Variational Analysis of the Phenyl + O2 and Phenoxy + O Reactions

Mechanism of formation of p-benzylenephenol peroxide radical (p-PhC(O2•)HPhOH)

CONTEXT

The reactions of radicals with O2 play the important role in the biological, medicinal, and industrial processes. The mechanism of this reaction is studied previously for the alkane, alkene, alkyne, phenols, and close-related radicals. According to these studies, the formation of intermediates in these reactions is predicted only for the aromatic radicals. Thus, the Van der Waals complexes of O2 with phenyl or benzyl radicals are predicted, as well as the π-π cluster for benzene. However, the possibility of the formation of such intermediate π-π clusters in the case of bisphenol radicals and the thermochemistry of its formation is not studied. Bisphenols are one of the main components of bio-oil, produced during pyrolysis of lignin-contained biomass. Synthetic bisphenols are used in polycarbonate plastics, epoxy resins, and thermal papers. Their mechanism of oxidation is important for the determination of fire safety of these materials, the possibility of using them as additives for fuels for the decreasing and the description of the ignition delays, as well as for the determination of its health risk assessment in medicine.

METHODS

The five DFT (M06-2X (i = 1), B3LYP (i = 2), wB97XD (i = 3), M08HX (i = 4), MN15 (i = 5)) approaches with 6-311 +  + G(d,p) basis set are used for the determination of standard enthalpies of atomization (ΔraHo(Yi)) of considered compounds (molecules, radicals, and transition states). These values of ΔraHo(Yi) are corrected using the empirical linear calibration dependencies, reported previously. The different calibration dependencies are used for the hydrocarbons (including the aromatics and simple oxygenated derivatives) and for the peroxides. The corrected values of ΔraHo(Yi, CORR) are used according to Hess's law for the determination of ΔfHo(Yi, CORR). The most consistent values of ΔfHo(Y, MEAN) are derived from the coordination of the values of ΔfHo(Yi, CORR) using the intersection of their values of standard deviations (3SDi). These values of ΔfHo(Y, MEAN), as well as the B3LYP values of So(Y), which are accounting the frequency correction and internal rotations, as well as their temperature dependencies, are used for the determination of thermochemistry of considered reactions and of the calculation, within transition state theory (TST), of the values of high-pressure limits of the rate constant. The values of Ho(Yi), So(Yi), and Go(Yi) are calculated using the Gaussian 16w program. The temperature dependencies of thermochemical properties and the values of rate constants are determined using the ChemRate program (v.1.5). The optimized structures are visualized using the Chemcraft.

Gas-Phase Phenyl Radical + O2 Reacts via a Submerged Transition State

In the gas-phase chemistry of the atmosphere and automotive fuel combustion, peroxyl radical intermediates are formed following O2 addition to carbon-centered radicals which then initiate a complex network of radical reactions that govern the oxidative processing of hydrocarbons. The rapid association of the phenyl radical-a fundamental radical related to benzene-with O2 has hitherto been modeled as a barrierless process, a common assumption for peroxyl radical formation. Here, we provide an alternate explanation for the kinetics of this reaction by deploying double-hybrid density functional theory (DFT), at the DSD-PBEP86-D3(BJ)/aug-cc-pVTZ level of theory, and locate a submerged adiabatic transition state connected to a prereaction complex along the reaction entrance pathway. Using this potential energy scheme, experimental rate coefficients k(T) for the addition of O2 to the phenyl radical are accurately reproduced within a microcanonical kinetic model. This work highlights that purportedly barrierless radical oxidation reactions may instead be modeled using stationary points, which in turn provides insight into pressure and temperature dependence.

A Reaction Kinetics Study on Benzene Oxidation in the Claus Process by Sulfur Monoxide

The Claus process is used in natural gas processing plants to treat H₂S-rich acid gas to recover sulfur, but the process suffers from catalytic deactivation when aromatic contaminants such as benzene, toluene, ethylbenzene, and xylene isomers (collectively called as BTEX) are present in the acid gas feed. To safeguard the catalytic reactors, it is desired to oxidize aromatic contaminants in the furnace that are present upstream of the catalytic reactors in the process by oxidants present in it. This work develops a reaction mechanism and evaluates the reaction kinetics for the oxidation of phenyl radical by SO using CBS-QBS for reaction energetics and RRKM and transition state theory for reaction kinetics. The mechanism explores the possible products that are formed from the barrierless addition of SO on phenyl through the O atom as well as through the S atom. The exothermicity of the addition reaction is higher when the addition of SO on the aromatic structure takes place through the S atom. The major products formed from phenyl oxidation by SO are cyclopentadienyl, cyclopentadienethiol and thiopyran radicals. A remarkable similarity between the pathways for phenyl radical oxidation by O₂ and its oxidation by SO at high temperatures is observed. The proposed reactions and their rate constants are used to conduct reactor simulations to determine the important reactions that contribute to the formation of major products during phenyl-SO reactions and the temperatures suitable for benzene oxidation by SO under process conditions similar to the Claus furnace.

Benzene decomposition by non-thermal plasma: A detailed mechanism study by synchrotron radiation photoionization mass spectrometry and theoretical calculations

Non-thermal Plasma (NTP) catalysis is considered as one of the most promising technologies to address a wide range of environmental needs, such as volatile organic compounds (VOCs) and NOx removal. To meet the updated environmental emission standard, the NTP catalysis reaction system needs to be better understood and further optimized. In this work, the degradation process of benzene in NTP, which is still regarded as a "black box" process, was explored by synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). For the first time, we observed over 20 representative species by PIMS and identified their structures accurately by photoionization efficiency (PIE) spectra. Phenol, acetylene and acrolein were recognized as the three main products. More intriguingly, concentration profiles demonstrated that a large amount of acrolein and also several higher-order products, which were usually neglected in previous research, were produced during the NTP destruction process. The details of the benzene degradation reaction mechanism, were finally established by the combination of SVUV-PIMS results, thermochemistry and theoretical calculations. This work helps to complete the mechanistic picture of plasma chemistry, which may be helpful on raveling the more complicated NTP catalysis mechanism in the future therefore contributing to design of improved NTP system for environmental applications.

Kinetic Analysis for Reaction of Cyclopentadiene with Hydroperoxyl Radical under Low- and Medium-Temperature Combustion

The kinetic data of cyclopentadiene C₅H₆ oxidation reactions are significant for the construction of aromatics oxidation mechanism because cyclopentadiene C₅H₆ has been proved to be an important intermediate in the aromatics combustion. Kinetics for the elementary reactions on the potential energy surface (PES) relevant for the C₅H₆ + HO₂ reaction are studied in this work. Stationary points on the PES are calculated by employing the CCSD(T)/cc-pVTZ//B3LYP/6-311G(d,p) level of theory. High-pressure limit and pressure-dependent rate constants for elementary reactions on this PES are calculated using conventional transition state theory (TST), variational transition-state theory (VTST) and Rice-Ramsberger-Kassel-Marcus/master equation (RRKM/ME) theory. In this work, the reaction channels for the C₅H₆ + HO₂ reaction_, which include H-abstraction channels from C₅H₆ by HO₂ to form the C₅H₅ + H₂O₂ and the addition channels through well-skipping pathways to form the bimolecular products C₅H₇ + O₂ or C₅H₆O + OH, or through C₅H₇O₂ stabilization and its unimolecular decomposition to form the bimolecular products C₅H₇ + O₂ or C₅H₆O + OH, namely sequential pathways, are studied. Also, the consuming reaction channels for the compounds C₅H₆O and C₅H₇ in the addition products are studied. The dominant reaction channels for these reactions are unraveled through comparing the energy barriers and rate constants of all elementary reactions and it is found: (1) HO₂ addition to cyclopentadiene C₅H₆ is more important than direct H-abstraction. (2) in the HO₂ addition channels, the well-skipping pathways and sequential pathways are competing and the well-skipping pathways will be favor in the higher pressures and the sequential pathways will be favor in the higher temperature. (3) The major consumption reaction channel for the five-member-ring compound C₅H₆O is the reaction channel to form C₄H₆ + CO and the major consumption reaction channel for the five-member-ring compound C₅H₇ is the reaction channel to form C₃H₅ + C₂H₂. High-pressure limit rate constants and pressure-dependent rate constants for elementary reactions on the PES are calculated, which will be useful in modeling the oxidation of aromatic compounds at low- and medium-temperatures.

An Intramolecular Hydrogen-Shift in a Peroxy Radical at Cryogenic Temperatures: The Reaction of 2-Hydroxyphenyl Radical with O2

Peroxy radical hydrogen-shifts are pivotal elementary reaction steps in the oxidation of small hydrocarbons in autoignition and the lower atmosphere. Although these reactions are typically associated with a substantial barrier, we demonstrate that the [1,5]H-shift in the peroxy species derived from the 2-hydroxyphenyl radical 1 is so facile that it even proceeds rapidly in an argon matrix at 35 K through a proton-coupled electron transfer mechanism. Hydrogen-bound complexes of o-benzoquinone are identified as the main reaction products by infrared spectroscopy although their formation through O-O bond scission is hampered by a barrier of 11.9 kcal mol^-1 at the ROCCSD(T)/cc-pVTZ/UB3LYP/6-311G(d,p) level of theory.

Oxidation of the para-Tolyl Radical by Molecular Oxygen under Single-Collison Conditions: Formation of the para-Toloxy Radical

Crossed molecular beam experiments were performed to elucidate the chemical dynamics of the para-tolyl (CH₃C₆H₄) radical reaction with molecular oxygen (O₂) at an average collision energy of 35.3 ± 1.4 kJ mol^-1. Combined with theoretical calculations, the results show that para-tolyl is efficiently oxidized by molecular oxygen to para-toloxy (CH₃C₆H₄O) plus ground-state atomic oxygen via a complex forming, overall exoergic reaction (experimental, -33 ± 16 kJ mol^-1; computational, -42 ± 8 kJ mol^-1). The reaction dynamics are analogous to those observed for the phenyl (C₆H₅) plus molecular oxygen system which suggests the methyl group is a spectator during para-tolyl oxidation and that application of phenyl thermochemistry and reaction rates to para-substituted aryls is likely a suitable approximation.

Theoretical Analysis of the Effect of C═C Double Bonds on the Low-Temperature Reactivity of Alkenylperoxy Radicals

Biodiesel contains a large proportion of unsaturated fatty acid methyl esters. Its combustion characteristics, especially its ignition behavior at low temperatures, have been greatly affected by these C═C double bonds. In this work, we performed a theoretical analysis of the effect of C═C double bonds on the low-temperature reactivity of alkenylperoxy radicals, the key intermediates from the low-temperature combustion of biodiesel. To understand how double bonds affect the fate of peroxy radicals, we selected three representative peroxy radicals from heptane, heptene, and heptadiene having zero, one, and two double C═C bonds, respectively, for study. The potential energy surfaces were explored at the CBS-QB3 level, and the reaction rate constants were computed using canonical/variational transition state theories. We have found that the double bond is responsible for the very different bond dissociation energies of the various types of C-H bonds, which in turn affect significantly the reaction kinetics of alkenylperoxy radicals.

Formation and stability of gas-phase o-benzoquinone from oxidation of ortho-hydroxyphenyl: a combined neutral and distonic radical study

The o-hydroxyphenyl radical reacts with O₂ to form o-benzoquinone + OH and cyclopentadienone is assigned as a secondary product.

Temperature and pressure dependent rate coefficients for the reaction of C2H4 + HO2 on the C2H4O2H potential energy surface

The potential energy surface (PES) for reaction C2H4 + HO2 was examined by using the quantum chemical methods. All rates were determined computationally using the CBS-QB3 composite method combined with conventional transition state theory(TST), variational transition-state theory (VTST) and Rice-Ramsberger-Kassel-Marcus/master-equation (RRKM/ME) theory. The geometries optimization and the vibrational frequency analysis of reactants, transition states, and products were performed at the B3LYP/CBSB7 level. The composite CBS-QB3 method was applied for energy calculations. The major product channel of reaction C2H4 + HO2 is the formation C2H4O2H via an OH(···)π complex with 3.7 kcal/mol binding energy which exhibits negative-temperature dependence. We further investigated the reactions related to this complex, which were ignored in previous studies. Thermochemical properties of the species involved in the reactions were determined using the CBS-QB3 method, and enthalpies of formation of species were compared with literature values. The calculated rate constants are in good agreement with those available from literature and given in modified Arrhenius equation form, which are serviceable in combustion modeling of hydrocarbons. Finally, in order to illustrate the effect for low-temperature ignition of our new rate constants, we have implemented them into the existing mechanisms, which can predict ethylene ignition in a shock tube with better performance.

Formation and chlorination of carbazole, phenoxazine, and phenazine

This contribution presents pathways for the formation of the three nitrogenated dioxin-like species, carbazole, phenoxazine, and phenazine via unimolecular rearrangements of diphenylamine (DPA) and its nitro substituents (NDPA). The latter represent major structural entities appearing in formulations of explosives and propellants. Intramolecular H transfer from the amine group to one of the two O atoms in the nitro group denotes the most accessible route in the unimolecular decomposition of NDPA. Further unimolecular rearrangements afford phenazine and carbazole. A loss of an ortho substituent from DPA, followed by addition of an oxygen molecule, prompts the formation of carbazole and phenoxazine in a facile mechanism. The consistency between trends in Fukui-based electrophilic indices and the experimental profiles of chlorinated carbazole, phenoxazine, and phenazine suggests the formation of these species by electrophilic substitution.

Formation of polybrominated dibenzofurans from polybrominated biphenyls

Decades after phasing out their production and use, especially in the formulations of brominated flame retardants (BFRs), polybrominated biphenyls (PBBs) still pose serious environmental and health problems. The oxidation of PBB has been hypothesised as a pathway for the formation of the notorious polybrominated dibenzofurans (PBDFs) and their dispersion in the environment. However, the exact reaction corridor remains misunderstood, with the existing mechanisms predicting the reaction to proceed via a high energy process that involves the breakage of C-C linkage (∼118.0 kcal mol(-1)) and the subsequent formation of bromophenols molecules, where the latter are supposed to act as precursors for the formation of PBDFs (∼40.0-60.0 kcal mol(-1)). Herein, we show that PBBs produce PBDFs in a facile mechanism through a series of highly exothermic reactions (i.e., overall barriers reside 8.2-10.0 kcal mol(-1) below the entrance channel). Whilst the fate of the ROO-type intermediates in oxidation of all aromatics is to emit CO or CO2, PBDFs constitute the dominant products from the oxidation of PBBs. Initially formed R-OO adduct evolves in a very exoergic mechanism to yield PBDFs. In view of the facile oxidative transformation of PBBs into PBDFs, we conclude that, it is unsafe to dispose BFRs in oxidation processes, as this practice generates high yields of toxic PBDFs.

Kinetics of homoallylic/homobenzylic rearrangement reactions under combustion conditions

Homoallylic/homobenzylic radicals refer to typical radicals with the radical site located at the β position from the vinyl/phenyl group. These radicals are largely involved in combustion systems, such as the pyrolysis or oxidation of alkenes, cycloalkanes, and aromatics. The 1,2-vinyl/phenyl migration via two steps (cyclization/fission) is a peculiar reaction type for the homoallylic/homobenzylic radicals, entitled homoallylic/homobenzylic rearrangement, which has been studied by theoretical calculations including the Hirshfeld atomic charge analysis in the present work. With the help of rate constant calculations, the competition between this reaction channel and other possible pathways under combustion temperatures (500-2000 K) were evaluated. Analogous 1,3- and 1,4-vinyl/phenyl migration reactions for similar radicals with the radical sites located at the γ and δ positions from the vinyl/phenyl group were also computed. The results indicate that the 1,2-vinyl/phenyl migration is particularly important for the kinetics of unimolecular reactions of homoallylic radicals under 1500 K; nevertheless, it still has noticeable contribution at higher temperature. For those radicals with the radical site at the γ or δ positions, the respective 1,3- or 1,4-vinyl/phenyl migration channel plays an insignificant role under combustion conditions.

Low temperature oxidation of benzene and toluene in mixture with n-decane

The oxidation of two blends, benzene/n-decane and toluene/n-decane, was studied in a jet-stirred reactor with gas chromatography analysis (temperatures from 500 to 1100 K, atmospheric pressure, stoichiometric mixtures). The studied hydrocarbon mixtures contained 75% of aromatics in order to highlight the chemistry of the low-temperature oxidation of these two aromatic compounds which have a very low reactivity compared to large alkanes. The difference of behavior between the two aromatic reactants is highly pronounced concerning the formation of derived aromatic products below 800 K. In the case of benzene, only phenol could be quantified. In the case of toluene, significant amounts of benzaldehyde, benzene, and cresols were also formed, as well as several heavy aromatic products such as bibenzyl, phenylbenzylether, methylphenylbenzylether, and ethylphenylphenol. A comparison with results obtained with neat n-decane showed that the reactivity of the alkane is inhibited by the presence of benzene and, to a larger extent, toluene. An improved model for the oxidation of toluene was developed based on recent theoretical studies of the elementary steps involved in the low-temperature chemistry of this molecule. Simulations using this model were successfully compared with the obtained experimental results.

Toxic pollutants emitted from thermal decomposition of phthalimide compounds

Phthalimide (PI) and tetrahydrophthalimide (THPI) are two structurally similar compounds extensively used as intermediates for the synthesis of variety of industrial chemicals. This paper investigates the thermal decomposition of PI and THPI under oxygen rich to oxygen lean conditions, quantifying the production of toxicants and explaining their formation pathways. The experiments involved a plug flow reactor followed by silica cartridges, activated charcoal trap and a condenser, with the decomposition products identified and quantified by Fourier transform infrared spectroscopy (FTIR), gas chromatography-mass spectrometry (GC-MS) and micro gas chromatography (μGC). The density functional theory (DFT) calculations served to obtain dissociation energies and reaction pathways, to elucidate the reaction mechanism. The oxidation of PI and THPI produced several toxic nitrogen-containing gases and volatile organic compounds, including hydrogen cyanide, isocyanic acid, nitrogen oxides, benzonitrile, maleimide and tentatively identified benzenemethanimine. The detection of dibenzo-p-dioxin (DD) and dibenzofuran (DF) suggests potential formation of the toxic persistent organic pollutants (POPs) in fires involving PI and THPI, in presence of a chlorine source. The oxidation of THPI produced 2-cyclohexen-1-one, a toxic unsaturated ketone. The results of the present study provide the data for quantitative risk assessments of emissions of toxicants in combustion processes involving PI and THPI.