Phenyl radical + propene: a prototypical reaction surface for aromatic-catalyzed 1,2-hydrogen-migration and subsequent resonance-stabilized radical formation

The C[subscript 9]H[subscript 11] potential energy surface (PES) was experimentally and theoretically explored because it is a relatively simple, prototypical alkylaromatic radical system. Although the C[subscript 9]H[subscript 11] PES has already been extensively studied both experimentally (under single-collision and thermal conditions) and theoretically, new insights were made in this work by taking a new experimental approach: flash photolysis combined with time-resolved molecular beam mass spectrometry (MBMS) and visible laser absorbance. The C[subscript 9]H[subscript 11] PES was experimentally accessed by photolytic generation of the phenyl radical and subsequent reaction with excess propene (C[subscript 6]H[subscript 5] + C[subscript 3]H[subscript 6]). The overall kinetics of C[subscript 6]H[subscript 5] + C[subscript 3]H[subscript 6] was measured using laser absorbance with high time-resolution from 300 to 700 K and was found to be in agreement with earlier measurements over a lower temperature range. Five major product channels of C[subscript 6]H[subscript 5] + C[subscript 3]H[subscript 6] were observed with MBMS at 600 and 700 K, four of which were expected: hydrogen (H)-abstraction (measured by the stable benzene, C[subscript 6]H[subscript 6], product), methyl radical (CH[subscript 3])-loss (styrene detected), H-loss (phenylpropene isomers detected) and radical adduct stabilization. The fifth, unexpected product observed was the benzyl radical, which was rationalized by the inclusion of a previously unreported pathway on the C[subscript 9]H[subscript 11] PES: aromatic-catalysed 1,2-H-migration and subsequent resonance stabilized radical (RSR, benzyl radical in this case) formation. The current theoretical understanding of the C[subscript 9]H[subscript 11] PES was supported (including the aromatic-catalyzed pathway) by quantitative comparisons between modelled and experimental MBMS results. At 700 K, the branching to styrene + CH[subscript 3] was 2-4 times greater than that of any other product channel, while benzyl radical + C[subscript 2]H[subscript 4] from the aromatic-catalyzed pathway accounted for ∼10% of the branching. Single-collision conditions were also simulated on the updated PES to explain why previous crossed molecular beam experiments did not see evidence of the aromatic-catalyzed pathway. This experimentally validated knowledge of the C[subscript 9]H[subscript 11] PES was added to the database of the open-source Reaction Mechanism Generator (RMG), which was then used to generalize the findings on the C[subscript 9]H[subscript 11] PES to a slightly more complicated alkylaromatic system.Think Global Education Trus

Measuring rate constants and product branching for reactions relevant to combustion and atmospheric chemistry

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, 2018.Cataloged from PDF version of thesis.Includes bibliographical references (pages [383]-406).Over the last century there have been countless experimental measurements of the overall reaction kinetics of gas-phase radicals, often with the aid of lasers. In more recent decades, ab initio predictions of product branching using quantum chemical calculations combined with modem rate theories have become common. However, there are few experimental measurements against which to validate predicted product branching, even for an important reaction system such as hydroxyl radical addition to acetylene that is critical to oxidation chemistry both in the atmosphere and in combustion. As a result, many of the kinetic parameters that appear in commonly used combustion mechanisms are based purely on predictions. The few experiments that do attempt to quantify product branching generally fall into two categories, each with unique advantages/disadvantages: crossed molecular beams (CMB) that simulate single collision conditions, or end-product analysis of a complex, thermalized process, such as pyrolysis. Laser flash photolysis (LFP) with molecular beam mass spectrometry (MBMS) offers a compromise between CMB and end-product experiments: the reaction conditions are thermalized but still simple enough that primary products can be quantified with confidence. This thesis describes a unique apparatus, and the improvements made to it, that combines LFP and MBMS for primary product branching quantification, as well as multiple-pass laser absorbance spectrometry (LAS) for accurate measurements of overall kinetics. The full capability of this LFP/MBMS/LAS apparatus is demonstrated for the chemically interesting phenyl radical + propene reaction system, which has been implicated as a potential source of second aromatic ring formation under combustion conditions. Overall kinetic measurements are also reported in this work either for systems that involve a newly discovered reactive species (various cycloaddition reactions of the simplest Criegee Intermediate formed in atmospheric ozonolysis) or that was disputed in the literature (vinyl radical + 1,3-butadiene, which has been implicated as a potential source of benzene in combustion). Finally, this thesis shows how detailed chemical insights made either experimentally or theoretically can be translated into applications via the Reaction Mechanism Generator (R4G). The application discussed is natural gas high temperature pyrolysis for the production of C2 commodity chemicals.by Zachary J. Buras.Ph. D

An Extended Group Additivity Method for Polycyclic Thermochemistry Estimation

Automatic kinetic mechanism generation, virtual high‐throughput screening, and automatic transition state search are currently trending applications requiring exploration of a large molecule space. Large‐scale search requires fast and accurate estimation of molecules' properties of interest, such as thermochemistry. Existing approaches are not satisfactory for large polycyclic molecules: considering the number of molecules being screened, quantum chemistry (even cheap density functional theory methods) can be computationally expensive, and group additivity, though fast, is not sufficiently accurate. This paper provides a fast and moderately accurate alternative by proposing a polycyclic thermochemistry estimation method that extends the group additivity method with two additional algorithms: similarity match and bicyclic decomposition. It significantly reduces Hf(298 K) estimation error from over 60 kcal/mol (group additivity method) to around 5 kcal/mol, Cp(298 K) error from 9 to 1 cal/mol/K, and S(298 K) error from 70 to 7 cal/mol/K. This method also works well for heteroatomic polycyclics. A web application for estimating thermochemistry by this method is made available at http://rmg.mit.edu/molecule_search.United States. Department of Energy (Grant DE-SC0014901

Temperature- and Pressure-Dependent Kinetics of CH2OO + CH3COCH3 and CH2OO + CH3CHO: Direct Measurements and Theoretical Analysis

The rate coefficients of the gas-phase reactions CH₂OO + CH₃COCH₃ and CH₂OO + CH₃CHO have been experimentally determined from 298–500 K and 4–50 Torr using pulsed laser photolysis with multiple-pass UV absorption at 375 nm, and products were detected using photoionization mass spectrometry at 10.5 eV. The CH₂OO + CH₃CHO reaction's rate coefficient is ∼4 times faster over the temperature 298–500 K range studied here. Both reactions have negative temperature dependence. The T dependence of both reactions was captured in simple Arrhenius expressions: kCH₂OO+CH₃CoCH₃(T) = (7 ± 2.5) x 10⁻¹⁵ cm3 molecule⁻¹s⁻¹ exp[+(9.3 ± 2.9)kJ mol⁻¹ /RT] kCH₂OO+CH₃CHO (T) = (3 ± 0.8 x 10⁻¹⁴ cm3 molecule⁻¹s⁻¹ exp[+(9.1 ± 2.7)kJ mol⁻¹ /RT] The rate of the reactions of CH₂OO with carbonyl compounds at room temperature is two orders of magnitude higher than that reported previously for the reaction with alkenes, but the A factors are of the same order of magnitude. Theoretical analysis of the entrance channel reveals that the inner 1,3-cycloaddition transition state is rate limiting at normal temperatures. Predicted rate-coefficients (RCCSD(T)-F12a/cc-pVTZ-F12//B3LYP/MG3S level of theory) in the low-pressure limit accurately reproduce the experimentally observed temperature dependence. The calculations only qualitatively reproduce the A factors and the relative reactivity between CH₃CHO and CH₃COCH₃. The rate coefficients are weakly pressure dependent, within the uncertainties of the current measurements. The predicted major products are not detectable with our photoionization source, but heavier species yielding ions with masses m/z = 104 and 89 are observed as products from the reaction of CH₂OO with CH₃COCH₃. The yield of m/z = 89 exhibits positive pressure dependence that appears to have already reached a high-pressure limit by 25 Torr.United States. Department of Energy (DESC0001198

Direct Determination of the Simplest Criegee Intermediate (CH₂OO) Self Reaction Rate

The rate of self-reaction of the simplest Criegee intermediate, CH₂OO, is of importance in many current laboratory experiments where CH₂OO concentrations are high, such as flash photolysis and alkene ozonolysis. Using laser flash photolysis while simultaneously probing both CH₂OO and I atom by direct absorption, we can accurately determine absolute CH₂OO concentrations as well as the UV absorption cross section of CH₂OO at our probe wavelength (λ = 375 nm), which is in agreement with a recently published value. Knowing absolute concentrations we can accurately measure <i>k</i>_self = 6.0 ± 2.1 × 10^–11cm³ molecule^–1 s^–1 at 297 K. We are also able to put an upper bound on the rate coefficient for CH₂OO + I of 1.0 × 10^–11 cm³ molecule^–1 s^–1. Both of these rate coefficients are at least a factor of 5 smaller than other recent measurements of the same reactions

From benzene to naphthalene: direct measurement of reactions and intermediates of phenyl radicals and acetylene

© 2019 the Owner Societies. Hydrogen-abstraction-C2H2-addition (HACA) is one of the most important pathways leading to the formation of naphthalene, the simplest two-ring polycyclic aromatic hydrocarbon (PAH). The major reaction channels for naphthalene formation have previously been calculated by Mebel et al., but few experiments exist to validate the theoretical predictions. In this work, time-resolved molecular beam mass spectrometry (MBMS) was used to investigate the time-dependent product formation in the reaction of a phenyl radical with C2H2 for the first time, at temperatures of 600 and 700 K and pressures of 10 and 50 Torr. A pressure-dependent model was developed with rate parameters derived from Mebel et al.'s calculations and from newly calculated pathways on the C8H7 PES at the G3(MP2,CC)//B3LYP/6-311G∗∗ level of theory. The model prediction is consistent with the MBMS product profiles at a mass-to-charge ratio (m/z) of 102 (corresponding to the H-loss product from C8H7, phenylacetylene), 103 (the initial C8H7 adduct and its isomers plus the 13C isotopologue of phenylacetylene), 128 (naphthalene), and 129 (C10H9 isomers plus the 13C isotopologue of naphthalene). An additional C8H7 isomer, bicyclo[4.2.0]octa-1,3,5-trien-7-yl, not considered by Mebel et al.'s calculations, contributes significantly to the signal at m/z 103 due to its stable energy and low reactivity. At high C2H2 concentrations, bimolecular reactions dominated the observed chemistry, and the m/z 128 and m/z 102 MBMS signal ratio was measured to directly determine the product branching ratio. At 600 K and 10 Torr, branching to the H-loss product (phenylacetylene) on the C8H7 PES accounted for 7.9% of phenyl radical consumption, increasing to 15.9% at 700 K and 10 Torr. At 50 Torr, the branching was measured to be 2.8% at 600 K and 6.2% at 700 K. Adduct stabilization is favored at higher pressure and lower temperature, which hinders the formation of the H-loss product. The pressure-dependent model predicted the observed branching ratios within the experimental uncertainty, indicating that the rate parameters reported here can be used in combustion mechanisms to provide insights into phenyl HACA reactions and PAH formation

Direct Kinetics and Product Measurement of Phenyl Radical + Ethylene

The phenyl + ethylene (C6H5 + C2H4) reaction network was explored experimentally and theoretically to understand the temperature dependence of the reaction kinetics and product distribution under various temperature and pressure conditions. The flash photolysis apparatus combining laser absorbance spectroscopy (LAS) and time-resolved molecular beam mass spectrometry (MBMS) was used to study reactions on the C8H9 potential energy surface (PES). In LAS experiments, 505.3 nm laser light selectively probed C6H5 decay, and we measured the total C6H5 consumption rate coefficients in the intermediate temperature region (400-800 K), which connects previous experiments performed in high-temperature (pyrolysis) and low-temperature (cavity-ring-down methods) regions. From the quantum chemistry calculations by Tokmakov and Lin using the G2M(RCC5)//B3LYP method, we constructed a kinetic model and estimated phenomenological pressure-dependent rate coefficients, k(T, P), with the Arkane package in the reaction mechanism generator. The MBMS experiments, performed at 600-800 K and 10-50 Torr, revealed three major product peaks: m/z = 105 (adducts, mostly 2-phenylethyl radical, but also 1-phenylethyl radical, ortho-ethyl phenyl radical, and a spiro-fused ring radical), 104 (styrene, co-product with a H atom), and 78 (benzene, co-product with C2H3 radical). Product branching ratios were predicted by the model and validated by experiments for the first time. At 600 K and 10 Torr, the yield ratio of the H-abstraction reaction (forming benzene + C2H3) is measured to be 1.1% and the H-loss channel (styrene + H) has a 2.5% yield ratio. The model predicts 1.0% for H-abstraction and 2.3% for H-loss, which is within the experimental error bars. The branching ratio and formation of styrene increase at high temperature due to the favored formally direct channel (1.0% at 600 K and 10 Torr, 5.8% at 800 K and 10 Torr in the model prediction) and the faster β-scission reactions of C8H9 isomers. The importance of pressure dependence in kinetics is verified by the increase in the yield of the stabilized adduct from radical addition from 80.2% (800 K, 10 Torr) to 88.9% (800 K, 50 Torr), at the expense of styrene + H. The pressure-dependent model developed in this work is well validated by the LAS and MBMS measurements and gives a complete picture of the C6H5 + C2H4 reaction. ©202

Direct Measurement of Radical-Catalyzed C₆H₆ Formation from Acetylene and Validation of Theoretical Rate Coefficients for C₂H₃+C₂H₂ and C₄H₅+C₂H₂ Reactions

The addition of vinylic radicals to acetylene is an important step contributing to the formation of polycyclic aromatic hydrocarbons in combustion. The overall reaction 3C2H2 → C6H6 could result in large benzene yields, but without accurate rate parameters validated by experiment, the extent of aromatic ring formation from this pathway is uncertain. The addition of vinyl radicals to acetylene was investigated using time-resolved photoionization time-of-flight mass spectrometry at 500 and 700 K and 5-50 Torr. The formation of C6H6 was observed at all conditions, attributed to sequential addition to acetylene followed by cyclization. Vinylacetylene (C4H4) was observed with increasing yield from 500 to 700 K, attributed to the β-scission of the thermalized 1,3-butadien-1-yl radical and the chemically activated reaction C2H3 + C2H2 → C4H4 + H. The measured kinetics and product distributions are consistent with a kinetic model constructed using pressure- A nd temperature-dependent reaction rate coefficients computed from previously reported ab initio calculations. The experiments provide direct measurements of the hypothesized C4H5 intermediates and validate predictions of pressure-dependent addition reactions of vinylic radicals to C2H2, which are thought to play a key role in soot formation.China Scholarship Council (Grant 201308120042)China. Ministry of Education. Program for New Century Excellent Talents (Grant NCET-13-0408

Modeling of aromatics formation in fuel-rich methane oxy-combustion with an automatically generated pressure-dependent mechanism

With the rise in production of natural gas, there is increased interest in homogeneous partial oxidation (POX) to convert methane to syngas (CO + H2), ethene (C2H4) and acetylene (C2H2). In POX, polycyclic aromatic hydrocarbons (PAH) are important undesired byproducts. To improve the productivity of such POX processes, it is necessary to have an accurate chemical mechanism for methane-rich combustion including PAH. A new mechanism was created to capture the chemistry from C0 to C12, incorporating new information derived from recent quantum chemistry calculations, with help from the Reaction Mechanism Generator (RMG) software. For better estimation of kinetics and thermochemistry of aromatic species, including reactions through carbene intermediates, new reaction families and additional data from quantum chemistry calculations were added to RMG-database. Many of the rate coefficients in the new mechanism are significantly pressure-dependent at POX conditions. The new mechanism was validated against electron-ionization molecular beam mass spectrometry (EI-MBMS) data from a high-temperature flow reactor reported by Kohler et al. In this work quantification of additional species from those experiments is reported including phenylacetylene (C8H6), indene (C9H8), naphthalene (C10H8) and acenaphthylene (C12H8) at many temperatures for several feed compositions. Comparison of the experimental species concentration data and the new kinetic model is satisfactory; the new mechanism is generally more accurate than other published mechanisms. Moreover, because the new mechanism is composed of elementary chemical reaction steps instead of global fitted kinetics, pathway analysis of species could be investigated step-by-step to understand PAH formation. For methane-rich combustion, the most important routes to key aromatics are propargyl recombination for benzene, reactions of the propargyl radical with the phenyl radical for indene, and hydrogen abstraction acetylene addition (HACA) for naphthalene.Saudi Arabia. Saudi Basic Industries Corporatio