6 research outputs found
Pressure-Dependent Competition among Reaction Pathways from First- and Second‑O<sub>2</sub> Additions in the Low-Temperature Oxidation of Tetrahydrofuran
We
report a combined experimental and quantum chemistry study of
the initial reactions in low-temperature oxidation of tetrahydrofuran
(THF). Using synchrotron-based time-resolved VUV photoionization mass
spectrometry, we probe numerous transient intermediates and products
at <i>P</i> = 10–2000 Torr and <i>T</i> = 400–700 K. A key reaction sequence, revealed by our experiments,
is the conversion of THF-yl peroxy to hydroperoxy-THF-yl radicals
(QOOH), followed by a second O<sub>2</sub> addition and subsequent
decomposition to dihydrofuranyl hydroperoxide + HO<sub>2</sub> or
to γ-butyrolactone hydroperoxide + OH. The competition between
these two pathways affects the degree of radical chain-branching and
is likely of central importance in modeling the autoignition of THF.
We interpret our data with the aid of quantum chemical calculations
of the THF-yl + O<sub>2</sub> and QOOH + O<sub>2</sub> potential energy
surfaces. On the basis of our results, we propose a simplified THF
oxidation mechanism below 700 K, which involves the competition among
unimolecular decomposition and oxidation pathways of QOOH
Hydroxyacetone Production From C<sub>3</sub> Criegee Intermediates
Hydroxyacetone
(CH<sub>3</sub>C(O)CH<sub>2</sub>OH) is observed as a stable end product
from reactions of the (CH<sub>3</sub>)<sub>2</sub>COO Criegee intermediate,
acetone oxide, in a flow tube coupled with multiplexed photoionization
mass spectrometer detection. In the experiment, the isomers at <i>m</i>/<i>z</i> = 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
Simulation of the VUV Absorption Spectra of Oxygenates and Hydrocarbons: A Joint Theoretical–Experimental Study
Vacuum UV absorption spectroscopy is regularly used to
provide
unambiguous identification of a target species, insight into the electronic
structure of molecules, and quantitative species concentrations. As
molecules of interest have become more complex, theoretical spectra
have been used in tandem with laboratory spectroscopic analysis or
as a replacement when experimental data is unavailable. However, it
is difficult to determine which theoretical methodologies can best
simulate experiment. This study examined the performance of EOM-CCSD
and 10 TD-DFT functionals (B3LYP, BH&HLYP, BMK, CAM-B3LYP, HSE,
M06-2X, M11, PBE0, ωB97X-D, and X3LYP) to produce reliable vacuum
UV absorption spectra for 19 small oxygenates and hydrocarbons using
vertical excitation energies. The simulated spectra were analyzed
against experiment using both a qualitative analysis and quantitative
metrics, including cosine similarity, relative integral change, mean
signed error, and mean absolute error. Based on our ranking system,
it was determined that M06-2X was consistently the top performing
TD-DFT method with BMK, CAM-B3LYP, and ωB97X-D also producing
reliable spectra for these small combustion species
Simulation of the VUV Absorption Spectra of Oxygenates and Hydrocarbons: A Joint Theoretical–Experimental Study
Vacuum UV absorption spectroscopy is regularly used to
provide
unambiguous identification of a target species, insight into the electronic
structure of molecules, and quantitative species concentrations. As
molecules of interest have become more complex, theoretical spectra
have been used in tandem with laboratory spectroscopic analysis or
as a replacement when experimental data is unavailable. However, it
is difficult to determine which theoretical methodologies can best
simulate experiment. This study examined the performance of EOM-CCSD
and 10 TD-DFT functionals (B3LYP, BH&HLYP, BMK, CAM-B3LYP, HSE,
M06-2X, M11, PBE0, ωB97X-D, and X3LYP) to produce reliable vacuum
UV absorption spectra for 19 small oxygenates and hydrocarbons using
vertical excitation energies. The simulated spectra were analyzed
against experiment using both a qualitative analysis and quantitative
metrics, including cosine similarity, relative integral change, mean
signed error, and mean absolute error. Based on our ranking system,
it was determined that M06-2X was consistently the top performing
TD-DFT method with BMK, CAM-B3LYP, and ωB97X-D also producing
reliable spectra for these small combustion species
Photoionization Mass Spectrometric Measurements of Initial Reaction Pathways in Low-Temperature Oxidation of 2,5-Dimethylhexane
Product formation from R + O<sub>2</sub> reactions relevant to
low-temperature autoignition chemistry was studied for 2,5-dimethylhexane,
a symmetrically branched octane isomer, at 550 and 650 K using Cl-atom
initiated oxidation and multiplexed photoionization mass spectrometry
(MPIMS). Interpretation of time- and photon-energy-resolved mass spectra
led to three specific results important to characterizing the initial
oxidation steps: (1) quantified isomer-resolved branching ratios for
HO<sub>2</sub> + alkene channels; (2) 2,2,5,5-tetramethyltetrahydrofuran
is formed in substantial yield from addition of O<sub>2</sub> to tertiary
2,5-dimethylhex-2-yl followed by isomerization of the resulting ROO
adduct to tertiary hydroperoxyalkyl (QOOH) and exhibits a positive
dependence on temperature over the range covered leading to a higher
flux relative to aggregate cyclic ether yield. The higher relative
flux is explained by a 1,5-hydrogen atom shift reaction that converts
the initial primary alkyl radical (2,5-dimethylhex-1-yl) to the tertiary
alkyl radical 2,5-dimethylhex-2-yl, providing an additional source
of tertiary alkyl radicals. Quantum-chemical and master-equation calculations
of the unimolecular decomposition of the primary alkyl radical reveal
that isomerization to the tertiary alkyl radical is the most favorable
pathway, and is favored over O<sub>2</sub>-addition at 650 K under
the conditions herein. The isomerization pathway to tertiary alkyl
radicals therefore contributes an additional mechanism to 2,2,5,5-tetramethyltetrahydrofuran
formation; (3) carbonyl species (acetone, propanal, and methylpropanal)
consistent with β-scission of QOOH radicals were formed in significant
yield, indicating unimolecular QOOH decomposition into carbonyl +
alkene + OH
Direct Measurements of Unimolecular and Bimolecular Reaction Kinetics of the Criegee Intermediate (CH<sub>3</sub>)<sub>2</sub>COO
The
Criegee intermediate acetone oxide, (CH<sub>3</sub>)<sub>2</sub>COO,
is formed by laser photolysis of 2,2-diiodopropane in the presence
of O<sub>2</sub> and characterized by synchrotron photoionization
mass spectrometry and by cavity ring-down ultraviolet absorption spectroscopy.
The rate coefficient of the reaction of the Criegee intermediate with
SO<sub>2</sub> was measured using photoionization mass spectrometry
and pseudo-first-order methods to be (7.3 ± 0.5) × 10<sup>–11</sup> cm<sup>3</sup> s<sup>–1</sup> at 298 K and
4 Torr and (1.5 ± 0.5) × 10<sup>–10</sup> cm<sup>3</sup> s<sup>–1</sup> at 298 K and 10 Torr (He buffer). These
values are similar to directly measured rate coefficients of <i>anti</i>-CH<sub>3</sub>CHOO with SO<sub>2</sub>, and in good
agreement with recent UV absorption measurements. The measurement
of this reaction at 293 K and slightly higher pressures (between 10
and 100 Torr) in N<sub>2</sub> from cavity ring-down decay of the
ultraviolet absorption of (CH<sub>3</sub>)<sub>2</sub>COO yielded
even larger rate coefficients, in the range (1.84 ± 0.12) ×
10<sup>–10</sup> to (2.29 ± 0.08) × 10<sup>–10</sup> cm<sup>3</sup> s<sup>–1</sup>. Photoionization mass spectrometry
measurements with deuterated acetone oxide at 4 Torr show an inverse
deuterium kinetic isotope effect, <i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> = (0.53 ± 0.06), for reactions with SO<sub>2</sub>, which may be consistent with recent suggestions that the
formation of an association complex affects the rate coefficient.
The reaction of (CD<sub>3</sub>)<sub>2</sub>COO with NO<sub>2</sub> has a rate coefficient at 298 K and 4 Torr of (2.1 ± 0.5) ×
10<sup>–12</sup> cm<sup>3</sup> s<sup>–1</sup> (measured
with photoionization mass spectrometry), again similar to rate for
the reaction of <i>anti</i>-CH<sub>3</sub>CHOO with NO<sub>2</sub>. Cavity ring-down measurements of the acetone oxide removal
without added reagents display a combination of first- and second-order
decay kinetics, which can be deconvolved to derive values for both
the self-reaction of (CH<sub>3</sub>)<sub>2</sub>COO and its unimolecular
thermal decay. The inferred unimolecular decay rate coefficient at
293 K, (305 ± 70) s<sup>–1</sup>, is similar to determinations
from ozonolysis. The present measurements confirm the large rate coefficient
for reaction of (CH<sub>3</sub>)<sub>2</sub>COO with SO<sub>2</sub> and the small rate coefficient for its reaction with water. Product
measurements of the reactions of (CH<sub>3</sub>)<sub>2</sub>COO with
NO<sub>2</sub> and with SO<sub>2</sub> suggest that these reactions
may facilitate isomerization to 2-hydroperoxypropene, possibly by
subsequent reactions of association products