Major Reaction Channel Research Articles

Mechanistic and kinetic insights of the formation of allene and propyne from the C3H3 reaction with water.

Allene (H2C = C = CH2) and propyne (CH3-C≡CH) are important compounds in the combustion chemistry. They can be created from the reaction of proparyl radicals with water. In this study, therefore, a computational study into the C3H3 + H2O potential energy landscape has been carefully conducted. The computed results indicate that the reaction paths forming the products (allene: CH2CCH2 + •OH) and (propyne: HCCCH3 + •OH) prevail under the 300-2000K temperature range, where the latter is much more predominant compared to the former. However, these two products are not easily formed under ambient conditions due to the high energy barriers. In the 300 - 2000K temperature range, the branching ratio for the propyne + •OH product declines from 100 to 86%, whereas the allene + •OH product shows an increase, reaching 14% at 2000K. The overall bimolecular rate constant of the title reaction can be presented by the modified Arrhenius expression of ktotal = 1.94 × 10-12 T0.14 exp[(-30.55kcal.mol-1)/RT] cm3 molecule-1s-1. The total rate constant at the ambient conditions in this work, 2.37 × 10-34 cm3 molecule-1s-1, was found to be over five orders of magnitude lower than the total rate constant of the C3H3 + NH3 reaction, 7.98 × 10-29 cm3 molecule-1s-1, calculated by Hue et al. (Int. J. Chem. Kinet. 2020, 4(2), 84-91). The results in this study contribute to elucidating the mechanism of allene and propylene formation from the C3H3 + H2O reaction, and they can be used for modeling C3H3-related systems under atmospheric and combustion conditions. All the geometric structures of the C3H3 + H2O system were optimized by the B3LYP method in conjunction with the 6-311 + + G(3df,2p) basis set. Single-point energies of these species were calculated at the CCSD(T)/6-311 + + G(3df,2p) level of theory. The CCSD(T)/CBS level has also been used to compute single-point energies for the two major reaction channels (C3H3 + H2O → allene + •OH and C3H3 + H2O → propyne + •OH). Rate constants and branching ratios of the key reaction channels were calculated in the 300-2000K temperature interval using the Chemrate software based on the transition state theory (TST) with Eckart tunneling corrections.

Insight into the reactions of n-, iso-, sec- and tert-Butylperoxy radicals with hydroperoxyl radical in the Atmosphere: Reaction Mechanism, thermodynamic analysis and kinetics

Butylperoxy radical (C4H9O2) is recognized today as an important atmospheric intermediate which formed by the oxidation of butane (C4H10), playing a crucial role in the cycling of carbon. This work gives a theoretical investigation of the reaction mechanism, thermodynamic analysis and kinetics of the reactions between HO2 and four isomers (n-, iso-, sec- and tert-Butylperoxy radicals) of C4H9O2, which has been computed using the CCSD(T)/aug-cc-pVDZ//B3LYP/6-311G(d,p) level of theory. As a result, the above four reactions proceed on both singlet and triplet potential energy surfaces have been demonstrated. Additionally, the formation of alkylhydroperoxides and 3O2via triplet hydrogen abstraction from the four reactions are the major reaction channels while other singlet product channels are negligibly small. Besides, rate coefficients for the title reactions were estimated using the transition state theory (TST) over the temperature range 258 – 378 K, and these results exhibit a negative temperature dependence.

Photochemistry and Photophysics of Cholesta-5,7,9(11)-trien-3β-ol in Ethanol.

Cholesta-5,7,9(11)-trien-3β-ol (9,11-dehydroprovitamin D3, CTL) is used as a fluorescent probe to track the presence and migration of cholesterol in vivo. We recently described the photochemistry and photophysics of CTL in degassed and air-saturated tetrahydrofuran (THF) solution, an aprotic solvent. The zwitterionic nature of the singlet excited state, 1CTL* is revealed in ethanol, a protic solvent. In ethanol, the products observed in THF are accompanied by ether photoadducts and by photoreduction of the triene moiety to four dienes, including provitamin D3. The major diene retains the conjugated s-trans-diene chromophore and the minor is unconjugated, involving 1,4-addition of H at the 7 and 11 positions. In the presence of air, peroxide formation is a major reaction channel as in THF. X-ray crystallography confirmed the identification of two of the new diene products as well as of a peroxide rearrangement product.

Theoretical studies of the reactions of methyl ethyl ether with hydrogen atom, triplet oxygen atom and methyl radical

An analysis of reaction mechanisms and kinetics of the CH3OC2H5 + H, CH3OC2H5 + CH3• and CH3OC2H5 + O(3P) reactions has been examined. The M06-2X/cc-pVTZ level of theory has been used to calculate the geometries of all species and the standard reaction Gibbs free energy was calculated at the same theory. Fifteen reaction channels have been described, including the H-abstraction channels and CH3-transfer channels. Electronic energies were refined by CCSD(T)/CBS method and the rate coefficients were calculated using CVT/SCT method. For the title reactions, the H-abstraction channels from the CH2 group were identified as major reaction channels.

A comprehensive study on low-temperature oxidation chemistry of cyclohexane. I. Conformational analysis and theoretical study of first and second oxygen addition

To understand the low-temperature oxidation chemistry of cyclohexane, conformational analysis and theoretical study of the first and second oxygen addition are performed using quantum chemical calculations and kinetic calculations. Pressure- and temperature-dependent rate constants and branching ratios for major reaction channels are determined with RRKM/master-equation simulations over 298–2000 K and 0.01–1000 bar. The theoretical results indicate that the rapid inversion-topomerization processes facilitate fast equilibrium between axial and equatorial conformers. This can greatly counterbalance the influence of initial positions of side-chain groups in ROO, QOOH, cis-OOγQOOH and trans-OOγQOOH conformers. Conformational effects are found to be influential on the chain branching reaction sequences in second oxygen addition. The carbon ring prevents the conventional intramolecular H-transfer of cis-OOγQOOH conformers to yield ketohydroperoxides, as well as the inversion-topomerization from cis-OOγQOOH conformer to trans-OOγQOOH conformers. cis-OOγQOOH conformers mainly undergo alternative isomerization channel (cis-OOγQOOH→γP(OOH)2→alkenylhydroperoxides+OH→oxy radical+OH+OH), while trans-OOγQOOH conformers have both conventional isomerization channel (trans-OOγQOOH→ketohydroperoxides+OH→oxy radical+OH+OH) and alternative isomerization channel (trans-OOγQOOH→γP(OOH)2→alkenylhydroperoxides+OH→oxy radical+OH+OH). Kinetic calculation results also support the application of the thumb rule widely used in acyclic alkane oxidation that the rate constant of QOOH+O2 is roughly half of that of R+O2 in cyclic alkane oxidation, while it is indicated that estimating the rate constants of OOQOOH reactions from similar reactions of ROO may cause significant uncertainties.

SiO2-Ag Composite as a Highly Virucidal Material: A Roadmap that Rapidly Eliminates SARS-CoV-2.

COVID-19, as the cause of a global pandemic, has resulted in lockdowns all over the world since early 2020. Both theoretical and experimental efforts are being made to find an effective treatment to suppress the virus, constituting the forefront of current global safety concerns and a significant burden on global economies. The development of innovative materials able to prevent the transmission, spread, and entry of COVID-19 pathogens into the human body is currently in the spotlight. The synthesis of these materials is, therefore, gaining momentum, as methods providing nontoxic and environmentally friendly procedures are in high demand. Here, a highly virucidal material constructed from SiO2-Ag composite immobilized in a polymeric matrix (ethyl vinyl acetate) is presented. The experimental results indicated that the as-fabricated samples exhibited high antibacterial activity towards Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) as well as towards SARS-CoV-2. Based on the present results and radical scavenger experiments, we propose a possible mechanism to explain the enhancement of the biocidal activity. In the presence of O2 and H2O, the plasmon-assisted surface mechanism is the major reaction channel generating reactive oxygen species (ROS). We believe that the present strategy based on the plasmonic effect would be a significant contribution to the design and preparation of efficient biocidal materials. This fundamental research is a precedent for the design and application of adequate technology to the next-generation of antiviral surfaces to combat SARS-CoV-2.

Kinetic studies of excited singlet oxygen atom O(1D) reactions with ethanol

AbstractThe multichannel reaction of excited singlet oxygen atom with ethanol, O(1D) + C2H5OH (1), was studied in a photolysis flow reactor coupled with mid‐infrared Faraday rotation spectroscopy (FRS) and UV‐IR direct absorption spectroscopy (DAS) at 297 K with reactor pressures of 60, 120, and 150 Torr (bath He). The excited singlet oxygen atom was generated through the photolysis of O3 at 266 nm. The photon flux and O(1D) concentrations were determined by in situ actinometry based on O3 depletion. Temporal profiles of OH and H2O were monitored via DAS signals at ca. 3568.62 and 3568.29 cm−1, while temporal profiles of HO2 were measured via FRS signals at ca. 1396.90 cm−1. The branching ratios of the target reaction (1) were determined by fitting temporal profiles to simulations from an in‐house reaction mechanism. Two major reaction channels were identified as CH3CHOH + OH and CH3O + CH2OH, and their branching ratios were determined as 0.46 ± 0.12 and 0.42 ± 0.11, respectively. A specific HO2 + RO2 reaction between HO2 and O2CH2CH2OH (β‐RO2) at the low‐temperature range is estimated in this work as HO2 + O2CH2CH2OH products with a rate constant of 7 × 10−12 cm3 molecule−1 s−1.

Dissecting transmetalation reactions at the molecular level: C-B versus F-B bond activation in phenyltrifluoroborate silver complexes.

The gas-phase unimolecular reactions of the silver(i) complex [Ag(PhBF3)2]-, formed via electrospray ionisation mass spectrometry of solutions containing the phenyltrifluoroborate salt and AgNO3, are examined. Upon collision induced dissociation (CID) three major reaction channels were observed for [Ag(PhBF3)2]-: Ph- group transfer via a transmetalation reaction to yield [PhAg(PhBF3)]-; F- transfer to produce [FAg(PhBF3)]-; and release of PhBF3-. The anionic silver product complexes of these reactions, [LAg(PhBF3)]- (where L = Ph and F), were then mass-selected and subjected to a further stage of CID. [PhAg(PhBF3)]- undergoes a Ph- group transfer via transmetalation to yield [Ph2Ag]- with loss of BF3. [FAg(PhBF3)]- solely fragments via loss of BF4-, a reaction that involves Ph- group transfer from B to Ag in conjunction with F- transfer from Ag to B. Density functional theory (DFT) calculations on the various competing pathways reveal that: (i) the overall endothermicities govern the experimentally observed product ion abundances; (ii) the Ph- group and F- transfer reactions proceed via late transition states; and (iii) formation of BF4- from [FAg(PhBF3)]- is a multistep reaction in which Ph- group transfer from B to Ag proceeds first to produce a [FAgPh(BF3)]- complex in which the BF3 moiety is initially weakly bound to the ipso-carbon of the phenyl group and then migrates across the linear [FAgPh]- moiety from C to Ag to F yielding [PhAg(BF4)]-, which can then dissociate via loss of PhAg.

A theoretical kinetics study on low-temperature oxidation of n-C4H9 radicals

The low-temperature oxidation mechanism of n‑butyl radicals (n-C4H9) has been investigated by high level quantum chemical calculations coupled with the Rice–Ramsperger–Kassel–Marcus/Master Equation (RRKM/ME) theory. The potential energy surfaces (PES) were explored at the QCISD(T)/CBS//B3LYP/6-311++G(d,p) level. The temperature- and pressure-dependent rate constants were computed and fitted in modified Arrhenius parameters. The major reaction channels were discussed to more deeply understand the competing relationships between chain branching, chain propagation and termination reactions. The results show that the 1,5 H-shift reaction is more competitive than the 1,6 H-shift and 1,4 H-shift for isomerization reactions of n‑butyl peroxy radicals, and the concerted HO2 elimination channel to form butene becomes more important at high temperatures. Furthermore, based on our calculations, a revised kinetic model was developed to describe n-butane oxidation. Good consistency between model predictions and experimental data was shown. This study enhances our understanding of the combustion mechanism of n-butane and can be used as a reliable reference for mechanistic understanding of larger alkanes.

Real-time analysis of intermediate products from non-thermal plasma degradation of ethyl acetate in air using PTR-MS: Performance evaluation and mechanism study

Non-thermal plasma (NTP) has developed into an emerging end-of-pipe technology for treating volatile organic compounds (VOCs) present in unhygienic point source of air streams. In this work, NTP oxidation of low-concentration ethyl acetate was performed in a coaxial double dielectric barrier discharge reactor. The effects of initial ethyl acetate concentration, gas flow rate, and external electrode length on ethyl acetate degradation were systematically investigated as a function of discharge power. In addition, detailed real-time and online proton transfer reaction mass spectrometry analysis was used to identify the transient species formation and transition in the various NTP oxidation periods of ethyl acetate. Based on the analysis of organic by-products, the degradation mechanism was speculated and the major reaction channels were presented. This study would deepen the understanding of plasma degradation of VOCs and reveal the plasma-chemical mechanism.

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.

Corporate social responsibility, market reaction and accounting conservatism

PurposeNon-financial information disclosure may reflect the quality of corporate financial reports or disclosure policy choices. The authors examine the relationship between corporate social responsibility (CSR) and accounting conservatism and also investigate channels through which such effects are transmitted. The purpose of this paper is to explore how CSR, as non-financial information that has received widespread attention, affects choices regarding corporate financial policy.Design/methodology/approachUsing ordinary least squares regression, the authors analyze China CSR Score data for 2010–2018. They control certain influencing variables related to the nature and characteristics of enterprises and discover that CSR can effectively increase accounting conservatism. Then, they extract the components of market reactions to CSR and study the market reaction path of CSR as it affects financial policy. They also conduct a robustness test to ensure that the results are not accidental in a complex environment.FindingsThe results reveal the influence of non-financial information on firms’ financial policy. In addition, the results confirm the attraction of liquidity and investor attention as the major market reaction channels by which CSR significantly promotes accounting conservatism. Additionally, other critical paths of influence deserve further exploration. The results remain robust for alternate measures of accounting conservatism, different components of CSR, other proxies on CSR, endogenous testing and alternate estimation methods.Originality/valueThe study represents the first analysis of the influence of CSR information disclosure on accounting conservatism in emerging markets, and it undertakes a preliminary exploration to clarify the mechanism of CSRs’ role in accounting conservatism. The results also provide a policy reference for external supervision and internal governance of enterprises. Thus, the results can help company managers maintain a favorable corporate image and establish a high-level investor protection mechanism.

OH-Initiated Reactions of para-Coumaryl Alcohol Relevant to the Lignin Pyrolysis. Part II. Kinetic Analysis.

Monolignols are precursor units and primary products of lignin pyrolysis. The currently available global (lumped) and semidetailed kinetic models, however, are lacking the comprehensive decomposition kinetics of these key intermediates in order to advance toward the fundamentally based detailed chemical-kinetic models of biomass pyrolysis. para-Coumaryl alcohol (HOPh-CH═CH-CH2OH, p-CMA) is the simplest of the three basic monolignols containing a typical side-chain double bond and both alkyl and phenolic type OH groups. The two other monomers additionally contain one and two methoxy groups, respectively, attached to the benzene ring. Previously, we developed a detailed fundamentally based mechanism for unimolecular decomposition of p-CMA (as well as its truncated allyl and cinnamyl alcohol models) and explored its reactivity toward H radicals generated during pyrolysis. The reactions of p-CMA with pyrolytic OH radicals is another set of key reactions particularly important for understanding the formation mechanisms of a wide variety of oxygenates in oxygen-deficit (anaerobic) conditions and the role of the lignin side groups in pyrolysis pathways. In Part I of the current study (J. Phys. Chem. A, 2019, 123, 2570-2585), we reported a detailed potential energy (enthalpy) surface analysis of the reaction OH + p-CMA with suggestions for a variety of chemically activated, unimolecular, and bimolecular reaction pathways. In Part II of our work, we provide a detailed kinetic analysis of the major reaction channels to evaluate their significance and possible impacts on product distributions. Temperature- and pressure-dependent rate constants are calculated using the quantum Rice-Ramsperger-Kassel method and the master equation analysis for falloff and stabilization. Enthalpies of formation, entropies, and heat capacities are calculated using density functional theory and higher-level composite methods for stable molecules, radicals, and transition-state species. A significant difference between well depths for the chemically activated adduct radicals, [p-CMA-OH]*, is found for the α- and β-carbon addition reactions to generate the 1,3- and 1,2-diol radicals, respectively. This is due to the synergistic effect from conjugation of the proximal radical center with the aromatic ring and the strong H-bonding interaction between vicinal OH groups in the β-adduct (1,2-diol radical). Both adducts undergo isomerization and low-energy transformations, however, with different kinetic efficiencies because of the difference in stabilization energies. Reaction pathways include dissociation, intramolecular abstraction, atom and group transfers, and elimination. Of particular interest is a roaming-like low-energy dehydration reaction to form O-centered intermediate radicals. The kinetic analysis demonstrated the feasible formation of various products detected in pyrolysis experiments, suggesting that the gas-phase reactions of OH radicals can be a key process to form major products and complex oxygenates during lignin pyrolysis. Our preliminary experiments involving pyrolysis of the vaporized monomers support this basic statement. A novel mechanism for the formation of benzofuran, identified in experimentation, is also provided based on the potential conversions of hydroxyphenylacetaldehyde and corresponding isomers, which are kinetically favored products.

A novel contrast of the reactions of 2,4,6-trinitrotoluene (TNT) in atmospheric-pressure O2 and N2 plasma: Experimental and theoretical study

Negative-mode ion/molecule reactions of 2,4,6-trinitrotoluene (TNT) generated in atmospheric-pressure O2 and N2 corona discharge plasmas were studied. In the O2 plasma, charge transfer reaction, O2−• + TNT → O2 + TNT−•, was found as the major reaction channel. The proton abstraction reaction, O2−• + TNT → O2H• + [TNT − H+]−, was negligible, which is in sharp contrast to the formation of [DNT − H+] and [NT − H+] (DNT: 2,4-dinitrotoluene and NT: 2-nitrotoluene). TNT−• further reacts with O3 to form [TNT−NO•]− and [TNT−OH•]−. In atmospheric-pressure N2 plasma, TNT−• was detected as the dominant ion with no fragment ions.Adiabatic electron affinity (EA) of TNT was estimated by density functional theory (DFT) calculations as 2.08 eV. The precise mechanisms of the charge transfer, O2−• + TNT → TNT−• + O2, and TNT−• + O3 were examined by the frontier-orbital theory. The charge transfer may be attained by the specific O2−• coordination on the benzene ring of TNT. Reaction paths of [TNT + O3]−• leading to [TNT − OH•]− and [TNT − NO•]− were investigated by bimolecular reaction of TNT−• with O3. The formation of [TNT−NO•]− was found to be energetically more favorable than that of [TNT−OH•]−.

Analysis of Photo Neutron Dose to Patient from a 15 MV Linear Accelerator

Medical Linear Accelerator produces high-intensity bremsstrahlung beam, with typical energy ranges from 4 to 18MV for treatment purposes. The materials including patient exposed to this high energy spectrum lead to a photonuclear reaction in them. In these Photon energy domains, photo neutron production (n, p, and γ, n) is the major reaction channel. This is important as Carbon, Oxygen, Hydrogen, and Nitrogen are the predominant elements in a typical human being and having low photon neutron threshold. Apart from this, the equipment involved in a radiation treatment including Head, Couch, immobilization accessories, etc contributes to photo neutron production. The present study was recorded and analyzed the photo neutron spectra due to 15 MV medical linear accelerator using CR-39 Solid State Nuclear Track Detector and image processing software.

The benchmark experiment on slab iron with D-T neutrons for validation of evaluated nuclear data

An experimental system for benchmark validation of nuclear data with slab samples has been set up at China Institute of Atomic Energy (CIAE). Neutron leakage spectra in the range of 0.8–16 MeV from iron slab samples were measured by time-of-flight technique using a D-T neutron source with the measured angle at 60° and 120°. The thicknesses of the slabs were chosen to be 5 cm, 10 cm and 15 cm. The experimental results were compared with the calculated ones by MCNP-4c simulation, using the evaluated data of iron from the CENDL-3.1, ENDF/B-VII.1 and JENDL-4.0 libraries.From the comparison of neutron spectra and the C/Es between the measured and the calculated results, it has been found that some discrepancies in the major reaction channels including (n, p), (n, n′) and (n, 2n) are existing. Detailed information about the neutron product distribution with the evaluated data has been presented to explain the discrepancies which turn out to the data of the above three libraries that needed to be improved as required in different energy ranges.

Tuning the Reactivities of the Heteronuclear [Aln V3-n O7-n ]+ (n=1, 2) Cluster Oxides towards Methane by Varying the Composition of the Metal Centers.

The thermal gas-phase reactions of [Al2 VO5 ]+ and [AlV2 O6 ]+ with methane have been explored by using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry complemented by high-level quantum chemical calculations. Both cluster ions chemisorbed methane as the major reaction channels at room temperature. [Al2 VO5 ]+ could break only one C-H bond to liberate CH3 , whereas [AlV2 O6 ]+ exhibited higher oxidizing ability such that it brings about the selective generation of formaldehyde. Mechanistic aspects are revealed and the crucial roles of the metal centers are discussed.

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

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.

Electromagnetic form factors of nucleon resonances from CLAS

Exclusive single and double meson photo- and electroproduction reactions are the largest sources of information on the spectrum and structure of nucleon resonances. The excited states of the nucleon manifest as a complex interplay between the inner core of three dressed quarks and the external meson-baryon cloud. Various N* with distinctively different structure appear as unique laboratory where many features of strong QCD can be explored. With combination of nearly 4π acceptance of the CLAS detector and continuous electron beam (Jefferson Lab, USA) it is possible to obtain physics observables of the major reaction channels in the N* excitation region. The results on the electromagnetic transition form factors of N(1440)1/2+ and N(1520)3/2- are presented.

Revisiting the chemical kinetics of CH3 + O2 and its impact on methane ignition

The reaction between the methyl radical and molecular oxygen is one of the most important combustion reactions—it plays a controlling role in methane ignition. Although the reaction kinetics of CH3 + O2 has been extensively studied (at least experimentally) in the literature, high-level theoretical studies are still desired to understand the chemistry of this “small but complicated” reaction system and provide rate constants over wide range of temperature and pressure. In the present study, the rate coefficients of three major reaction channels of CH3 + O2, i.e., CH3OO stabilization (R1), CH3O + O channel (R2), and CH2O + OH channel (R3), were computed with temperature ranging from 300 to 2500 K and pressure from 0.01 to 100 atm by combining high-level quantum chemical calculations (CCSD(T)-F12/cc-pVQZ-F12//QCISD/6-311++G(2df,2p)) and RRKM/master equation simulations (with variable reaction coordinate transition state theory simulation for the barrierless channels). The computational results were compared with previously reported rate constants based on a thorough literature review. They show satisfactory agreement with those most recent measurements over the limited experimental temperature ranges—the discrepancies are within a factor of two for the three dominant reactions. The rate constants of R2 and R3 have very high sensitivity coefficients to the ignition delay time of methane. The impact of this reaction kinetics on methane ignition was evaluated by putting our currently computed rate coefficients into two recently reported kinetic models for methane combustion. There was a significant difference on the simulated ignition delay for an RCM experiment by replacing the rate constants of R2 and R3 with the newly computed results. It indicates the risk of estimating rate constants by simple extrapolation from experimental measurements over limited temperature ranges. The modified Arrhenius representations of the reaction channels of both CH3 + O2 and CH3OO dissociation were provided for utility in combustion modeling.