Hydroxyacetone (CH3C(O)CH2OH) is observed as a stable end product from reactions of the (CH3)2COO Criegee intermediate, acetone oxide, in a flow tube coupled with multiplexed photoionization mass spectrometer detection. In the experiment, the isomers at m/z = 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.
Lignocellulosic-derived biofuels represent an important part of sustainable transportation en- ergy and often contain oxygenated functional groups due to the mono- and polysaccharide content in cellulose and hemicellulose. The yields of conjugate alkene + HO2 formation in low-temperature tetrahydropyran oxidation were studied and the influence of oxygen heteroatoms in cyclic hydrocarbons on the associated chain-termination pathways stemming from R + O2 were examined. Relative to the initial radical concentration the trend in conjugate alkene branching fraction showed monotonic positive temperature dependence in both cyclohexane and tetrahydropyran except for tetrahydropyran at 10 torr where increasing the temperature to 700 K caused a decrease. Conjugate alkene branching fractions measured at 1520 torr for cyclohexane and tetrahydropyran followed monotonic positive temperature dependence. In contrast to the results at higher temperature where ring-opening of tetrahydropyranyl radicals interrupted R + O2chemistry and reduces the formation of conjugate alkenes branching fractions measured below 700 K were higher in tetrahydropyran compared to cyclohexane at 10 torr.
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 P = 10-2000 Torr and T = 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 O2 addition and subsequent decomposition to dihydrofuranyl hydroperoxide + HO2 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 + O2 and QOOH + O2 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.
Time-resolved two-tone frequency modulation (TTFM) absorption spectroscopy has been used to measure, in situ and quantitatively, hydroperoxy (HO2) radical in fuel oxidation reactions at the first overtone transitions (2v1) of HO2 near 1509nm. Typical HO2 detection limit is on the order of 1011 molecule cm-3, which corresponds to a relative absorption of 10-5. TTFM method successfully removes low frequency thermal lensing noise in measured HO2 kinetic time traces, which is a general noise source in fuel oxidation absorption experiments. Compared with previous works, we have upgraded the TTFM experiment with a NIR distributed feedback (DFB) diode laser, a fiber-coupled broadband phase modulator, and a two-channel wave generator, which have improve the performance of our experiment substantially.
Oxidation of organic compounds in combustion and in Earth's troposphere is mediated by reactive species formed by the addition of molecular oxygen (O2) to organic radicals. Among the most crucial and elusive of these intermediates are hydroperoxyalkyl radicals, often denoted "QOOH." These species and their reactions with O2 are responsible for the radical chain branching that sustains autoignition and are implicated in tropospheric autoxidation that can form low-volatility, highly oxygenated organic aerosol precursors. We report direct observation and kinetics measurements of a QOOH intermediate in the oxidation of 1,3-cycloheptadiene, a molecule that offers insight into both resonance-stabilized and nonstabilized radical intermediates. The results establish that resonance stabilization dramatically changes QOOH reactivity and, hence, that oxidation of unsaturated organics can produce exceptionally long-lived QOOH intermediates.
Cymenes are monoterpene derivatives composed of a benzene ring with iso-propyl and methyl substituents, and the proximity of the two alkyl groups enables relevant analysis into the ortho-effect of polysubstituted aromatics, in which low-temperature autoignition is more facile for ortho-substitution. The initial steps of ROO-related reactions from Cl-initiated oxidation of ortho-, meta- and para-cymene were studied at low pressure (8 Torr) over the temperature range 450-750 K using multiplexed photoionization mass spectrometry (MPIMS). Ratios of cyclic ether formation (related to chain-propagation and OH formation) relative to HO2-loss (related to chain-termination) were measured to characterize the ortho-effect as a function of temperature. The main results are twofold: Cyclic ether measurements indicate significant chain-propagation below 700 K only in o-cymene oxidation; above 700 K, chain-propagation of the three cymene isomers converge.The competition between chain-propagation channels stemming from resonance- and non-resonance-stabilized initial cymene radicals changes significantly with temperature, with chain-propagation near 700 K arising predominantly from non-resonance-stabilized R radicals. Cyclic ether yields in m- and p-cymene oxidation are negligible below 650 K, indicating minimal chain-propagation. In contrast, o-cymene exhibits significant cyclic ether formation, attributed to the favorable 6- and 7-membered-ring transition states in the formation of hydroperoxyalkyl (QOOH) intermediates from peroxy radicals (ROO). The cyclic ether/HO2-loss ratio in o-cymene oxidation is defined as unity at 450 K and decreases to ∼0.10 at 700 K. The ratio converges to ∼0.10 at 700 K for the three cymene isomers, indicating an upper limit of temperature for the ortho-effect. Photoionization spectra of cyclic ether formation from o-cymene oxidation indicate a competition between both resonance- and non-resonance-stabilized initial radicals at the lower range of temperature. With increasing temperature, cyclic ether formation shifts from pathways involving resonance-stabilized initial radicals towards pathways solely from non-resonance-stabilized initial radicals because of back-dissociation of weakly bound ROO adducts.
Product formation from R + O2 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 HO2 + alkene channels; (2) 2,2,5,5-tetramethyltetrahydrofuran is formed in substantial yield from addition of O2 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 O2-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. (Chemical Equation Pesented).
Combined with a Herriott-type multi-pass slow flow reactor, high-resolution differential direct absorption spectroscopy has been used to probe, in situ and quantitatively, hydroxyl (OH), hydroperoxy (HO 2 ) and formaldehyde (CH 2 O) molecules in fuel oxidation reactions in the reactor, with a time resolution of about 1 micro-second. While OH and CH 2 O are probed in the mid-infrared (MIR) region near 2870nm and 3574nm respectively, HO 2 can be probed in both regions: near-infrared (NIR) at 1509nm and MIR at 2870nm. Typical sensitivities are on the order of 10 10 - 10 11 molecule cm -3 for OH at 2870nm, 10 11 molecule cm -3 for HO 2 at 1509nm, and 10 11 molecule cm -3 for CH 2 O at 3574nm. Measurements of multiple important intermediates (OH and HO 2 ) and product (CH 2 O) facilitate to understand and further validate chemical mechanisms of fuel oxidation chemistry.