Theoretical kinetic study of the reactions of cycloalkylperoxy radicals

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 (2νOH) 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.

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 (C6H11O2), building on higher-level reference calculations for the smaller ethane oxidation (C2H5O2) 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.

High-Pressure Limit and Pressure-Dependent Rate Rules for β-Scission Reaction Class of Hydroperoxyl Alkyl Hydroperoxyl Radicals (•P(OOH)2) in Normal-Alkyl Cyclohexanes Combustion

Chemical kinetic studies of the β-scission reaction class of hydroperoxyl alkyl hydroperoxyl radicals (•P(OOH)2) from normal-alkyl cyclohexanes are carried out systematically through high-level ab initio calculations. Geometry optimizations and frequency calculations for all species involved in the reactions are performed at the B3LYP/CBSB7 level of theory. Electronic single-point energy calculations are calculated at the CBS-QB3 level of theory. Rate constants for the reactions of β-scission, in the temperature range of 500-1500 K and the pressure range of 0.01-100 atm, are calculated using transition state theory (TST) and Rice-Ramsberger-Kassel-Marcus/Master-Equation (RRKM/ME) theory taking asymmetric Eckart tunneling corrections and the one-dimensional hindered rotor approximation into consideration. The rate rules are obtained by averaging the rate constants of the representative reactions of this class. These rate rules can greatly assist in constructing more accurate low-temperature combustion mechanisms for normal-alkyl cyclohexanes.

Experimental and Kinetic Modeling Study on High-Temperature Autoignition of Cyclohexene

Cyclohexene is an important intermediate during the oxidation of cycloalkanes, which comprise a significant portion of real fuels. Thus, experimental data sets and kinetic models of cyclohexene play an important role in the understanding of the combustion of cycloalkanes and real fuels. In this work, an experimental and kinetic modeling study of the high-temperature ignition of cyclohexene is performed. Ignition delay time (IDT) measurements are carried out in a high-pressure shock tube (HPST). The studied pressures are 5, 10, and 20 bar; the equivalence ratios are 0.5, 1.0, and 2.0; and the temperatures range from 980 to 1400 K for IDT in HPST. It is shown that the IDTs of cyclohexene exhibit Arrhenius behaviors as a function of temperature, and the IDTs decrease as the equivalence ratio and pressure increase. The experimental results are simulated using three previous detailed kinetic mechanisms and an updated detailed mechanism in this work. The updated detailed kinetic mechanism exhibits good agreement with experimental results. Reaction path analysis and sensitivity analysis are performed to provide insights into the chemical kinetics controlling the ignition of cyclohexene. The results demonstrate that different detailed kinetic mechanisms are significantly different, and there are still no unified conclusions about the major reaction path for cyclohexene oxidation. However, it is worth noting that the abstraction reaction by oxygen at the allylic site and the submechanism of cyclopentene are of significant importance for the accurate prediction of IDTs of cyclohexene. The present experimental data set and kinetic model should be valuable to improve our understanding of the combustion chemistry of cycloalkanes.

Develop the low-temperature oxidation mechanism of cyclopentane: an experimental and theoretical study

At present, the reactivity of cyclic alkanes is estimated by analogy comparison with non-cyclic hydrocarbons. Due to the difference in the structure of cycloalkanes and non-cycloalkanes, the thermodynamic data obtained by analogy is not applicable. In this study, a molecular beam sampling vacuum ultraviolet photoionization time-of-flight mass spectrometer (MB-VUV-PI-TOFMS) was applied to study the low-temperature oxidation of cyclopentane (CPT) at a total pressure range from 1-3 atm and low-temperature range between 500 K and 800 K. The low-temperature reaction products including cyclic olefins, cyclic ethers, highly oxygenated intermediates (e.g., ketohydroperoxide KHP, keto-dihydroperoxide KDHP, olefinic hydroperoxides OHP and ketone structure products) were observed. Further investigation of the oxidation of CPT, the electronic structure calculations at the UCCSD(T)-F12a/aug-cc-pVDZ//B3LYP/6-31+ G(d,p) level were carried out to explore the reactivity of O2 molecules adding sequentially to cyclopentyl radicals. Experimental and theoretical observations showed that the dominant product channel in the reaction of CPT radicals with O2 is HO2 elimination, yielding cyclopentene. The pathways of second and third O2 addition, the dissociation of hydroperoxide were further confirmed. The results of this study will develop the low-temperature oxidation mechanism of CPT, which can be used for future research on accurately simulating the combustion process of CPT.

High-Pressure-Limit and Pressure-Dependent Rate Rules for Unimolecular Reactions Related to Hydroperoxy Alkyl Radicals in Normal-Alkyl Cyclohexane Combustion. 2. Cyclization Reaction Class

The hydroperoxy alkyl radicals are important intermediates in the low-temperature combustion for normal-alkyl cycloalkanes, and the cyclization reactions of hydroperoxy alkyl radicals to form cyclic ethers are responsible for a major fraction of the OH formation, which has the potential to promote ignition. In most of the previous modeling studies for normal-alkyl cycloalkane combustion, the kinetic data of the cyclization reactions in the detailed combustion mechanism were mainly taken from the analogous reactions in cyclohexane, methyl cyclohexane, and alkanes in published literature studies. In this work, the kinetics of the cyclization reaction class of hydroperoxy alkyl radicals in normal-alkyl cycloalkanes is studied, where the reaction class is divided into subclasses depending upon the ring size of the transition states, the types of the carbons on which the -OOH site is located and the types of the carbons on which the radical site is located, and the positions of the cyclization (on the alkyl side chain, on the cycle, or between the alkyl side chain and the cycle). Energy barriers and high-pressure-limit site rate constants and pressure-dependent rates for reactions in all subclasses are calculated, and rate rules for all subclasses are developed. The high-pressure-limit rate constants are determined from CBS-QB3 electronic structure calculations combined with canonical transition-state theory calculations, and pressure-dependent rate constants are calculated by using the Rice-Ramsberger-Kassel-Marcus/Master Equation theory at pressures varying from 0.01 to 100 atm. Comparisons of the rate constants for cyclization reactions of hydroperoxy alkyl cyclohexylperoxy radicals calculated in this work with the values of the corresponding reactions in some of the popular combustion mechanisms show that it is unreasonable to use the kinetic data of analogous reactions in alkanes, cyclohexanes, or smaller normal-alkyl cyclohexanes. Therefore, the accurate kinetic calculations and the construction of rate rules for normal-alkyl cycloalkanes are necessary and significant for the reliable modeling of the low-temperature combustion of normal-alkyl cyclohexanes.

Quantitative Detection of Products and Radical Intermediates in Low-Temperature Oxidation of Cyclopentane

We present a combined experimental and theoretical investigation of the autoignition chemistry of a prototypical cyclic hydrocarbon, cyclopentane. Experiments using a high-pressure photolysis reactor coupled to time-resolved synchrotron VUV photoionization mass spectrometry directly probe the short-lived radical intermediates and products in cyclopentane oxidation reactions. We detect key peroxy radical intermediates ROO and OOQOOH, as well as several hydroperoxides, formed by second O₂ addition. Automated quantum chemical calculations map out the R + O₂ + O₂ reaction channels and demonstrate that the detected intermediates belong to the dominant radical chain-branching pathway: ROO (+ O₂) → γ-QOOH + O₂ → γ-OOQOOH → products. ROO, OOQOOH, and hydroperoxide products of second-O₂ addition undergo extensive dissociative ionization, making their experimental assignment challenging. We use photoionization dynamics calculations to aid in their characterization and report the absolute photoionization spectra of isomerically pure ROO and γ-OOQOOH. A global statistical fit of the observed kinetics enables reliable quantification of the time-resolved concentrations of these elusive, yet critical species, paving the way for detailed comparisons with theoretical predictions from master-equation-based models.

Low Temperature Oxidation of Cyclohexane: Uncertainty of Important Thermo-Chemical Properties

The study of the standard formation enthalpy, entropy, and heat capacity for key species relevant to the low-temperature combustion of cyclohexane has been performed by applying the group additivity method of Benson. The properties of 18 Benson groups (8 of them for the first time), and 10 ring correction factors for cyclic species were estimated through different empirical and semi-empirical methods. The method validation proceeded through comparison of predicted values for certain number of newly estimated groups and available literature data derived from quantum chemistry estimations. Further validations of the estimated properties of groups have been provided by comparing estimated properties of test species with data in literature and kinetic databases. Also the standard deviation between prediction and reported values has been evaluated for each validation case. A similar approach has been applied for validation of the estimated ring correction groups. For selected well-studied cyclic molecules the predicted values and the literature data have been compared with each other, and the standard deviations have been also reported. The evaluated properties of the cyclohexane relevant species were also compared with similar ones available in other kinetic models and in databases. At the end the estimated properties have been presented in a tabulated form of NASA polynomial coefficients with extrapolation up to 3500 K.

The multi-channel reaction of the OH radical with 5-hydroxymethylcytosine: a computational study

The hydroxyl radical may attack the new cytosine derivative 5-hydroxymethylcytosine (5-hmCyt), causing DNA oxidative damage. Two distinct mechanisms have been explored and our results provide some evidence between 5-hmCyt and tumor development.

Effects of an acid–alkaline environment on the reactivity of 5-carboxycytosine with hydroxyl radicals

The addition of ˙OH to C5C6 double bond and abstraction of H5 from 5-caCyt are more favourable in neutral, acidic and alkaline conditions.

An experimental and modeling study of the low- and high-temperature oxidation of cyclohexane

The experimental study of the oxidation of cyclohexane has been performed in a jet-stirred reactor at temperatures ranging from 500 to 1100 K (low- and intermediate temperature zones including the negative temperature-coefficient area), at a residence time of 2 s and for dilute mixtures with equivalence ratios of 0.5, 1, and 2. Experiments were carried out at quasi-atmospheric pressure (1.07 bar). The fuel and reaction product mole fractions were measured using online gas chromatography. A total of 34 reaction products have been detected and quantified in this study. Typical reaction products formed in the low-temperature oxidation of cyclohexane include cyclic ethers (1,2-epoxycyclohexane and 1,4-epoxycyclohexane), 5-hexenal (formed from the rapid decomposition of 1,3-epoxycyclohexane), cyclohexanone, and cyclohexene, as well as benzene and phenol. Cyclohexane displays high low-temperature reactivity with well-marked negative temperature-coefficient (NTC) behavior at equivalence ratios 0.5 and 1. The fuel-rich system (ϕ = 2) is much less reactive in the same region and exhibits no NTC. To the best of our knowledge, this is the first jet-stirred reactor study to report NTC in cyclohexane oxidation. Laminar burning velocities were also measured by the heated burner method at initial gas temperatures of 298, 358, and 398 K and at 1 atm. The laminar burning velocity values peak at ϕ = 1.1 and are measured as 40 and 63.1 cm/s for Ti = 298 and 398 K, respectively. An updated detailed chemical kinetic model including low-temperature pathways was used to simulate the present (jet-stirred reactor and laminar burning velocity) and literature experimental (laminar burning velocity, rapid compression machine, and shock tube ignition delay times) data. Reasonable agreement is observed with most of the products observed in our reactor, as well as the literature experimental data considered in this paper.

PROGRESS IN DETAILED KINETIC MODELING OF THE COMBUSTION OF OXYGENATED COMPONENTS OF BIOFUELS

Due to growing environmental concerns and diminishing petroleum reserves, a wide range of oxygenated species has been proposed as possible substitutes to fossil fuels: alcohols, methyl esters, acyclic and cyclic ethers. After a short review the major detailed kinetic models already proposed in the literature for the combustion of these molecules, the specific classes of reactions considered for modeling the oxidation of acyclic and cyclic oxygenated molecules respectively, are detailed.