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Note: Absolute photoionization cross-section of the vinyl radical

Journal of Chemical Physics

Savee, John D.; Lockyear, Jessica F.; Borkar, Sampada; Eskola, Arkke J.; Welz, Oliver W.; Taatjes, Craig A.; Osborn, David L.

This work measures the absolute photoionization cross-section of the vinyl radical (σvinyl(E)) between 8.1 and 11.0 eV. Two different methods were used to obtain absolute cross-section measurements: 193 nm photodissociation of methyl vinyl ketone (MVK) and 248 nm photodissociation of vinyl iodide (VI). The values of the photoionization cross-section for the vinyl radical using MVK, σvinyl(10.224 eV) = (6.1 ± 1.4) Mb and σvinyl(10.424 eV) = (8.3 ± 1.9) Mb, and using VI, σvinyl(10.013 eV) = (4.7 ± 1.1) Mb, σ vinyl(10.513 eV) = (9.0 ± 2.1) Mb, and σ vinyl(10.813 eV) = (12.1 ± 2.9) Mb, define a photoionization cross-section that is ∼1.7 times smaller than a previous determination of this value. © 2013 AIP Publishing LLC.

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TYPE Journal Article YEAR 2013
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Product branching fractions of the CH + propene reaction from synchrotron photoionization mass spectrometry

Journal of Physical Chemistry A

Trevitt, Adam J.; Prendergast, Matthew B.; Goulay, Fabien; Savee, John D.; Osborn, David L.; Taatjes, Craig A.; Leone, Stephen R.

The CH(X2Π) + propene reaction is studied in the gas phase at 298 K and 4 Torr (533.3 Pa) using VUV synchrotron photoionization mass spectrometry. The dominant product channel is the formation of C 4H6 (m/z 54) + H. By fitting experimental photoionization spectra to measured spectra of known C4H6 isomers, the following relative branching fractions are obtained: 1,3-butadiene (0.63 ± 0.13), 1,2-butadiene (0.25 ± 0.05), and 1-butyne (0.12 ± 0.03) with no detectable contribution from 2-butyne. The CD + propene reaction is also studied and two product channels are observed that correspond to C 4H6 (m/z 54) + D and C4H5D (m/z 55) + H, formed at a ratio of 0.4 (m/z 54) to 1.0 (m/z 55). The D elimination channel forms almost exclusively 1,2-butadiene (0.97 ± 0.20) whereas the H elimination channel leads to the formation of deuterated 1,3-butadiene (0.89 ± 0.18) and 1-butyne (0.11 ± 0.02); photoionization spectra of undeuterated species are used in the fitting of the measured m/z 55 (C 4H5D) spectrum. The results are generally consistent with a CH cycloaddition mechanism to the C-C bond of propene, forming 1-methylallyl followed by elimination of a H atom via several competing processes. The direct detection of 1,3-butadiene as a reaction product is an important validation of molecular weight growth schemes implicating the CH + propene reaction, for example, those reported recently for the formation of benzene in the interstellar medium (Jones, B. M. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 452-457). © 2013 American Chemical Society.

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TYPE Journal Article YEAR 2013
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Directly measuring reaction kinetics of QOOH-a crucial but elusive intermediate in hydrocarbon autoignition

Physical Chemistry Chemical Physics

Zador, Judit Z.; Huang, Haifeng H.; Welz, Oliver W.; Zetterberg, Johan; Osborn, David L.; Taatjes, Craig A.

Hydrocarbon autoignition has long been an area of intense fundamental chemical interest, and is a key technological process for emerging clean and efficient combustion strategies. Carbon-centered radicals containing an -OOH group, commonly denoted QOOH radicals, are produced by isomerization of the alkylperoxy radicals that are formed in the first stages of oxidation. These QOOH radicals are among the most critical species for modeling autoignition, as their reactions with O2 are responsible for chain branching below 1000 K. Despite their importance, no QOOH radicals have ever been observed by any means, and only computational and indirect experimental evidence has been available on their reactivity. Here, we directly produce a QOOH radical, 2-hydroperoxy-2-methylprop-1-yl, and experimentally determine rate coefficients for its unimolecular decomposition and its association reaction with O 2. The results are supported by high-level theoretical kinetics calculations. Our experimental strategy opens up a new avenue to study the chemistry of QOOH radicals in isolation. © 2013 the Owner Societies.

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TYPE Journal Article YEAR 2013
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Low-temperature combustion chemistry of biofuels: Pathways in the low-temperature (550-700 K) oxidation chemistry of isobutanol and tert-butanol

Proceedings of the Combustion Institute

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

Butanol isomers are promising next-generation biofuels. Their use in internal combustion applications, especially those relying on low-temperature autoignition, requires an understanding of their low-temperature combustion chemistry. Whereas the high-temperature oxidation chemistry of all four butanol isomers has been the subject of substantial experimental and theoretical efforts, their low-temperature oxidation chemistry remains underexplored. In this work we report an experimental study on the fundamental low-temperature oxidation chemistry of two butanol isomers, tert-butanol and isobutanol, in low-pressure (4-5.1 Torr) experiments at 550 and 700 K. We use pulsed-photolytic chlorine atom initiation to generate hydroxyalkyl radicals derived from tert-butanol and isobutanol, and probe the chemistry of these radicals in the presence of an excess of O2 by multiplexed time-resolved tunable synchrotron photoionization mass spectrometry. Isomer-resolved yields of stable products are determined, providing insight into the chemistry of the different hydroxyalkyl radicals. In isobutanol oxidation, we find that the reaction of the a-hydroxyalkyl radical with O2 is predominantly linked to chain-terminating formation of HO2. The Waddington mechanism, associated with chain-propagating formation of OH, is the main product channel in the reactions of O2 with b-hydroxyalkyl radicals derived from both tert-butanol and isobutanol. In the tert-butanol case, direct HO2 elimination is not possible in the b-hydroxyalkyl + O2 reaction because of the absence of a beta C-H bond; this channel is available in the b-hydroxyalkyl + O2 reaction for isobutanol, but we find that it is strongly suppressed. Observed evolution of the main products from 550 to 700 K can be qualitatively explained by an increasing role of hydroxyalkyl radical decomposition at 700 K. © 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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TYPE Journal Article YEAR 2013
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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
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Results 76–100 of 140
Results 76–100 of 140