Low Temperature Chlorine-Initiated Oxidation of Small-Chain Methyl Esters: Quantification of Chain-Terminating HO₂‑Elimination Channels

Cl-initiated oxidation reactions of three small-chain methyl esters, methyl propanoate (CH₃CH₂COOCH₃; MP), methyl butanoate (CH₃CH₂CH₂COOCH₃; MB), and methyl valerate (CH₃CH₂CH₂CH₂COOCH₃; MV), are studied at 1 or 8 Torr and 550 and 650 K. Products are monitored as a function of mass, time, and photoionization energy using multiplexed photoionization mass spectrometry coupled to tunable synchrotron photoionization radiation. Pulsed photolysis of molecular chlorine is the source of Cl radicals, which remove an H atom from the ester, forming a free radical. In each case, after addition of O₂ to the initial radicals, chain-terminating HO₂-elimination reactions are observed to be important. Branching ratios among competing HO₂-elimination channels are determined via absolute photoionization spectra of the unsaturated methyl ester coproducts. At 550 K, HO₂-elimination is observed to be selective, resulting in nearly exclusive production of the conjugated methyl ester coproducts, methyl propenoate, methyl-2-butenoate, and methyl-2-pentenoate, respectively. However, in MV, upon raising the temperature to 650 K, other HO₂-elimination pathways are observed that yield methyl-3-pentenoate and methyl-4-pentenoate. In each methyl ester oxidation reaction, a peak is observed at a mass consistent with cyclic ether formation, indicating chain-propagating OH loss/ring formation pathways via QOOH intermediates. Evidence is observed for the participation of resonance-stabilized QOOH in the most prominent cyclic ether pathways. Stationary point energies for HO₂-elimination pathways and select cyclic ether formation channels are calculated at the CBS-QB3 level of theory and assist in the assignment of reaction pathways and final products

Infrared Spectra of Gas-Phase 1- and 2‑Propenol Isomers

Fourier transform infrared spectra of isolated 1-propenol and 2-propenol in the gas-phase have been collected in the range of 900–3800 cm^–1, and the absolute infrared absorption cross sections reported for the first time. Both <i>cis</i> and <i>trans</i> isomers of 1-propenol were observed with the <i>trans</i> isomer in greater abundance. <i>Syn</i> and <i>anti</i> conformers of both 1- and 2-propenol were also observed, with abundance consistent with thermal population. The FTIR spectrum of the smaller ethenol (vinyl alcohol) was used as a benchmark for our computational results. As a consequence, its spectrum has been partially reassigned resulting in the first report of the <i>anti</i>-ethenol conformer. Electronic structure calculations were used to support our experimental results and assign vibrational modes for the most abundant isomers, <i>syn-trans</i>-1-propenol and <i>syn</i>-2-propenol

Synchrotron Photoionization Study of Mesitylene Oxidation Initiated by Reaction with Cl(²P) or O(³P) Radicals

This work studies the oxidation of mesitylene (1,3,5-trimethylbenzene) initiated by O­(³P) or Cl­(²P) atoms. The O­(³P) initiated mesitylene oxidation was investigated at room temperature and 823 K, whereas the Cl-initiated reaction was carried out at room temperature only. Products were probed by a multiplexed chemical kinetics photoionization mass spectrometer using the synchrotron radiation produced at the Advanced Light Source (ALS) of Lawrence Berkeley National Laboratory. Reaction products and intermediates are identified on the basis of their time behavior, mass-to-charge ratio, ionization energies, and photoionization spectra. Branching yields are derived for the O-initiated reaction at 823 K and the Cl-initiated reaction at room temperature. Reaction schematics are proposed and presented

Synchrotron Photoionization Measurements of OH-Initiated Cyclohexene Oxidation: Ring-Preserving Products in OH + Cyclohexene and Hydroxycyclohexyl + O₂ Reactions

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 H₂O₂ photolysis in the presence of C₆D₁₀ 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-<i>d</i>₄ confirms this result and allows mass discrimination of different abstraction pathways. Products of 2-hydroxycyclohexyl-<i>d</i>₁₀ reaction with O₂ are observed upon adding a large excess of O₂ to the OH + C₆D₁₀ system

Time- and Isomer-Resolved Measurements of Sequential Addition of Acetylene to the Propargyl Radical

Soot formation in combustion is a complex process in which polycyclic aromatic hydrocarbons (PAHs) are believed to play a critical role. Recent works concluded that three consecutive additions of acetylene (C₂H₂) to propargyl (C₃H₃) create a facile route to the PAH indene (C₉H₈). However, the isomeric forms of C₅H₅ and C₇H₇ intermediates in this reaction sequence are not known. We directly investigate these intermediates using time- and isomer-resolved experiments. Both the resonance stabilized vinylpropargyl (<i>vp</i>-C₅H₅) and 2,4-cyclopentadienyl (<i>c</i>-C₅H₅) radical isomers of C₅H₅ are produced, with substantially different intensities at 800 K vs 1000 K. In agreement with literature master equation calculations, we find that <i>c</i>-C₅H₅ + C₂H₂ produces only the tropyl isomer of C₇H₇ (<i>tp</i>-C₇H₇) below 1000 K, and that <i>tp</i>-C₇H₇ + C₂H₂ terminates the reaction sequence yielding C₉H₈ (indene) + H. This work demonstrates a pathway for PAH formation that does not proceed through benzene

Facile Rearrangement of 3‑Oxoalkyl Radicals is Evident in Low-Temperature Gas-Phase Oxidation of Ketones

The pulsed photolytic chlorine-initiated oxidation of methyl-<i>tert</i>-butyl ketone (MTbuK), di-<i>tert</i>-butyl ketone (DTbuK), and a series of partially deuterated diethyl ketones (DEK) is studied in the gas phase at 8 Torr and 550–650 K. Products are monitored as a function of reaction time, mass, and photoionization energy using multiplexed photoionization mass spectrometry with tunable synchrotron ionizing radiation. The results establish that the primary 3-oxoalkyl radicals of those ketones, formed by abstraction of a hydrogen atom from the carbon atom in γ-position relative to the carbonyl oxygen, undergo a rapid rearrangement resulting in an effective 1,2-acyl group migration, similar to that in a Dowd–Beckwith ring expansion. Without this rearrangement, peroxy radicals derived from MTbuK and DTbuK cannot undergo HO₂ elimination to yield a closed-shell unsaturated hydrocarbon coproduct. However, not only are these coproducts observed, but they represent the dominant oxidation channels of these ketones under the conditions of this study. For MTbuK and DTbuK, the rearrangement yields a more stable tertiary radical, which provides the thermodynamic driving force for this reaction. Even in the absence of such a driving force in the oxidation of partially deuterated DEK, the 1,2-acyl group migration is observed. Quantum chemical (CBS-QB3) calculations show the barrier for gas-phase rearrangement to be on the order of 10 kcal mol^–1. The MTbuK oxidation experiments also show several minor channels, including β-scission of the initial radicals and cyclic ether formation

Resonance Stabilization Effects on Ketone Autoxidation: Isomer-Specific Cyclic Ether and Ketohydroperoxide Formation in the Low-Temperature (400–625 K) Oxidation of Diethyl Ketone

The pulsed photolytic chlorine-initiated oxidation of diethyl ketone [DEK; (CH₃CH₂)₂CO], 2,2,4,4-<i>d</i>₄-DEK [<i>d</i>₄-DEK; (CH₃CD₂)₂CO], and 1,1,1,5,5,5-<i>d</i>₆-DEK [<i>d</i>₆-DEK; (CD₃CH₂)₂CO] is studied at 8 torr and 1–2 atm and from 400–625 K. Cl atoms produced by laser photolysis react with diethyl ketone to form either primary (3-pentan-on-1-yl, R_P) or secondary (3-pentan-on-2-yl, R_S) radicals, which in turn react with O₂. Multiplexed time-of-flight mass spectrometry, coupled to either a hydrogen discharge lamp or tunable synchrotron photoionizing radiation, is used to detect products as a function of mass, time, and photon energy. At 8 torr, the nature of the chain propagating cyclic ether + OH channel changes as a function of temperature. At 450 K, the production of OH is mainly in conjunction with formation of 2,4-dimethyloxetan-3-one, resulting from reaction of the resonance-stabilized secondary R_S with O₂. In contrast, at 550 K and 8 torr, 2-methyl-tetrahydrofuran-3-one, originating from oxidation of the primary radical (R_P), is observed as the dominant cyclic ether product. Formation of both of these cyclic ether production channels proceeds via a resonance-stabilized hydroperoxy alkyl (QOOH) intermediate. Little or no ketohydroperoxide (KHP) is observed under the low-pressure conditions. At higher O₂ concentrations and higher pressures (1–2 atm), a strong KHP signal appears as the temperature is increased above 450 K. Definitive isomeric identification from measurements on the deuterated DEK isotopologues indicates the favored pathway produces a γ-KHP via resonance-stabilized alkyl, QOOH, and HOOPOOH radicals. Time-resolved measurements reveal the KHP formation becomes faster and signal more intense upon increasing temperature from 450 to 575 K before intensity drops significantly at 625 K. The KHP time profile also shows a peak followed by a gradual depletion for the extent of experiment. Several tertiary products exhibit a slow accumulation in coincidence with the observed KHP decay. These products can be associated with decomposition of KHP by β-scission pathways or via isomerization of a γ-KHP into a cyclic peroxide intermediate (Korcek mechanism). The oxidation of <i>d</i>₄-DEK, where kinetic isotope effects disfavor γ-KHP formation, shows greatly reduced KHP formation and associated signatures from KHP decomposition products

Synchrotron Photoionization Mass Spectrometry Measurements of Product Formation in Low-Temperature <i>n</i>‑Butane Oxidation: Toward a Fundamental Understanding of Autoignition Chemistry and <i>n</i>‑C₄H₉ + O₂/<i>s</i>‑C₄H₉ + O₂ Reactions

Product formation in the laser-initiated low-temperature (575–700 K) oxidation of <i>n</i>-butane was investigated by using tunable synchrotron photoionization time-of-flight mass spectrometry at low pressure (∼4 Torr). Oxidation was triggered either by 351 nm photolysis of Cl₂ and subsequent fast Cl + <i>n</i>-butane reaction or by 248 nm photolysis of 1-iodobutane or 2-iodobutane. Iodobutane photolysis allowed isomer-specific preparation of either <i>n</i>-C₄H₉ or <i>s</i>-C₄H₉ radicals. Experiments probed the time-resolved formation of products and identified isomeric species by their photoionization spectra. For stable primary products of butyl + O₂ reactions (e.g., butene or oxygen heterocycles) bimodal time behavior is observed; the initial prompt formation, primarily due to chemical activation, is followed by a slower component arising from the dissociation of thermalized butylperoxy or hydroperoxybutyl radicals. In addition, time-resolved formation of C₄-ketohydroperoxides (C₄H₈O₃, <i>m</i>/<i>z</i> = 104) was observed in the <i>n</i>-C₄H₉ + O₂ and Cl-initiated oxidation experiments but not in the <i>s</i>-C₄H₉ + O₂ measurements, suggesting isomeric selectivity in the combined process of the “second” oxygen addition to hydroperoxybutyl radicals and subsequent internal H-abstraction/dissociation leading to ketohydroperoxide + OH. To further constrain product isomer identification, Cl-initiated oxidation experiments were also performed with partially deuterated <i>n</i>-butanes (CD₃CH₂CH₂CD₃ and CH₃CD₂CD₂CH₃). From these experiments, the relative yields of butene product isomers (<i>cis</i>-2-butene, <i>trans</i>-2-butene, and 1-butene) from C₄H₈ + HO₂ reaction channels and oxygenated product isomers (2,3-dimethyloxirane, 2-methyloxetane, tetrahydrofuran, ethyloxirane, butanal, and butanone) associated with OH formation were determined. The current measurements show substantially different isomeric selectivity for oxygenated products than do recent jet-stirred reactor studies but are in reasonable agreement with measurements from butane addition to reacting H₂/O₂ mixtures at 753 K

Facile Rearrangement of 3‑Oxoalkyl Radicals is Evident in Low-Temperature Gas-Phase Oxidation of Ketones

The pulsed photolytic chlorine-initiated oxidation of methyl-<i>tert</i>-butyl ketone (MTbuK), di-<i>tert</i>-butyl ketone (DTbuK), and a series of partially deuterated diethyl ketones (DEK) is studied in the gas phase at 8 Torr and 550–650 K. Products are monitored as a function of reaction time, mass, and photoionization energy using multiplexed photoionization mass spectrometry with tunable synchrotron ionizing radiation. The results establish that the primary 3-oxoalkyl radicals of those ketones, formed by abstraction of a hydrogen atom from the carbon atom in γ-position relative to the carbonyl oxygen, undergo a rapid rearrangement resulting in an effective 1,2-acyl group migration, similar to that in a Dowd–Beckwith ring expansion. Without this rearrangement, peroxy radicals derived from MTbuK and DTbuK cannot undergo HO₂ elimination to yield a closed-shell unsaturated hydrocarbon coproduct. However, not only are these coproducts observed, but they represent the dominant oxidation channels of these ketones under the conditions of this study. For MTbuK and DTbuK, the rearrangement yields a more stable tertiary radical, which provides the thermodynamic driving force for this reaction. Even in the absence of such a driving force in the oxidation of partially deuterated DEK, the 1,2-acyl group migration is observed. Quantum chemical (CBS-QB3) calculations show the barrier for gas-phase rearrangement to be on the order of 10 kcal mol^–1. The MTbuK oxidation experiments also show several minor channels, including β-scission of the initial radicals and cyclic ether formation

Low-Temperature Combustion Chemistry of <i>n-</i>Butanol: Principal Oxidation Pathways of Hydroxybutyl Radicals

Reactions of hydroxybutyl radicals with O₂ were investigated by a combination of quantum-chemical calculations and experimental measurements of product formation. In pulsed-photolytic Cl-initiated oxidation of <i>n</i>-butanol, the time-resolved and isomer-specific product concentrations were probed using multiplexed tunable synchrotron photoionization mass spectrometry (MPIMS). The interpretation of the experimental data is underpinned by potential energy surfaces for the reactions of O₂ with the four hydroxybutyl isomers (1-hydroxybut-1-yl, 1-hydroxybut-2-yl, 4-hydroxybut-2-yl, and 4-hydroxybut-1-yl) calculated at the CBS-QB3 and RQCISD(T)/cc-pV∞Z//B3LYP/6-311++G­(d,p) levels of theory. The observed product yields display substantial temperature dependence, arising from a competition among three fundamental pathways: (1) stabilization of hydroxybutylperoxy radicals, (2) bimolecular product formation in the hydroxybutyl + O₂ reactions, and (3) decomposition of hydroxybutyl radicals. The 1-hydroxybut-1-yl + O₂ reaction is dominated by direct HO₂ elimination from the corresponding peroxy radical forming butanal as the stable coproduct. The chemistry of the other three hydroxybutylperoxy radical isomers mainly proceeds via alcohol-specific internal H-atom abstractions involving the H atom from either the −OH group or from the carbon attached to the −OH group. We observe evidence of the recently reported water elimination pathway (Welz et al. <i>J. Phys. Chem. Lett.</i> <b>2013</b>, <i>4</i> (3), 350–354) from the 4-hydroxybut-2-yl + O₂ reaction, supporting its importance in γ-hydroxyalkyl + O₂ reactions. Experiments using the 1,1-<i>d</i>₂ and 4,4,4-<i>d</i>₃ isotopologues of <i>n</i>-butanol suggest the presence of yet unexplored pathways to acetaldehyde