Engineering topics

Klippenstein, Stephen J.

Publications and source records attributed to Klippenstein, Stephen J..

At least 19 records

DOE OSTI2024

A Photochemical Phosphorus-Hydrogen-Oxygen Network for Hydrogen-dominated Exoplanet Atmospheres

Due to the detection of phosphine (PH 3 ) in the solar system gas giants Jupiter and Saturn, PH 3 has long been suggested to be detectable in exosolar substellar atmospheres too. However, to date, direct detection of phosphine has proven to be elusive in exoplanet atmosphere surveys. We construct an updated phosphorus-hydrogen-oxygen (PHO) photochemical network suitable for the simulation of gas giant hydrogen-dominated atmospheres. Using this network, we examine PHO photochemistry in hot Jupiter and warm Neptune exoplanet atmospheres at solar and enriched metallicities. Our results show for HD 189733b-like hot Jupiters that HOPO, PO, and P 2 are typically the dominant P carriers at pressures important for transit and emission spectra, rather than PH 3 . For GJ1214b-like warm Neptune atmospheres our results suggest that at solar metallicity PH 3 is dominant in the absence of photochemistry, but is generally not in high abundance for all other chemical environments. At 10 and 100 times solar, small oxygenated phosphorus molecules such as HOPO and PO dominate for both thermochemical and photochemical simulations. The network is able to reproduce well the observed PH 3 abundances on Jupiter and Saturn. Despite progress in improving the accuracy of the PHO network, large portions of the reaction rate data remain with approximate, uncertain, or missing values, which could change the conclusions of the current study significantly. Improving understanding of the kinetics of phosphorus-bearing chemical reactions will be a key undertaking for astronomers aiming to detect phosphine and other phosphorus species in both rocky and gaseous exoplanetary atmospheres in the near future.

atmospheric composition↗

DOE OSTI2024

An experimental, theoretical and kinetic modeling study of the N 2 O-H 2 system: Implications for N 2 O + H

The reaction of N 2 O with His the key step in consumption of nitrous oxide in thermal processes. The major product channel is N 2 + OH, while NH + NO constitute minor products. In addition, a pathway involving HNNO, initiated by N 2 O + H (+M)$\rightleftarrows$HNNO (+M) (R3, R4), has been inferred from experiment and theory by Burke and coworkers. At longer reaction times, the reaction may reach partial equilibration, and in addition to k 3 and k 4 the importance of this channel depends on the thermodynamic properties of HNNO and its consumption reactions, mainly HNNO + H. In the present work, we re-examined the thermochemistry of HNNO and calculated rate constants and branching fractions for the HNNO + H reaction. Experiments on the N 2 O-H 2 system were conducted in a high-pressure flow reactor at 100 atm as a function of temperature (600-925 K) and stoichiometry and explained in terms of an updated chemical kinetic model. The results support the importance of the HNNO pathway, which results in inhibition of N 2 O consumption and formation of NH 3 . In addition, selected literature results on the N 2 O-H 2 system are re-examined and the implications for the other product channels of N 2 O + H, in particular NH + NO, are discussed.

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DOE OSTI2024

Infrared Fingerprint and Unimolecular Decay Dynamics of the Hydroperoxyalkyl Intermediate (•QOOH) in Cyclopentane Oxidation

A transient carbon-centered hydroperoxyalkyl intermediate (•QOOH) in the oxidation of cyclopentane is identified by IR action spectroscopy with time-resolved unimolecular decay to hydroxyl (OH) radical products that are detected by UV laser-induced fluorescence. Two nearly degenerate •QOOH isomers, β- and γ-QOOH, are generated by H atom abstraction of the cyclopentyl hydroperoxide precursor. Fundamental and first overtone OH stretch transitions and combination bands of •QOOH are observed and compared with anharmonic frequencies computed by second-order vibrational perturbation theory. An OH stretch transition is also observed for a conformer arising from torsion about a low-energy CCOO barrier. Definitive identification of the β-QOOH isomer relies on its significantly lower transition state (TS) barrier to OH products, which results in rapid unimolecular decay and near unity branching to OH products. A benchmarking approach is utilized to compute high-accuracy stationary point energies, most importantly TS barriers, for cyclopentane oxidation (C 5 H 9 O 2 ), building on higher level reference calculations for ethane oxidation (C 2 H 5 O 2 ). The experimental OH product appearance rates are compared with computed statistical microcanonical rates using RRKM theory, including heavy-atom tunneling, thereby validating the computed TS barrier. The results are extended to thermal unimolecular decay rate constants at temperatures and pressures relevant to cyclopentane combustion via master-equation modeling. Furthermore, the various torsional and ring puckering states of the wells and transition states are explicitly considered in these calculations.

Chemical calculations↗

DOE OSTI2024

On the prediction of pressure effects for the combination kinetics of two alkyl radicals with the geometric mean rule

Here, alkyl combination reactions, along with their reverse alkane unimolecular decomposition reactions, are widely present in pyrolysis and combustion systems. These reactions occur mainly at temperatures at which their rate coefficients depend on pressure, and only a few studies have explored this dependence for large size alkyls. The accurate calculation of these kinetic data is limited by the size of the alkyl fragments involved, since the theoretical methods used are computationally expensive. Therefore, little reliable data is available for the modeling of real fuels alkane decomposition kinetics. In this paper, we propose an alternative approach that allows an accurate estimation of pressure-dependent kinetic rates for the combination of long-chain alkyl radicals, based on tabulated reaction models. The rates of combination of linear alkyls radicals forming ethane to pentane were calculated using the CASPT2-F12/cc-pVDZ-F12 level within VRC-TST theory combined with master-equation simulations. The computed high-pressure limit rate coefficients showed that n-propyl could be a minimum-size representative structure for larger n-alkyl chain, but this approach fails to capture the pressure dependence of the reaction that remains size-dependent for larger alkyls. The Geometric Mean Rule (GMR), a simple rule that links the self and cross-reactions of three alkyl radicals, is used to address this issue. Our theoretical calculations show for the first time that the GMR applies not only at the high-pressure limit, but also for every pressure explored. The GMR is shown to be accurate, with a maximum 22 % deviation in the falloff region for the estimation of rates for small alkyl combinations. Using this method, we propose a tabulated reaction rate rule for the combination of alkyl radicals up to C5+C5. The use of computed rate coefficients with the GMR approximation opens ways to develop pressure-dependent reaction rate rules for large n-alkyl combinations, based on accurate and computationally expensive theoretical calculations on smaller alkyl fragments.

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DOE OSTI2024

Systematic exploration of the thermochemistry for a set of peroxy hydroperoxy-alkyl radicals

Here, the thermochemistry of peroxy hydroperoxy-alkyl ($\overline{O}$OQOOH) radicals has a significant influence on the reactivity of fuels and on the formation of highly oxygenated molecules (HOMs) in the atmosphere. Theoretical characterization of these radicals can be arduous due to their molecular size and complex fundamental interactions, such as hydrogen-bonding and torsional anharmonicity, and difficult to validate in the absence of any direct experimental thermochemical data. In this work, we systematically explore the thermochemistry of a set of $\overline{O}$OQOOH radicals with five increasingly affordable approaches with considerations for these interactions. In doing so, we present a novel conformer selection approach suited to predict properties at combustion temperatures. As a corollary, we also highlight the shortcomings in the standard choice of the ground conformer as the reference. The set of molecules is comprised of 149 C 2 -C 8 $\overline{O}$OQOOH radicals, selected to encompass a wide variety of branching and substitution patterns. Comparisons amongst the approaches help quantify the errors arising from various simplifying assumptions. For the largest of these radicals, and with the most affordable approach, final predictions of Gibbs energies are assigned a 2 σ uncertainty of 4 kcal mol -1 in the negative temperature coefficient region.

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DOE OSTI2024

High pressure ammonia/methanol oxidation up to 100 atm

Here, high pressure ammonia/methanol oxidation and NOx formations were investigated using a recently developed supercritical pressure jet-stirred reactor (SP-JSR) at 20 and 100 atm with temperatures between 550 and 950 K and equivalence ratios of 0.138 and 1.15. The experimental results show that NH 3 oxidation at high pressure is significantly accelerated by the active OH radicals produced from CH 3 OH oxidation. Furthermore, the kinetic interactions between NH 3 and CH 3 OH are governed mainly by the reactions CH 3 OH + NH 2 = CH 2 OH + NH 3 , CH 3 OH + NH 2 = CH 3 O + NH 3 , and CH 2 O + NH 2 = HCO + NH 3 . A HP-Mech model for high-pressure NH 3 /CH 3 OH oxidation was developed in this study. It consists of the most recent NH 3 and CH 3 OH models including some new reactions and updated rate constants from the literature as well as NH 3 -CH 3 OH interactions where rate constants of CH 3 OH + NH 2 = CH 2 OH + NH 3 , CH 3 OH + NH 2 = CH 3 O + NH 3 , NH 2 + CH 2 O = NH 3 + HCO, and NH 2 + CH 2 O = NH 2 CHO + H were theoretically calculated in this study. Our model with these updates improves the prediction for the measured N 2 O/NO x temperature dependence at 100 atm. In addition, the reaction pathway and sensitivity analysis show that N 2 O/NO x /HONO interactions with HO 2 are very important, especially for a fuel-lean mixture at 100 atm. The HONO mole fraction for the fuel-lean mixture at 100 atm was then measured by off-axis integrated cavity output spectroscopy (ICOS) at wavenumber of 6638.26 cm -1 . The experimental data show a significant HONO formation at intermediate temperature that is strongly underpredicted by numerical simulation at 100 atm. Therefore, the HONO related reactions with notable uncertainty at high pressure such as NO + OH (+M) = HONO (+M) and H 2 NO + NO 2 = HONO + HNO need deeper exploration in the future.

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DOE OSTI2024

Infrared signature of the hydroperoxyalkyl intermediate (·QOOH) in cyclohexane oxidation: An isomer-resolved spectroscopic study

Infrared (IR) action spectroscopy is utilized to characterize carbon-centered hydroperoxy-cyclohexyl radicals (·QOOH) transiently formed in cyclohexane oxidation. The oxidation pathway leads to three nearly degenerate ·QOOH isomers, β-, γ-, and δ-QOOH, which are generated in the laboratory by H-atom abstraction from the corresponding ring sites of the cyclohexyl hydroperoxide (CHHP) precursor. The IR spectral features of jet-cooled and stabilized ·QOOH radicals are observed from 3590 to 7010 cm −1 (∼10–20 kcal mol −1 ) at energies in the vicinity of the transition state (TS) barrier leading to OH radicals that are detected by ultraviolet laser-induced fluorescence. The experimental approach affords selective detection of β-QOOH, arising from its significantly lower TS barrier to OH products compared to γ and δ isomers, which results in rapid unimolecular decay and near unity branching to OH products. The observed IR spectrum of β-QOOH includes fundamental and overtone OH stretch transitions, overtone CH stretch transitions, and combination bands involving OH or CH stretch with lower frequency modes. The assignment of β-QOOH spectral features is guided by anharmonic frequencies and intensities computed using second-order vibrational perturbation theory. The overtone OH stretch (2ν OH ) of β-QOOH is shifted only a few wavenumbers from that observed for the CHHP precursor, yet they are readily distinguished by their prompt vs slow dissociation rates to OH products.

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DOE OSTI2024

Quasi-Classical Trajectory Calculation of Rate Constants Using an Ab Initio Trained Machine Learning Model (aML-MD) with Multifidelity Data

Machine learning (ML) provides a great opportunity for the construction of models with improved accuracy in classical molecular dynamics (MD). However, the accuracy of a ML trained model is limited by the quality and quantity of the training data. Generating large sets of accurate ab initio training data can require significant computational resources. Furthermore, inconsistent or incompatible data with different accuracies obtained using different methods may lead to biased or unreliable ML models that do not accurately represent the underlying physics. Recently, transfer learning showed its potential for avoiding these problems as well as for improving the accuracy, efficiency, and generalization of ML models using multifidelity data. In this work, ab initio trained ML-based MD (aML-MD) models are developed through transfer learning using DFT and multireference data from multiple sources with varying accuracy within the Deep Potential MD framework. Further, the accuracy of the force field is demonstrated by calculating rate constants for the H + HO 2 → H 2 + 3 O 2 reaction using quasi-classical trajectories. We show that the aML-MD model with transfer learning can accurately predict the rate constants while reducing the computational cost by more than five times compared to the use of more expensive quantum chemistry training data sets. Hence, the aML-MD model with transfer learning shows great potential in using multifidelity data to reduce the computational cost involved in generating the training set for these potentials.

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DOE OSTI2024

Isomer-resolved unimolecular dynamics of the hydroperoxyalkyl intermediate (•QOOH) in cyclohexane oxidation

The oxidation of cycloalkanes is important in the combustion of transportation fuels and in atmospheric secondary organic aerosol formation. A transient carbon-centered radical intermediate (•QOOH) in the oxidation of cyclohexane is identified through its infrared fingerprint and time- and energy-resolved unimolecular dissociation dynamics to hydroxyl (OH) radical and bicyclic ether products. Although the cyclohexyl ring structure leads to three nearly degenerate •QOOH isomers (β-, γ-, and δ-QOOH), their transition state (TS) barriers to OH products are predicted to differ considerably. Selective characterization of the β-QOOH isomer is achieved at excitation energies associated with the lowest TS barrier, resulting in rapid unimolecular decay to OH products that are detected. A benchmarking approach is employed for the calculation of high-accuracy stationary point energies, in particular TS barriers, for cyclohexane oxidation (C 6 H 11 O 2 ), building on higher-level reference calculations for the smaller ethane oxidation (C 2 H 5 O 2 ) system. The isomer-specific characterization of β-QOOH is validated by comparison of experimental OH product appearance rates with computed statistical microcanonical rates, including significant heavy-atom tunneling, at energies in the vicinity of the TS barrier. Master-equation modeling is utilized to extend the results to thermal unimolecular decay rate constants at temperatures and pressures relevant to cyclohexane combustion.

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DOE OSTI2024

The role of stereochemistry in combustion processes

Stereochemical effects significantly influence chemical processes, yet it is not well understood if they are a leading source of uncertainty in combustion modeling. Stereochemistry influences a combustion model (i) at the earliest stage of its construction when mapping the reaction network, (ii) in the computation of individual thermochemical and rate parameters, and (iii) in the prediction of combustion observables. The present work reviews the importance of enumerating stereochemical species and reactions at each of these steps. Further, it analyzes the separate influence of several types of stereochemistry, including geometric, optical, and fleeting transition state diastereomers. Three reaction networks serve to examine which stages of low-temperature oxidation are most affected by stereochemistry, including the first and second oxidation of n-butane, the third oxidation of n-pentane, and the early stages of pyrolysis of 1- and 2-pentene. The 149 reactions in the n-butane mechanism are expanded to 183 reactions when accounting for diastereomerism. Each of these 183 reactions is parameterized with ab initio kinetics computations to determine that, for the n-butane mechanism, the median factor of diastereomeric deviation is 3.5 at 360 K for rate constants and as high as 1.6 for mechanism reactivity, in terms of ignition delay times, as opposed to a mechanism without stereochemical expansion.

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DOE OSTI2024

An experimental, theoretical, and kinetic modeling study of post-flame oxidation of ammonia

The post -flame oxidation rate of ammonia was investigated in a novel atmospheric pressure flow reactor at temperatures of 1280 ± 16 K and as a function of residence time and mixture composition (1-10% O 2 , dry and moist). The experimental results, as well as selected data from literature, were analyzed using an updated detailed chemical kinetic model. The medium temperature, very lean conditions enhance the importance of reactions of the nitroxyl (HNO) intermediate. High-level theory was used to calculate the rate constant for HNO + NH 2 , indicating that this step is significantly faster than values used in literature. Furthermore, a trajectory based approach was used to determine collision efficiencies for selected bath gases for HNO + M. The experimental results show that the NH 3 oxidation rate increases with temperature and O 2 concentration, while the presence of water vapor slightly inhibits reaction. Formation of NO and N 2 O was strongly promoted at higher levels of O 2 . Modeling results agreed well with the measurements, except at the lowest level of O 2 . The predicted oxidation rate of NH 3 was shown to result from a delicate balance between chain branching and terminating steps involving NH 2 , H 2 NO, and HNO. Recent theoretical work on reactions of these species by Klippenstein and coworkers and Stagni et al. was instrumental in improving modeling predictions. After initiation, NO reached a pseudo -steady-state level, where the pathways to NO were largely balanced by the NH 2 + NO reaction. Nitric oxide was partly oxidized to NO 2 , with the NH 2 + NO 2 reaction responsible for most of the N 2 O formation. Novelty and significance statement: This study provides the first detailed kinetic analysis of the lean postflame oxidation of ammonia, based on time -resolved flow reactor data in a novel reactor. In addition to the post -flame oxidation rate of ammonia, data for formation of NO and N 2 O were compared with modeling predictions. The medium temperature, very lean conditions enhance the importance of reactions of the HNO and H 2 NO intermediates. Inclusion in the model of results from recent high-level theoretical work, including present calculations for HNO + NH 2 and HNO + M, was crucial for capturing the observed behavior. It is argued that the post -initiation steady-state NH 3 oxidation rates constitute important data for model validation, along with ignition delays and laminar flame speeds.

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DOE OSTI2024

Competing Radical and Molecular Channels in the Unimolecular Dissociation of Methylformate

The thermal dissociation of methylformate has been characterized by numerous experimental and theoretical studies that all seem to agree that the lowest energetically accessible process is a 3-center H-atom transfer that leads to the molecular products CH3OH 3 OH + CO. However, these literature studies seem to be at odds with regards to the role of other competing molecular eliminations and bond-fission processes and therefore a complete and resolved mechanistic picture of methylformate thermal dissociation still eludes us. In this work, we have performed high-level electronic structure theory calculations to characterize the energetics of other overlooked molecular and radical processes that can originate on the complex CH3OCHO 3 OCHO potential energy surface (PES). The present calculations do indeed confirm that the lowest energy process accessible on this PES is molecular elimination to form CH3OH 3 OH + CO. However, unlike prior theoretical studies, the present calculations reveal that the second lowest energy process is a 5-center concerted elimination process that leads to the direct formation of H2 2 + CH2O 2 O + CO. We also note that HCO2 2 from the lowest-lying bond fission has two energetically comparable electronic states (of 2 B 2 and 2 A 1 symmetry). Furthermore, the barrier for H-atom migration of CH3OCHO 3 OCHO to form the carbene (CH3OCOH) 3 OCOH) is similar to that for the direct 4-center elimination process leading to CH2O 2 O + CH2O 2 O characterized in prior literature studies. Radical and molecular pathways to CH3 3 + HOCO and CH4 4 + CO2 2 can also be facilitated from this carbene. Master equation calculations were performed to characterize the competition between the various molecular and radical processes on this more elaborate CH3OCHO 3 OCHO PES. The results of the present theoretical analyses were used to resolve outstanding questions on the role of secondary radical- initiated reactions in characterizing prior literature experimental studies.

Chemical Kinetics↗

DOE OSTI2023

OH Roaming during the Ozonolysis of α-Pinene: A New Route to Highly Oxygenated Molecules?

The formation of low-volatility organic compounds in the ozonolysis of α-pinene, the dominant atmospheric monoterpene, provides an important route to aerosol formation. Here, in this work, we consider a previously unexplored set of pathways for the formation of highly oxygenated molecules in α-pinene ozonolysis. Pioneering, direct experimental observations of Lester and co-workers have demonstrated a significant production of hydroxycarbonyl products in the dissociation of Criegee intermediates. Theoretical analyses indicate that this production arises from OH roaming-induced pathways during the OO fission of the vinylhydroperoxides (VHPs), which in turn come from internal H transfers in the Criegee intermediates. Ab initio kinetics computations are used here to explore the OH roaming-induced channels that arise from the ozonolysis of α-pinene. For computational reasons, the calculations consider a surrogate for α-pinene, where two spectator methyl groups are replaced with H atoms. Multireference electronic structure calculations are used to illustrate a variety of energetically accessible OH roaming pathways for the four VHPs arising from the ozonolysis of this α-pinene surrogate. Ab initio transition-state theory-based master equation calculations indicate that for the dissociation of stabilized VHPs, these OH roaming pathways are kinetically significant with a branching that generally increases from ~20% at room temperature up to ~70% at lower temperatures representative of the troposphere. For one of the VHPs, this branching already exceeds 60% at room temperature. For the overall ozonolysis process, these branching ratios would be greatly reduced by a limited branching to the stabilized VHP, although there would also be some modest roaming fraction for the nonthermal VHP dissociation process. The strong exothermicities of the roaming-induced isomerizations/additions and abstractions suggest new routes to fission of the cyclobutane rings. Such ring fissions would facilitate further autoxidation reactions, thereby providing a new route for producing highly oxygenated nonvolatile precursors to aerosol formation.

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DOE OSTI2023

Modeling–Experiment–Theory Analysis of Reactions Initiated from Cl + Methyl Formate

Methyl formate (MF; CH 3 OCHO) is the smallest representative of esters, which are common components of biodiesel. The present study characterizes the thermal dissociation kinetics of the radicals formed by H atom abstraction from MF—CH 3 OCO and CH 2 OCHO—through a combination of modeling, experiment, and theory. For the experimental effort, excimer laser photolysis of Cl 2 was used as a source of Cl atoms to initiate reactions with MF in the gas phase. Time-resolved species profiles of MF, Cl 2 , HCl, CO 2 , CH 3 , CH 3 Cl, CH 2 O, and CH 2 ClOCHO were measured and quantified using photoionization mass spectrometry at temperatures of 400–750 K and 10 Torr. The experimental data were simulated using a kinetic model, which was informed by ab initio-based theoretical kinetics calculations and included chlorine chemistry and secondary reactions of radical decomposition products. Here, we calculated the rate coefficients for the H-abstraction reactions Cl + MF → HCl + CH 3 OCO (R1a) and Cl + MF → HCl + CH 2 OCHO (R1b): k 1a,theory = 6.71 × 10 –15 ·T 1.14 ·exp(—606/T) cm 3 /molecule·s; k 1b,theory = 4.67 × 10 –18 ·T 2.21 ·exp(—245/T) cm 3 /molecule·s over T = 200–2000 K. Electronic structure calculations indicate that the barriers to CH 3 OCO and CH 2 OCHO dissociation are 13.7 and 31.6 kcal/mol and lead to CH 3 + CO 2 (R3) and CH 2 O + HCO (R5), respectively. The master equation-based theoretical rate coefficients are k 3,theory (P = ∞) = 2.94 × 10 9 ·T 1.21 ·exp(—6209/T) s –1 and k 5,theory (P = ∞) = 8.45 × 10 8 ·T 1.39 ·exp(—15132/T) s –1 over T = 300–1500 K. The calculated branching fractions into R1a and R1b and the rate coefficient for R5 were validated by modeling of the experimental species time profiles and found to be in excellent agreement with theory. Additionally, we found that the bimolecular reactions CH 2 OCHO + Cl, CH 2 OCHO + Cl 2 , and CH 3 + Cl 2 were critical to accurately model the experimental data and constrain the kinetics of MF-radicals. Inclusion of the kinetic parameters determined in this study showed a significant impact on combustion simulations of larger methyl esters, which are considered as biodiesel surrogates.

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DOE OSTI2023

Quantum and anharmonic effects in non-adiabatic transition state theory

Quantitative descriptions of non-adiabatic transition rates at intermediate temperatures are challenging due to the simultaneous importance of quantum and anharmonic effects. Here, in this paper, the interplay between quantum effects—for motion across or along the seam of crossing—and anharmonicity in the seam potential is considered within the weak coupling limit. The well-known expression for quantized 1-D motion across the seam (i.e., tunneling) in the linear terms approximation is derived in the thermal domain using the Lagrangian formalism, which is then applied to the case when tunneling is distributed along the seam of crossing (treating motion along the seam classically). For high-frequency quantum modes, a vibrationally adiabatic (VA) approach is developed that introduces to the non-adiabatic rate constant a factor associated with high-frequency wavefunction overlap; this approach treats the high-frequency motion along the seam quantum mechanically. To test these methodologies, the reaction N 2 O ↔ N 2 + O( 3 P) was chosen. CCSD(T)-F12b/cc-pVTZ-F12 explorations of the 3 A'- 1 A' seam of N 2 O revealed that seam anharmonicity has a strong effect on the rate constant (a factor of ~20 at 2000 K). Several quantum effects were found to be significant at intermediate/lower temperatures, including the quantum N–N vibration that was coupled with seam anharmonicity using the VA approach. Finally, a 1-D approximation to non-adiabatic instanton theory is presented to estimate the validity limit of the linear terms model at low temperatures (⁠~ 250 K for N 2 O). We recommend that the assumptions built into many statistical theories for non-adiabatic reactions—harmonic behavior, classical motion, linear terms, and weak coupling—should be verified on a case-by-case basis.

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DOE OSTI2023

Bimolecular Reaction of Methyl-Ethyl-Substituted Criegee Intermediate with SO 2

Methyl-ethyl-substituted Criegee intermediate (MECI) is a four-carbon carbonyl oxide that is formed in the ozonolysis of some asymmetric alkenes. MECI is structurally similar to the isoprene-derived methyl vinyl ketone oxide (MVK-oxide) but lacks resonance stabilization, making it a promising candidate to help us unravel the effects of size, structure, and resonance stabilization that influence the reactivity of atmospherically important, highly functionalized Criegee intermediates. We present experimental and theoretical results from the first bimolecular study of MECI in its reaction with SO 2 , a reaction that shows significant sensitivity to the Criegee intermediate structure. Using multiplexed photoionization mass spectrometry, we obtain a rate coefficient of (1.3 ± 0.3) × 10 –10 cm 3 s –1 (95% confidence limits, 298 K, 10 Torr) and demonstrate the formation of SO 3 under our experimental conditions. Through high-level theory, we explore the effect of Criegee intermediate structure on the minimum energy pathways for their reactions with SO 2 and obtain modified Arrhenius fits to our predictions for the reaction of both syn and anti conformers of MECI with SO 2 (k syn = 4.42 × 10 11 T –7.80 exp(-1401/T) cm 3 s –1 and k anti = 1.26 × 10 11 T –7.55 exp(-1397/T) cm 3 s –1 ). In conclusion, our experimental and theoretical rate coefficients (which are in reasonable agreement at 298 K) show that the reaction of MECI with SO 2 is significantly faster than MVK-oxide + SO 2 , demonstrating the substantial effect of resonance stabilization on Criegee intermediate reactivity.

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DOE OSTI2023

Automated identification and calculation of prompt effects in kinetic mechanisms using statistical models

The kinetics of prompt dissociation involves rovibrationally excited species (generally formed by exothermic reactions) which may dissociate or isomerize prior to thermalization via collisions with the bath gas. Treating such rovibrationally excited species (so-called "hot" species) with standard kinetic phenomenology may result in incorrect macroscopic representation of their reactivity. Here this work presents the first fully automated methodology for the calculation of prompt effects of a chosen species in a kinetic mechanism, including (i) reaction selection; (ii) theoretical calculation of rate constants and prompt branching fractions; and (iii) final rate constant fitting. The energy partition between hot fragments is estimated using a variety of statistical models, including a new physically sound microcanonical statistical model based on the rovibrational density of states of the fragments. The methodology is validated against literature data for the prompt dissociations of HCO and C 3 H 7 radicals. The microcanonical statistical model is in better agreement with trajectory simulations for larger species and is thus applicable for practical systems that typically involve large molecules, for which direct dynamics calculations are impractical. The automated workflow is applied to the evaluation of the effects of prompt dissociation for two isomeric radicals C 4 H 7 1-3 (1-methylallyl) and C 4 H 7 1-4 (3-buten-1-yl). Twelve H-atom abstraction reactions are selected and the corresponding rate constants are computed with first principles theory. The microcanonical statistical model predicts that prompt dissociations of C 4 H 7 1-3 and C 4 H 7 1-4 are already significant at 1000K, resulting in differences of up to an order of magnitude at 2000K with respect to the phenomenological thermal rate constants. To illustrate the effects of prompt dissociation on simulations of experimental data, the calculated prompt rate constants are implemented in both CRECK and C3MechV3.3 kinetic mechanisms. Simulations of experimental flame data illustrate the noticeable impact of prompt dissociation kinetics on the high-temperature combustion reactivity of C 4 H 8 -1 and C 4 H 8 -2.

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DOE OSTI2023

Uncovering novel liquid organic hydrogen carriers: a systematic exploration of chemical compound space using cheminformatics and quantum chemical methods

We present a comprehensive, in silico-based discovery approach to identifying novel liquid organic hydrogen carrier (LOHC) candidates using cheminformatics methods and quantum chemical calculations. We screened over 160 billion molecules from ZINC15 and GDB-17 chemical databases for structural similarity to known LOHCs and employed a data-driven selection criterion connecting molecular features with dehydrogenation enthalpy. This scoring criterion effectively predicts dehydrogenation enthalpies from SMILES strings, streamlining the LOHC screening process. After rigorous screening and down-selection, we compiled a database of 3000 dehydrogenation reactions for the most promising LOHC candidates, setting the stage for future selection based on kinetics and catalysis. This work demonstrates the significant impact of integrating quantum chemistry and cheminformatics in materials discovery, accelerating the selection process while reducing experimental efforts and time. By proposing new molecules as prospective LOHC candidates, our study provides a valuable resource for researchers and engineers in the development of advanced LOHC systems and showcases a successful approach for high-throughput discovery, contributing to more efficient and sustainable energy storage solutions.

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