Publications

Results 101–140 of 140
Skip to search filters

Synchrotron photoionization measurements of OH-initiated cyclohexene oxidation: Ring-preserving products in OH + cyclohexene and hydroxycyclohexyl + O 2 reactions

Journal of Physical Chemistry A

Ray, Amelia W.; Taatjes, Craig A.; Welz, Oliver W.; Osborn, David L.; Meloni, Giovanni

Earlier synchrotron photoionization mass spectrometry experiments suggested a prominent ring-opening channel in the OH-initiated oxidation of cyclohexene, based on comparison of product photoionization spectra with calculated spectra of possible isomers. The present work re-examines the OH + cyclohexene reaction, measuring the isomeric products of OH-initiated oxidation of partially and fully deuterated cyclohexene. In particular, the directly measured photoionization spectrum of 2-cyclohexen-1-ol differs substantially from the previously calculated Franck-Condon envelope, and the product spectrum can be fit with no contribution from ring-opening. Measurements of H 2O 2 photolysis in the presence of C 6D 10 establish that the addition-elimination product incorporates the hydrogen atom from the hydroxyl radical reactant and loses a hydrogen (a D atom in this case) from the ring. Investigation of OH + cyclohexene-4,4,5,5-d 4 confirms this result and allows mass discrimination of different abstraction pathways. Products of 2-hydroxycyclohexyl-d 10 reaction with O 2 are observed upon adding a large excess of O 2 to the OH + C 6D 10 system. © 2012 American Chemical Society.

More Details
TYPE Journal Article YEAR 2012
ScopusOSTI

Low-temperature combustion chemistry of biofuels: Pathways in the initial low-temperature (550 K-750 K) oxidation chemistry of isopentanol

Physical Chemistry Chemical Physics

Welz, Oliver W.; Zador, Judit Z.; Savee, John D.; Ng, Martin Y.; Meloni, Giovanni; Fernandes, Ravi X.; Sheps, Leonid S.; Simmons, Blake S.; Lee, Taek S.; Osborn, David L.; Taatjes, Craig A.

The branched C 5 alcohol isopentanol (3-methylbutan-1-ol) has shown promise as a potential biofuel both because of new advanced biochemical routes for its production and because of its combustion characteristics, in particular as a fuel for homogeneous-charge compression ignition (HCCI) or related strategies. In the present work, the fundamental autoignition chemistry of isopentanol is investigated by using the technique of pulsed-photolytic Cl-initiated oxidation and by analyzing the reacting mixture by time-resolved tunable synchrotron photoionization mass spectrometry in low-pressure (8 Torr) experiments in the 550-750 K temperature range. The mass-spectrometric experiments reveal a rich chemistry for the initial steps of isopentanol oxidation and give new insight into the low-temperature oxidation mechanism of medium-chain alcohols. Formation of isopentanal (3-methylbutanal) and unsaturated alcohols (including enols) associated with HO 2 production was observed. Cyclic ether channels are not observed, although such channels dominate OH formation in alkane oxidation. Rather, products are observed that correspond to formation of OH via β-C-C bond fission pathways of QOOH species derived from β- and γ-hydroxyisopentylperoxy (RO 2) radicals. In these pathways, internal hydrogen abstraction in the RO 2 QOOH isomerization reaction takes place from either the -OH group or the C-H bond in α-position to the -OH group. These pathways should be broadly characteristic for longer-chain alcohol oxidation. Isomer-resolved branching ratios are deduced, showing evolution of the main products from 550 to 750 K, which can be qualitatively explained by the dominance of RO 2 chemistry at lower temperature and hydroxyisopentyl decomposition at higher temperature. © 2012 The Owner Societies.

More Details
TYPE Journal Article YEAR 2012
ScopusOSTI

Temperature-dependent kinetics of the vinyl radical (C2H3) self-reaction

Proposed for publication in the Journal of Physical Chemistry A.

Taatjes, Craig A.; Zador, Judit Z.; Osborn, David L.; Selby, Talitha S.; Jusinski, Leonard E.

The rate coefficient for the self-reaction of vinyl radicals has been measured by two independent methods. The rate constant as a function of temperature at 20 Torr has been determined by a laser-photolysis/laser absorption technique. Vinyl iodide is photolyzed at 266 nm, and both the vinyl radical and the iodine atom photolysis products are monitored by laser absorption. The vinyl radical concentration is derived from the initial iodine atom concentration, which is determined by using the known absorption cross section of the iodine atomic transition to relate the observed absorption to concentration. The measured rate constant for the self-reaction at room temperature is approximately a factor of 2 lower than literature recommendations. The reaction displays a slightly negative temperature dependence, which can be represented by a negative activation energy, (E{sub a}/R) = -400 K. The laser absorption results are supported by independent experiments at 298 K and 4 Torr using time-resolved synchrotron-photoionization mass-spectrometric detection of the products of divinyl ketone and methyl vinyl ketone photolysis. The photoionization mass spectrometry experiments additionally show that methyl + propargyl are formed in the vinyl radical self-reaction, with an estimated branching fraction of 0.5 at 298 K and 4 Torr.

More Details
TYPE Journal Article YEAR 2008
OSTI

The vinyl + NO Reaction: Determining the products with time-resolved fourier transform spectroscopy

Journal of Physical Chemistry A

Zou, Peng; Klippenstein, Stephen J.; Osborn, David L.

We have studied the vinyl + NO reaction using time-resolved Fourier transform emission spectroscopy, complemented by electronic structure and microcanonical RRKM rate coefficient calculations. To unambiguously determine the reaction products, three precursors are used to produce the vinyl radical by laser photolysis: vinyl bromide, methyl vinyl ketone, and vinyl iodide. The emission spectra and theoretical calculations indicate that HCN + CH 2O is the only significant product channel for the C 2H 3 + NO reaction near room temperature, in contradiction to several reports in the literature. Although CO emission is observed when vinyl bromide is used as the precursor, it arises from the reaction of NO with photofragments other than vinyl. This conclusion is supported by the absence of CO emission when vinyl iodide or methyl vinyl ketone is used. Prompt emission from vibrationally excited NO is evidence of the competition between back dissociation and isomerization of the initially formed nitrosoethylene adduct, consistent with previous work on the pressure dependence of this reaction. Our calculations indicate that production of products is dominated by the low energy portion of the energy distribution. The calculation also predicts an upper bound of 0.19% for the branching ratio of the H 2CNH + CO channel, which is consistent with our experimental results. © 2005 American Chemical Society.

More Details
TYPE Journal Article YEAR 2005
ScopusOSTI
Results 101–140 of 140
Results 101–140 of 140