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Unconventional peroxy chemistry in alcohol oxidation: The water elimination pathway

Journal of Physical Chemistry Letters

Welz, Oliver W.; Klippenstein, Stephen J.; Harding, Lawrence B.; Taatjes, Craig A.; Zador, Judit Z.

Predictive simulation for designing efficient engines requires detailed modeling of combustion chemistry, for which the possibility of unknown pathways is a continual concern. Here, we characterize a low-lying water elimination pathway from key hydroperoxyalkyl (QOOH) radicals derived from alcohols. The corresponding saddle-point structure involves the interaction of radical and zwitterionic electronic states. This interaction presents extreme difficulties for electronic structure characterizations, but we demonstrate that these properties of this saddle point can be well captured by M06-2X and CCSD(T) methods. Experimental evidence for the existence and relevance of this pathway is shown in recently reported data on the low-temperature oxidation of isopentanol and isobutanol. In these systems, water elimination is a major pathway, and is likely ubiquitous in low-temperature alcohol oxidation. These findings will substantially alter current alcohol oxidation mechanisms. Moreover, the methods described will be useful for the more general phenomenon of interacting radical and zwitterionic states. © 2013 American Chemical Society.

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TYPE Conference YEAR 2013
ScopusOSTI

Synchrotron photoionization measurements of fundamental autoignition reactions: Product formation in low-temperature isobutane oxidation

Proceedings of the Combustion Institute

Eskola, Arkke J.; Welz, Oliver W.; Savee, John D.; Osborn, David L.; Taatjes, Craig A.

Product formation in laser-photolytic Cl-initiated low-temperature (550-700 K) oxidation of isobutane in a slow-flow reactor was investigated by tunable synchrotron photoionization mass spectrometry. These experiments probed the time-resolved formation of products following photolytic initiation of the oxidation, and identify isomeric species by their photoionization spectra. The relative yields of oxygenated product isomers (2,2-dimethyloxirane, methylpropanal, and 3-methyloxetane) are in reasonable concord with measurements from Walker and co-workers (J. Chem. Soc. Faraday Trans. 74 (1) (1978) 2229-2251) at higher temperature. Oxidation of isotopically labeled isobutane, (CH3)3CD, suggests that methylpropanal formation can proceed from both (CH3)2CCH2OOH and CH 3CH(CH2)CH2OOH isomers. Bimodal time behavior is observed for product formation; the initial prompt formation reflects "formally direct" channels, principally chemical activation, and the longer-timescale "delayed" component arises from dissociation of thermalized ROO and QOOH radicals. The proportion of prompt to delayed signal is smaller for the oxygenated products than for the isobutene product. This channel-specific behavior can be qualitatively understood by considering the different energetic distributions of ROO and QOOH in formally direct vs. thermal channels and the fact that the transition states involved in the formation of oxygenated products are "tighter" than that for isobutene formation. © 2012 Published by Elsevier Inc. on behalf of The Combustion Institute.

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TYPE Conference YEAR 2013
OSTIScopus

New mechanistic insights to the O(3P) + propene reaction from multiplexed photoionization mass spectrometry

Physical Chemistry Chemical Physics

Savee, John D.; Welz, Oliver W.; Taatjes, Craig A.; Osborn, David L.

The reaction of O(3P) with propene (C3H6) has been examined using tunable vacuum ultraviolet radiation and time-resolved multiplexed photoionization mass spectrometry at 4 Torr and 298 K. The temporal and isomeric resolution of these experiments allow the separation of primary from secondary reaction products and determination of branching ratios of 1.00, 0.91 ± 0.30, and 0.05 ± 0.04 for the primary product channels CH3 + CH2CHO, C2H5 + HCO, and H2 + CH3CHCO, respectively. The H + CH3CHCHO product channel was not observable for technical reasons in these experiments, so literature values for the branching fraction of this channel were used to convert the measured product branching ratios to branching fractions. The results of the present study, in combination with past experimental and theoretical studies of O(3P) + C3H6, identify important pathways leading to products on the C3H6O potential energy surface (PES). The present results suggest that up to 40% of the total product yield may require intersystem crossing from the initial triplet C3H6O PES to the lower-lying singlet PES. © the Owner Societies.

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TYPE Conference YEAR 2012
OSTIScopus
Results 126–150 of 236
Results 126–150 of 236