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

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

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

Product Branching Fractions of the CH + Propene Reaction from Synchrotron Photoionization Mass Spectrometry

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 C₄H₆ (<i>m</i>/<i>z</i> 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 C₄H₆ (<i>m</i>/<i>z</i> 54) + D and C₄H₅D (<i>m</i>/<i>z</i> 55) + H, formed at a ratio of 0.4 (<i>m</i>/<i>z</i> 54) to 1.0 (<i>m</i>/<i>z</i> 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 <i>m</i>/<i>z</i> 55 (C₄H₅D) 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)

Isomer Specific Product Detection in the Reaction of CH with Acrolein

The products formed in the reaction between the methylidene radical (CH) and acrolein (CH₂CHCHO) are probed at 4 Torr and 298 K employing tunable vacuum-ultraviolet synchrotron light and multiplexed photoionization mass-spectrometry. The data suggest a principal exit channel of H loss from the adduct to yield C₄H₄O, accounting for (78 ± 10)% of the products. Examination of the photoionization spectra measured upon reaction of both CH and CD with acrolein reveals that the isomeric composition of the C₄H₄O product is (60 ± 12)% 1,3-butadienal and (17 ± 10)% furan. The remaining 23% of the possible C₄H₄O products cannot be accurately distinguished without more reliable photoionization spectra of the possible product isomers but most likely involves oxygenated butyne species. In addition, C₂H₂O and C₃H₄ are detected, which account for (14 ± 10)% and (8 +10, −8)% of the products, respectively. The C₂H₂O photoionization spectrum matches that of ketene and the C₃H₄ signal is composed of (24 ± 14)% allene and (76 ± 22)% propyne, with an upper limit of 8% placed on the cyclopropene contribution. The reactive potential energy surface is also investigated computationally, and specific rate coefficients are calculated with RRKM theory. These calculations predict overall branching fractions for 1,3-butadienal and furan of 27% and 12%, respectively, in agreement with the experimental results. In contrast, the calculations predict a prominent CO + 2-methylvinyl product channel that is at most a minor channel according to the experimental results. Studies with the CD radical strongly suggest that the title reaction proceeds predominantly via cycloaddition of the radical onto the CO bond of acrolein, with cycloaddition to the CC bond being the second most probable reactive mechanism

Product Detection of the CH Radical Reaction with Acetaldehyde

The reaction of the methylidyne radical (CH) with acetaldehyde (CH₃CHO) is studied at room temperature and at a pressure of 4 Torr (533.3 Pa) using a multiplexed photoionization mass spectrometer coupled to the tunable vacuum ultraviolet synchrotron radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory. The CH radicals are generated by 248 nm multiphoton photolysis of CHBr₃ and react with acetaldehyde in an excess of helium and nitrogen gas flow. Five reaction exit channels are observed corresponding to elimination of methylene (CH₂), elimination of a formyl radical (HCO), elimination of carbon monoxide (CO), elimination of a methyl radical (CH₃), and elimination of a hydrogen atom. Analysis of the photoionization yields versus photon energy for the reaction of CH and CD radicals with acetaldehyde and CH radical with partially deuterated acetaldehyde (CD₃CHO) provides fine details about the reaction mechanism. The CH₂ elimination channel is found to preferentially form the acetyl radical by removal of the aldehydic hydrogen. The insertion of the CH radical into a C–H bond of the methyl group of acetaldehyde is likely to lead to a C₃H₅O reaction intermediate that can isomerize by β-hydrogen transfer of the aldehydic hydrogen atom and dissociate to form acrolein + H or ketene + CH₃, which are observed directly. Cycloaddition of the radical onto the carbonyl group is likely to lead to the formation of the observed products, methylketene, methyleneoxirane, and acrolein

VUV Photoionization Cross Sections of HO₂, H₂O₂, and H₂CO

The absolute vacuum ultraviolet (VUV) photoionization spectra of the hydroperoxyl radical (HO₂), hydrogen peroxide (H₂O₂), and formaldehyde (H₂CO) have been measured from their first ionization thresholds to 12.008 eV. HO₂, H₂O₂, and H₂CO were generated from the oxidation of methanol initiated by pulsed-laser-photolysis of Cl₂ in a low-pressure slow flow reactor. Reactants, intermediates, and products were detected by time-resolved multiplexed synchrotron photoionization mass spectrometry. Absolute concentrations were obtained from the time-dependent photoion signals by modeling the kinetics of the methanol oxidation chemistry. Photoionization cross sections were determined at several photon energies relative to the cross section of methanol, which was in turn determined relative to that of propene. These measurements were used to place relative photoionization spectra of HO₂, H₂O₂, and H₂CO on an absolute scale, resulting in absolute photoionization spectra

New Insights into Low-Temperature Oxidation of Propane from Synchrotron Photoionization Mass Spectrometry and Multiscale Informatics Modeling

Low-temperature propane oxidation was studied at <i>P</i> = 4 Torr and <i>T</i> = 530, 600, and 670 K by time-resolved multiplexed photoionization mass spectrometry (MPIMS), which probes the reactants, intermediates, and products with isomeric selectivity using tunable synchrotron vacuum UV ionizing radiation. The oxidation is initiated by pulsed laser photolysis of oxalyl chloride, (COCl)₂, at 248 nm, which rapidly generates a ∼1:1 mixture of 1-propyl (<i>n</i>-propyl) and 2-propyl (<i>i</i>-propyl) radicals via the fast Cl + propane reaction. At all three temperatures, the major stable product species is propene, formed in the propyl + O₂ reactions by direct HO₂ elimination from both <i>n</i>- and <i>i</i>-propyl peroxy radicals. The experimentally derived propene yields relative to the initial concentration of Cl atoms are (20 ± 4)% at 530 K, (55 ± 11)% at 600 K, and (86 ± 17)% at 670 K at a reaction time of 20 ms. The lower yield of propene at low temperature reflects substantial formation of propyl peroxy radicals, which do not completely decompose on the experimental time scale. In addition, C₃H₆O isomers methyloxirane, oxetane, acetone, and propanal are detected as minor products. Our measured yields of oxetane and methyloxirane, which are coproducts of OH radicals, suggest a revision of the OH formation pathways in models of low-temperature propane oxidation. The experimental results are modeled and interpreted using a multiscale informatics approach, presented in detail in a separate publication (Burke, M. P.; Goldsmith, C. F.; Klippenstein, S. J.; Welz, O.; Huang H.; Antonov I. O.; Savee J. D.; Osborn D. L.; Zádor, J.; Taatjes, C. A.; Sheps, L. Multiscale Informatics for Low-Temperature Propane Oxidation: Further Complexities in Studies of Complex Reactions. <i>J. Phys. Chem A.</i> <b>2015</b>, DOI: 10.1021/acs.jpca.5b01003). The model predicts the time profiles and yields of the experimentally observed primary products well, and shows satisfactory agreement for products formed mostly via secondary radical–radical reactions

Radical–Radical Reactions in Molecular Weight Growth: The Phenyl + Propargyl Reaction

The mechanism for hydrocarbon ring growth in sooting environments is still the subject of considerable debate. The reaction of phenyl radical (C6H5) with propargyl radical (H2CCCH) provides an important prototype for radical–radical ring-growth pathways. We studied this reaction experimentally over the temperature range of 300–1000 K and pressure range of 4–10 Torr using time-resolved multiplexed photoionization mass spectrometry. We detect both the C9H8 and C9H7 + H product channels and report experimental isomer-resolved product branching fractions for the C9H8 product. We compare these experiments to theoretical kinetics predictions from a recently published study augmented by new calculations. These ab initio transition state theory-based master equation calculations employ high-quality potential energy surfaces, conventional transition state theory for the tight transition states, and direct CASPT2-based variable reaction coordinate transition state theory (VRC-TST) for the barrierless channels. At 300 K only the direct adducts from radical–radical addition are observed, with good agreement between experimental and theoretical branching fractions, supporting the VRC-TST calculations of the barrierless entrance channel. As the temperature is increased to 1000 K we observe two additional isomers, including indene, a two-ring polycyclic aromatic hydrocarbon, and a small amount of bimolecular products C9H7 + H. Our calculated branching fractions for the phenyl + propargyl reaction predict significantly less indene than observed experimentally. We present further calculations and experimental evidence that the most likely cause of this discrepancy is the contribution of H atom reactions, both H + indenyl (C9H7) recombination to indene and H-assisted isomerization that converts less stable C9H8 isomers into indene. Especially at low pressures typical of laboratory investigations, H-atom-assisted isomerization needs to be considered. Regardless, the experimental observation of indene demonstrates that the title reaction leads, either directly or indirectly, to the formation of the second ring in polycyclic aromatic hydrocarbons

Multiscale Informatics for Low-Temperature Propane Oxidation: Further Complexities in Studies of Complex Reactions

The present paper describes further development of the multiscale informatics approach to kinetic model formulation of Burke et al. (Burke, M. P.; Klippenstein, S. J.; Harding, L. B. <i>Proc. Combust. Inst.</i> <b>2013</b>, <i>34</i>, 547–555) that directly incorporates elementary kinetic theories as a means to provide reliable, physics-based extrapolation of kinetic models to unexplored conditions. Here, we extend and generalize the multiscale informatics strategy to treat systems of considerable complexityinvolving multiwell reactions, potentially missing reactions, nonstatistical product branching ratios, and non-Boltzmann (i.e., nonthermal) reactant distributions. The methodology is demonstrated here for a subsystem of low-temperature propane oxidation, as a representative system for low-temperature fuel oxidation. A multiscale model is assembled and informed by a wide variety of targets that include <i>ab initio</i> calculations of molecular properties, rate constant measurements of isolated reactions, and complex systems measurements. Active model parameters are chosen to accommodate both “parametric” and “structural” uncertainties. Theoretical parameters (e.g., barrier heights) are included as active model parameters to account for parametric uncertainties in the theoretical treatment; experimental parameters (e.g., initial temperatures) are included to account for parametric uncertainties in the physical models of the experiments. RMG software is used to assess potential structural uncertainties due to missing reactions. Additionally, branching ratios among product channels are included as active model parameters to account for structural uncertainties related to difficulties in modeling sequences of multiple chemically activated steps. The approach is demonstrated here for interpreting time-resolved measurements of OH, HO₂, <i>n</i>-propyl, <i>i</i>-propyl, propene, oxetane, and methyloxirane from photolysis-initiated low-temperature oxidation of propane at pressures from 4 to 60 Torr and temperatures from 300 to 700 K. In particular, the multiscale informed model provides a consistent quantitative explanation of both <i>ab initio</i> calculations and time-resolved species measurements. The present results show that interpretations of OH measurements are significantly more complicated than previously thoughtin addition to barrier heights for key transition states considered previously, OH profiles also depend on additional theoretical parameters for R + O₂ reactions, secondary reactions, QOOH + O₂ reactions, and treatment of non-Boltzmann reaction sequences. Extraction of physically rigorous information from those measurements may require more sophisticated treatment of all of those model aspects, as well as additional experimental data under more conditions, to discriminate among possible interpretations and ensure model reliability