Reflected Shock Tube Studies of High-Temperature Rate Constants for OH + CH4 → CH3 + H2O and CH3 + NO2 → CH3O + NO

Spin current in the early stage of radical reactions and its mechanisms

We study the electronic spin flux (atomic-scale flow of the spin density in molecules) by a perturbation analysis and ab initio nonadiabatic calculations. We derive a general perturbative expression of the charge and spin fluxes and identify the driving perturbation of the fluxes to be the time derivative of the electron-nucleus interaction term in the Hamiltonian. We then expand the expression in molecular orbitals so as to identify relevant components of the fluxes. Our perturbation theory describes the electronic fluxes in the early stage of reactions in an intuitively clear manner. The perturbation theory is then applied to an analysis of the spin flux obtained in ab initio calculations of the radical reaction of O2 and CH3· starting from three distinct spin configurations; (a) CH3· and triplet O2 with total spin of the system set Stot=1/2 (b) CH3· and singlet O2, Stot=1/2, and (c) CH3· and triplet O2, Stot=3/2. Further analysis of the time-dependent behaviors of the spin flux in these numerical simulations reveals (i) the spin flux induces rearrangement of the local spin structure, such as reduction of the spin polarization arising from the triplet O2 and (ii) the spin flux flows from O2 to CH3· in the reaction starting from spin configuration (a) and from CH3· to O2 in that starting from configuration (b), whereas no major intermolecular spin flux was observed in that starting from configuration (c). Our study thus establishes the mechanism of the spin flux that rearranges the local spin structures associated with chemical bonds.

New Stable and Fast Ring-Polymer Molecular Dynamics for Calculating Bimolecular Rate Coefficients with an Example of OH + CH4

The accurate and efficient calculation of the rate coefficients of chemical reactions is a key issue in the research of chemical dynamics. In this work, by applying the dimension-free ultrastable Cayley propagator, the thermal rate coefficients of a prototypic high dimensional chemical reaction OH + CH₄ → H₂O + CH₃ in the temperature range of 200 to 1500 K are investigated with ring polymer molecular dynamics (RPMD) on a highly accurate full-dimensional potential energy surface. Kinetic isotope effects (KIEs) for three isotopologues of the title reaction are also studied. The results demonstrate excellent agreement with experimental data, even in the deep tunneling region. Especially, the Cayley propagator shows a high calculation efficiency with little loss of accuracy. The present results confirmed the applicability of the RPMD method, particularly the speed-up using a Cayley propagator, in theoretical calculations of bimolecular reaction rates.

Catalytic Effect of CO2 and H2O Molecules on •CH3 + 3O2 Reaction

The methyl (•CH3) + 3O2 radical is an important reaction in both atmospheric and combustion processes. We investigated potential energy surfaces for the effect of CO2 and H2O molecules on a •CH3+ O2 system. The mechanism for three reaction systems, i.e., for •CH3 + 3O2, •CH3 + 3O2 (+CO2) and •CH3 + 3O2 (+H2O), were explored using ab initio/DFT methods [CCSD(T)//M062X/6-311++G(3df,3pd)] in combination with a Rice−Ramsperger−Kassel−Marcus (RRKM)/master-equation (ME) simulation between a temperature range of 500 to 1500 K and a pressure range of 0.0001 to 10 atm. When a CO2 and H2O molecule is introduced in a •CH3 + 3O2 reaction, the reactive complexes, intermediates, transition states and post complexes become thermodynamically more favorable. The calculated rate constant for the •CH3 + 3O2 (3 × 10−15 cm3 molecule−1 s−1 at 1000 K) is in good agreement with the previously reported experimentally measured values (~1 × 10−15 cm3 molecule−1 s−1 at 1000 K). The rate constant for the effect of CO2 (3 × 10−16 cm3 molecule−1 s−1 at 1000 K) and H2O (2 × 10−17 cm3 molecule−1 s−1 at 1000 K) is at least one–two-order magnitude smaller than the free reaction (3 × 10−15 cm3 molecule−1 s−1 at 1000 K). The effect of CO2 and H2O on •CH3 + 3O2 shows non-RRKM behavior, however, the effect on •CH3 + 3O2 shows RRKM behavior. Our results also demonstrate that a single CO2 and H2O molecule has the potential to accelerate a gas-phase reaction at temperature higher than >1300 K and slow the reaction at a lower temperature. The result is unique and observed for the first time.

Effect of Water and Formic Acid on ·OH + CH4 Reaction: An Ab Initio/DFT Study

In this work, we used ab initio/DFT method coupled with statistical rate theory to answer the question of whether or not formic acid (HCOOH) and water molecules can catalyze the most important atmospheric and combustion prototype reaction, i.e., ·OH (OH radical) + CH4. The potential energy surface for ·OH + CH4 and ·OH + CH4 (+X) (X = HCOOH, H2O) reactions were calculated using the combination of hybrid-density functional theory and coupled-cluster theory with Pople basis set [(CCSD(T)/ 6-311++G(3df,3pd)//M06-2X/6-311++G(3df,3pd)]. The results of this study show that the catalytic effect of HCOOH (FA) and water molecules on the ·OH + CH4 reaction has a major impact when the concentration of FA and H2O is not included. In this situation the rate constants for the CH4 + HO···HCOOH (3 × 10−9 cm3 molecule−1 s−1) reaction is ~105 times and for CH4 + H2O···HO reaction (3 × 10−14 cm3 molecule−1 s−1 at 300 K) is ~20 times higher than ·OH + CH4 (~6 × 10−15 cm3 molecule−1 s−1). However, the total effective rate constants, which include the concentration of both species in the kinetic calculation has no effect under atmospheric condition. As a result, the total effective reaction rate constants are smaller. The rate constants when taking the account of the FA and water for CH4 + HO···HCOOH (4.1 × 10−22 cm3 molecule−1 s−1) is at least seven orders magnitude and for the CH4 + H2O···HO (7.6 × 10−17 cm3 molecule−1 s−1) is two orders magnitude smaller than ·OH + CH4 reaction. These results are also consistent with previous experimental and theoretical studies on similar reaction systems. This study helps to understand how FA and water molecules change the reaction kinetic under atmospheric conditions for ·OH + CH4 reaction.

Dual Fuel Reaction Mechanism 2.0 including NOx Formation and Laminar Flame Speed Calculations Using Methane/Propane/n-Heptane Fuel Blends

This study presents the further development of the TU Wien dual fuel mechanism, which was optimized for simulating ignition and combustion in a rapid compression expansion machine (RCEM) in dual fuel mode using diesel and natural gas at pressures higher than 60 bar at the start of injection. The mechanism is based on the Complete San Diego mechanism with n-heptane extension and was attuned to the RCEM measurements to achieve high agreement between experiments and simulation. This resulted in a specific application area. To obtain a mechanism for a wider parameter range, the Arrhenius parameter changes performed were analyzed and updated. Furthermore, the San Diego nitrogen sub-mechanism was added to consider NOx formation. The ignition delay time-reducing effect of propane addition to methane was closely examined and improved. To investigate the propagation of the flame front, the laminar flame speed of methane–air mixtures was simulated and compared with measured values from literature. Deviations at stoichiometric and fuel-rich conditions were found and by further mechanism optimization reduced significantly. To be able to justify the parameter changes performed, the resulting reaction rate coefficients were compared with data from the National Institute of Standards and Technology chemical kinetics database.

A Novel Dual Fuel Reaction Mechanism for Ignition in Natural Gas–Diesel Combustion

In this study, a reaction mechanism is presented that is optimized for the simulation of the dual fuel combustion process using n-heptane and a mixture of methane/propane as surrogate fuels for diesel and natural gas, respectively. By comparing the measured and calculated ignition delay times (IDTs) of different homogeneous methane–propane–n-heptane mixtures, six different n-heptane mechanisms were investigated and evaluated. The selected mechanism was used for computational fluid dynamics (CFD) simulations to calculate the ignition of a diesel spray injected into air and a natural gas–air mixture. The observed deviations between the simulation results and the measurements performed with a rapid compression expansion machine (RCEM) and a combustion vessel motivated the adaptation of the mechanism by adjusting the Arrhenius parameters of individual reactions. For the identification of the reactions suitable for the mechanism adaption, sensitivity and flow analyzes were performed. The adjusted mechanism is able to describe ignition phenomena in the context of natural gas–diesel, i.e., dual fuel combustion.

Seasonal influences on surface ozone variability in continental South Africa and implications for air quality

Although elevated surface ozone (O₃) concentrations are observed in many areas within southern Africa, few studies have investigated the regional atmospheric chemistry and dominant atmospheric processes driving surface O₃ formation in this region. Therefore, an assessment of comprehensive continuous surface O₃ measurements performed at four sites in continental South Africa was conducted. The regional O₃ problem was evident, with O₃ concentrations regularly exceeding the South African air quality standard limit, while O₃ levels were higher compared to other background sites in the Southern Hemisphere. The temporal O₃ patterns observed at the four sites resembled typical trends for O₃ in continental South Africa, with O₃ concentrations peaking in late winter and early spring. Increased O₃ concentrations in winter were indicative of increased emissions of O₃ precursors from household combustion and other low-level sources, while a spring maximum observed at all the sites was attributed to increased regional biomass burning. Source area maps of O₃ and CO indicated significantly higher O₃ and CO concentrations associated with air masses passing over a region with increased seasonal open biomass burning, which indicated CO associated with open biomass burning as a major source of O₃ in continental South Africa. A strong correlation between O₃ on CO was observed, while O₃ levels remained relatively constant or decreased with increasing NO _x , which supports a VOC-limited regime. The instantaneous production rate of O₃ calculated at Welgegund indicated that ~ 40 % of O₃ production occurred in the VOC-limited regime. The relationship between O₃ and precursor species suggests that continental South Africa can be considered VOC limited, which can be attributed to high anthropogenic emissions of NO _x in the interior of South Africa. The study indicated that the most effective emission control strategy to reduce O₃ levels in continental South Africa should be CO and VOC reduction, mainly associated with household combustion and regional open biomass burning.

Bimolecular Reactions between Dimethylnitramine and Its Radical Decomposition Products

Bimolecular reactions between intact nitramines and their radical decomposition products can accelerate thermal decomposition, yet the detailed mechanisms of such reactions are not well understood. We have used density functional theory at the M06/6-311++G(3df,3pd) level to locate transition structures and compute 0 K activation barriers for various gas-phase reactions that may contribute to radical-assisted decomposition of dimethylnitramine (DMNA, (CH₃)₂NNO₂). Our calculations indicate that H abstraction from DMNA is the lowest-barrier mechanism for most radicals and a subsequent N-N β-scission in the alkyl radical 3 leads to an imine intermediate and NO₂. H abstraction is thus responsible for conversion of most radicals to NO₂. Also, among the nine radicals considered, NO is found to be least reactive and its reactions with DMNA yield dimethylnitrosoamine (DMNSA, (CH₃)₂NNO), a known product of DMNA decomposition.

Recent advances in quantum scattering calculations on polyatomic bimolecular reactions

This review surveys quantum scattering calculations on chemical reactions of polyatomic molecules in the gas phase published in the last ten years.

Quantum instanton calculation of rate constant for CH4 + OH → CH3 + H2O reaction: torsional anharmonicity and kinetic isotope effect

Thermal rate constants for the title reaction are calculated by using the quantum instanton approximation within the full dimensional Cartesian coordinates. The results reveal that the quantum effect is remarkable for the reaction at both low and high temperatures, and the obtained rates are in good agreement with experimental measurements at high temperatures. Compared to the harmonic approximation, the torsional anharmonic effect of the internal rotation has a little influence on the rates at low temperatures, however, it enhances the rate by about 20% at 1000 K. In addition, the free energy barriers for the isotopic reactions and the temperature dependence of kinetic isotope effects are also investigated. Generally speaking, for the title reaction, the replacement of OH with OD will reduce the free energy barrier, while substituting D for H (connected to C) will increase the free energy barrier.

Reflected Shock Tube and Theoretical Studies of High-Temperature Rate Constants for OH + CF3H ⇆ CF3 + H2O and CF3 + OH → Products

The reflected shock tube technique with multipass absorption spectrometric detection of OH radicals at 308 nm, using either 36 or 60 optical passes corresponding to total path lengths of 3.25 or 5.25 m, respectively, has been used to study the bimolecular reactions, OH+CF3H-->CF3+H2O (1) and CF3+H2O-->OH+CF3H (-1), between 995 and 1663 K. During the course of the study, estimates of rate constants for CF3+OH-->products (2) could also be determined. Experiments on reaction -1 were transformed through equilibrium constants to k1, giving the Arrhenius expression k1=(9.7+/-2.1)x10(-12) exp(-4398+/-275K/T) cm3 molecule(-1) s(-1). Over the temperature range, 1318-1663 K, the results for reaction 2 were constant at k2=(1.5+/-0.4)x10(-11) cm3 molecule(-1) s(-1). Reactions 1 and -1 were also studied with variational transition state theory (VTST) employing QCISD(T) properties for the transition state. These a priori VTST predictions were in good agreement with the present experimental results but were too low at the lower temperatures of earlier experiments, suggesting that either the barrier height was overestimated by about 1.3 kcal/mol or that the effect of tunneling was greatly underestimated. The present experimental results have been combined with the most accurate earlier studies to derive an evaluation over the extended temperature range of 252-1663 K. The three parameter expression k1=2.08x10(-17) T1.5513 exp(-1848 K/T) cm3 molecule(-1) s(-1) describes the rate behavior over this temperature range. Alternatively, the expression k1,th=1.78x10(-23) T3.406 exp(-837 K/T) cm3 molecule(-1) s(-1) obtained from empirically adjusted VTST calculations over the 250-2250 K range agrees with the experimental evaluation to within a factor of 1.6. Reaction 2 was also studied with direct CASPT2 variable reaction coordinate transition state theory. The resulting predictions for the capture rate are found to be in good agreement with the mean of the experimental results and can be represented by the expression k2,th=2.42x10(-11) T-0.0650 exp(134 K/T) cm3 molecule(-1) s(-1) over the 200-2500 K temperature range. The products of this reaction are predicted to be CF2O+HF.

High-temperature rate constants for CH3OH + Kr --> products, OH + CH3OH --> products, OH + (CH3)(2)CO --> CH2COCH3 + H2O, and OH + CH3 --> CH) + H2O

The reflected shock tube technique with multipass absorption spectrometric detection of OH radicals at 308 nm (corresponding to a total path length of approximately 4.9 m) has been used to study the dissociation of methanol between 1591 and 2865 K. Rate constants for two product channels [CH3OH + Kr --> CH3 + OH + Kr (1) and CH3OH + Kr --> 1CH2 + H2O + Kr (2)] were determined. During the course of the study, it was necessary to determine several other rate constants that contributed to the profile fits. These include OH + CH3OH --> products, OH + (CH3)2CO --> CH2COCH3 + H2O, and OH + CH3 --> 1,3CH2 + H2O. The derived expressions, in units of cm(3) molecule(-1) s(-1), are k(1) = 9.33 x 10(-9) exp(-30857 K/T) for 1591-2287 K, k(2) = 3.27 x 10(-10) exp(-25946 K/T) for 1734-2287 K, kOH+CH3OH = 2.96 x 10-16T1.4434 exp(-57 K/T) for 210-1710 K, k(OH+(CH3)(2)CO) = (7.3 +/- 0.7) x 10(-12) for 1178-1299 K and k(OH+CH3) = (1.3 +/- 0.2) x 10(-11) for 1000-1200 K. With these values along with other well-established rate constants, a mechanism was used to obtain profile fits that agreed with experiment to within <+/-10%. The values obtained for reactions 1 and 2 are compared with earlier determinations and also with new theoretical calculations that are presented in the preceding article in this issue. These new calculations are in good agreement with the present data for both (1) and (2) and also for OH + CH3 --> products.

High pressure effects on the mutual sensitization of the oxidation of NO and CH4–C2H6 blends

The mutual sensitization of the oxidation of NO and a CH(4)-C(2)H(6) (10 : 1) simulated natural gas (NG) blend was studied under fuel lean conditions (Phi = 0.5) at 50 atm and 1000-1500 K in the UIC high pressure shock tube (HPST). New experimental results were also obtained for the mutual sensitization of methane and the NG blend in the CNRS jet stirred reactor (JSR) at 10 atm. A detailed chemical kinetic model was assembled to describe the observed changes in reactivity in the CH(4) and NG blends, with and without NO, in the HPST and the JSR. The data and the validated model (tested against a variety of targets) show a reduced difference of reactivity between methane and NG blends in the presence of NO at characteristic reaction times for the JSR (250-1000 micros). However the HPST data and subsequent simulations using the validated model have revealed that at higher pressures and in the millisecond time scale regime representative of the HPST experiments (and practical combustion devices) there still persists a significant difference in reactivity between methane and NG blends in the presence of NO. The experimental data, the model development and validations and its predictions and utility as a tool to probe the NO-hydrocarbon sensitization effects under practical combustion conditions is discussed.

Reflected shock tube studies of high-temperature rate constants for OH + C2H2 and OH + C2H4

The reflected shock tube technique with multi-pass absorption spectrometric detection of OH-radicals at 308 nm (corresponding to a total path length of approximately 4.9 m) has been used to study the reactions, OH + C(2)H(2)--> products (1) and OH + C(2)H(4)--> C(2)H(3) + H(2)O (2). The present optical configuration gives a S/N ratio of approximately 1 at approximately 0.5-1.0 x 10(12) radicals cm(-3). Hence, kinetics experiments could be performed at [OH](0) = approximately 4-20 ppm thereby minimizing secondary reactions. OH was produced rapidly from the dissociations of either CH(3)OH or NH(2)OH (hydroxylamine). A mechanism was then used to obtain profile fits that agreed with the experiment to within <+/-5%. The derived Arrhenius expressions, in units of cm(3) molecule(-1) s(-1) are: k(1) = (1.03 +/- 0.24) x 10(-10) exp(-7212 +/- 417 K/T) for 1509-2362 K and k(2) = (10.2 +/- 5.8) x 10(-10) exp(-7411 +/- 871 K/T) for 1463-1931 K. The present study is the first ever direct measurement for reaction (1) at temperatures >1275 K while the present results extend the temperature range for (2) by approximately 700 K. These values are compared with earlier determinations and with recent theoretical calculations. The calculations agree with the present data for both reactions to within +/-10% over the entire T-range.

Product pair correlation in bimolecular reactions

The vast majority of chemical reactions involve polyatomic species as reactants and/or products. The added degree of complexity offers opportunities to address dynamical questions other than those already encountered in a typical atom + diatom reaction. Product pair correlation is one of them. This article introduces the basic concept, outlines the experimental approach we developed and then highlights some of the applications to bimolecular reaction dynamics. Particular emphasis is placed on the information contents and unique insights gained from this type of measurements, which otherwise would have been lost by the conventional approach.

Reflected Shock Tube Studies of High-Temperature Rate Constants for OH + NO2 → HO2 + NO and OH + HO2 → H2O + O2

The motivation for the present study comes from the preceding paper where it is suggested that accepted rate constants for OH + NO2 --> NO + HO2 are high by approximately 2. This conclusion was based on a reevaluation of heats of formation for HO2, OH, NO, and NO2 using the Active Thermochemical Table (ATcT) approach. The present experiments were performed in C2H5I/NO2 mixtures, using the reflected shock tube technique and OH-radical electronic absorption detection (at 308 nm) and using a multipass optical system. Time-dependent profile decays were fitted with a 23-step mechanism, but only OH + NO2, OH + HO2, both HO2 and NO2 dissociations, and the atom molecule reactions, O + NO2 and O + C2H4, contributed to the decay profile. Since all of the reactions except the first two are known with good accuracy, the profiles were fitted by varying only OH + NO2 and OH + HO2. The new ATcT approach was used to evaluate equilibrium constants so that back reactions were accurately taken into account. The combined rate constant from the present work and earlier work by Glaenzer and Troe (GT) is k(OH+NO2) = 2.25 x 10(-11) exp(-3831 K/T) cm3 molecule(-1) s(-1), which is a factor of 2 lower than the extrapolated direct value from Howard but agrees well with NO + HO2 --> OH + NO2 transformed with the updated equilibrium constants. Also, the rate constant for OH + HO2 suitable for combustion modeling applications over the T range (1200-1700 K) is (5 +/- 3) x 10(-11) cm3 molecule(-1) s(-1). Finally, simulating previous experimental results of GT using our updated mechanism, we suggest a constant rate for k(HO2+NO2) = (2.2 +/- 0.7) x 10(-11) cm3 molecule(-1) s(-1) over the T range 1350-1760 K.

Imaging the Reaction Dynamics of OH + CD4. 2. Translational Energy Dependencies

The reaction of OH + CD4 is investigated in a crossed-beam experiment over the collisional energies ranging from reaction threshold of about 5 to 16 kcal/mol. Exploiting a time-sliced ion velocity imaging detection scheme, the coincident information on the two polyatomic product pairs, HOD and CD3, is revealed in a state-correlated manner. The recently discovered vibrational mode-correlation between the two products is found to persist over the full range of collision energies of this study. In addition, the energy dependencies of the correlated cross section, state distribution, and angular distribution are elucidated, providing an unprecedented insight into this important reaction.

Reflected Shock Tube Studies of High-Temperature Rate Constants for CH3 + O2, H2CO + O2, and OH + O2

The reflected shock tube technique with multipass absorption spectrometric detection of OH-radicals at 308 nm, corresponding to a total path length of approximately 2.8 m, has been used to study the reaction CH3 + O2 CH2O + OH. Experiments were performed between 1303 and 2272 K, using ppm quantities of CH3I (methyl source) and 5-10% O2, diluted with Kr as the bath gas at test pressures less than 1 atm. We have also reanalyzed our earlier ARAS measurements for the atomic channel (CH3 + O2 --> CH3O + O) and have compared both these results with other earlier studies to derive a rate expression of the Arrhenius form. The derived expressions, in units of cm3 molecule(-1) s(-1), are k = 3.11 x 10(-13) exp(-4953 K/T) over the T-range 1237-2430 K, for the OH-channel, and k = 1.253 x 10(-11) exp(-14241 K/T) over the T-range 1250-2430 K, for the O-atom channel. Since CH2O is a major product in both reactions, reliable rates for the reaction CH2O + O2 --> HCO + HO2 could be derived from [OH]t and [O]t experiments over the T-range 1587-2109 K. The combined linear least-squares fit result, k = 1.34 x 10(-8) exp(-26883 K/T) cm3 molecule(-1) s(-1), and a recent VTST calculation clearly overlap within the uncertainties in both studies. Finally, a high sensitivity for the reaction OH + O2 --> HO2 + O was noted at high temperature in the O-atom data set simulations. The values for this obtained by fitting the O-atom data sets at later times (approximately 1.2 ms) again follow the Arrhenius form, k = 2.56 x 10(-10) exp(-24145 K/T) cm3 molecule(-1) s(-1), over the T-range, 1950-2100 K.