry and computational kinetics of critical reactions with rate-equation models of the global chemical system. Comprehensive mechanisms for autoignition generally ignore the pressure dependence of branching fractions in the important alkyl + O reaction systems; however we have demonstrated that pressure-dependent 'formally direct' pathways persist at in-cylinder pressures.
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ABSTRACT: Low-temperature partial oxidation of methane was investigated using reactive molecular dynamics (MD) and quantum mechanical (QM) methods. In particular, the ReaxFF hydrocarbon force field [Chenoweth, K.; et al. J. Phys. Chem. A 2008, 112, 1040] was employed to simulate a [20 CH(4) + 10 O(2)] model system at 500 degrees C. The chemical mechanism of the partial oxidation of methane was proposed on the basis of analysis of the computed trajectory of this model system. The partial oxidation of methane was observed to be initiated by the abstraction of hydrogen from CH(4) by O(2) and the atomization of CH(4) itself. Subsequent radical recombination between hydrogen atoms and the dehydrogenation of CH(4) were the primary pathways by which H(2) was formed. In agreement with current models of low-temperature combustion, radicals including H(3)C-OO and H(2)C-OO were also observed during the MD simulation. The observed reaction mechanism was subsequently analyzed using QM methods. For instance, structural features of prominent radical species observed during the MD simulation were analyzed using density functional theory (DFT) and coupled-cluster (CCSD(T)) methods. Enthalpies of reaction of all observed chemical processes were calculated using DFT and the W1 composite method. Where possible, comparisons with experimental data were made.The Journal of Physical Chemistry A 02/2009; 113(8):1539-47. DOI:10.1021/jp809576k · 2.78 Impact Factor