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New Insight into Competition between Decomposition Pathways of Hydroperoxymethyl Formate in Low Temperature DME Oxidation
Abstract
Hydroperoxymethyl formate is a crucial intermediate formed during the low-temperature oxidation of dimethyl ether. The decomposition pathways of HOOCH2OCHO were calculated at QCISD(T)/CBS//B3LYP/6-311++G(d,p) level. The temperature- and pressure-dependent rate constants are computed using microcanonical variational transition state theory coupled with the RRKM/master equation calculations. The calculations show that a pathway leads to the formation of formic acid and a Criegee intermediate does exist, besides the direct dissociation channel to OH and OCH2OCHO radicals. However, formation of the Criegee intermediate has never been considered as an intermediate in dimethyl ether combustion before. The computed rate constants indicate that the newly confirmed pathway is competitive to the direct dissociation route and it is promising to reduce the low-temperature oxidation reactivity. Also electronic effect of groups, e.g. -CHO and O atom, is taken into account. Moreover, Hirshfeld atomic charge and natural bond order analysis are performed to explain this phenomenon from a perspective of chemical nature.
... The concentration of HPMF in the atmosphere can reach the ppt level according to the results of model simulation ( Khan et al., 2018b ), and maybe underestimated as there are still many unknown sources for both CH 2 OO and HCOOH (HPMF is the main product of the reaction of CH 2 OO and HCOOH). Also, early studies suggested HPMF was a crucial intermediate formed during the oxidation of dimethyl ether (DME) Carter, 2003, 2006 ;Xing et al., 2015 ). There is still a lack of HPMF concentration measurement data in the actual observation. ...
... Previous studies have been carried out to study the degradation of HPMF by theoretical methods. Compared with the bimolecular reaction discussed in this study, the degradation of HPMF has higher energy barrier ( Genossar et al., 2020 ;Xing et al., 2015 ). HPMF has a rigid structure, and the occurrence of monomolecular degradation means that intramolecular hydrogen bonding must be overcome first ( Chung et al., 2019 ). ...
The reaction mechanism and kinetics of the simplest Criegee intermediate CH2OO reaction with hydroperoxymethyl formate (HPMF) was investigated at high-level quantum chemistry calculations. HPMF has two reactive functional groups, -C(O)OH and -OOH. The calculated results of thermodynamic data and rate constants indicated that the insertion reactions of CH2OO with –OOH group of HPMF were more favorable than the reactions of CH2OO with -C(O)OH group. The calculated overall rate constant was 2.33 × 10⁻¹³ cm³/(molecule⋅sec) at 298 K and the rate constants decreased as the temperature increased from 200 to 480 K. In addition, we also proved the polymerization reaction mechanism between CH2OO and -OOH of HPMF. This theoretical study interpreted the previous experimental results, and supplied the structures of the intermediate products that couldn't be detected during the experiment.
... In this process, hydroxymethyl hydroperoxide can be produced [23,50]. Similarly, the bimolecular reaction of the CI with formic acid could lead to hydroperoxymethyl formate (HPMF, HOOCH2OCHO) [51][52][53][54][55][56]. The experimental measurements reveal a very fast rate for the reaction of the CI with carboxylic acids [57,58] For larger alkenes ( Fig. 2(b)), the formed CIs could undergo a 1,4-hydrogen shift to form an α-hydroperoxide alkene (i.e., a vinylhydroperoxide (VHP) intermediate [59]). ...
... In addition to these channels, the Korcek reaction of HPMF could also produce formic acid [157,180]. However, other theoretical calculation revealed that HMPF can easily dissociate into formic acid, and the smallest Criegee intermediate (CH2OO) [51][52][53][54], instead of dissociation via the Korcek mechanism (Section 4.2.2). This pathway could be another source of formic acid in DME low-temperature oxidation. ...
The aim of this paper is to review recent progress in detection and quantification of hydroperoxides, and to understand their reaction kinetics in combustion environments. Hydroperoxides, characterized by an OOH group, are ubiquitous in the atmospheric oxidation of volatile organic compounds (∼300 K), and in the liquid and gas phase oxidation of fuel components at elevated temperatures (∼400–1000 K). They are responsible for two-stage fuel ignition in internal combustion engines and they play an important role in the formation and evolution of secondary organic aerosols in the atmosphere. The introduction outlines the importance of hydroperoxide chemistry in combustion reaction processes. In addition to this main topic, the role of hydroperoxides in atmospheric and liquid phase oxidation chemistry is also introduced, for a more general perspective. The second part of this paper briefly reviews the mechanistic insights of hydroperoxide chemistry in combustion systems, including experimental detection of these reactive species before 2010. Since that time significant progress has been made by advanced diagnostic techniques like tunable synchrotron vacuum ultraviolet photoionization mass spectrometry and infrared cavity ring-down spectroscopy. The third chapter of this work summarizes progress in gas phase oxidation experiments to measure hydrogen peroxide, alkyl hydroperoxides, olefinic hydroperoxides, ketohydroperoxides, and more complex hydroperoxides that include as many as five oxygen atoms. The fourth section details recent advances in understanding the combustion chemistry of hydroperoxides, involving the formation of carboxylic acids and diones, as well as the development of oxidation models that include a third O2 addition reaction mechanism. Finally, challenges are discussed, and perspectives are offered regarding the future of accurately measuring molecule-specific hydroperoxide concentrations and understanding their respective reaction kinetics.
Hydroperoxymethyl formate (HPMF), a carbonyl-hydroperoxide-like species, plays a vital role in the low temperature oxidation of DME because HPMF is formed with ȮH as a co-product, and it can also decompose to generate ȮH radicals. Unfortunately there is neither theoretical nor experimental evidence nor an established explanation of HPMF kinetics in the literature. A theoretical understanding of the potential energy surface of HPMF dissociation kinetics is evaluated at the CCSD(T)/CBS//M06-2X/6-311++G(d,p) level of theory. Microcanonical variational transition state theory and Rice-Ramsperger-Kassel-Marcus/Master Equation calculations were performed to obtain temperature- and pressure-dependent rate coefficients. Two main reaction pathways, (1) the widely recognized channel involving O–O bond fission to produce ȮH + ȮCH2OCHO radicals, and (2) a novel channel producing HOCHO + CH2OO, were considered to be the most competitive reaction channels. Calculation results reveal that there are significant differences between the current calculations and literature values, and the pressure-dependent behavior of HPMF decomposition cannot be ignored, particularly at low temperatures (T < 600 K). The orders of magnitude difference between the model recommendations and theoretical calculations make it reasonable to doubt the previous kinetic description of HPMF O–O bond fission, with its mechanism remaining unreliable. Generally, this study serves as theoretical evidence to provide new insights into HPMF kinetics but also questions our current understanding of the low temperature DME oxidation mechanism.
To obtain insight into the catalytic reaction mechanism of biodiesels over ZSM-5 zeolites, the pyrolysis and catalytic pyrolysis of methyl butanoate, a biodiesel surrogate, with H-type ZSM-5 (HZSM-5) were performed in a flow re-actor under atmospheric pressure. The pyrolysis products were identified and quantified using gas chromatography-mass spectrometry (GC-MS). Kinetic modelling and experimental results revealed that H-atom abstraction in the gas phase was the primary pathway for methyl butanoate decomposition during pyrolysis, but dissociating to ketene and methanol over HZSM-5 was the primary pathway for methyl butanoate consumption during catalytic pyrolysis. The initial decomposition temperature of methyl butanoate was reduced by approximately 300 K over HZSM-5 compared to that for the uncatalyzed reaction. In addition, the apparent activation energies of methyl butanoate under catalytic pyrolysis and homogeneous pyrolysis conditions were obtained using the Arrhenius equation. The significantly reduced apparent activation energy confirmed the catalytic performance of HZSM-5 for methyl butanoate pyrolysis. The activation temperature may also affect some catalytic properties of HZSM-5. Overall, this study can be used to guide subsequent catalytic combustion for practical biodiesel fuels.
The objective of this work is to present a unified collection of structural and chemical information on a series of neutral chemical tri-elemental species up to a molecular formula C2H2O8, which may be used for validation purposes, for deep structured learning or indeed more simply for basic data of a single species. Such a collection vastly is tightly focused in terms of its component parts, contains novel results, and covers a number of chemical classes including stable molecules, radicals, carbenes, dipolar species, and excited states. Wherever possible, comparisons are made to the experimental and quantum chemical literature of gas-phase molecules, but the paucity of such means that there is only a very limited scope for validation. The primary data consist of structural information in the form of Cartesian coordinates, rotational constants together with vibrational frequencies, and anharmonicity coefficients, all obtained through density functional, B3LYP, calculations with the cc-pVTZ+d basis set. Standard statistical thermodynamic relations are then used to compute entropy, specific heat at constant pressure, and an enthalpy function over temperatures from 298.15 K to 2000 K. Supplementary material contains all the information necessary to carry out these calculations over different conditions as required as well as the raw species data. High-level quantum mechanical computations employing composite model chemistries, including CBS-QB3, CBS-APNO, G3, G4, W1BD, WMS, W2X, and W3X-L, are used to derive formation enthalpies via atomization and/or isodesmic calculations as appropriate.
Criegee intermediates have raised much attention in atmospheric chemistry because of its significance in ozonolysis mechanism. The simplest Criegee intermediate, CH2OO, and its reactions with acrylic acid including cycloadditions and insertions as main entrance channels have been investigated at CCSD(T)/cc-pVTZ//M06-2X/6-31G(d,p) level. Temperature- and pressure-dependent kinetics were predicted by solving the time-dependent master equations based on Rice-Ramsperger-Kassel-Marcus theory using MESS program, with temperature from 200 to 500 K, pressure from 0.001 to 1000 atm. Variational transition state theory (VTST) were used for barrierless pathways, and conventional transition state theory (CTST) for pathways with distinct barriers. Results indicate that hydroperoxymethyl acrylate is the dominant product at atmospheric condition. Combination of two reactants will reduce the volatility and makes a possible factor that induces formation of secondary organic aerosols, which suggests CH2OO’s entangled role in ever-increasing air pollution.
The potential energy surfaces (PES) for the reactions of methyl crotonate (MC) with hydroxyl radical, including H-abstraction and OH-addition, were explored by the QCISD(T)/CBS//M062x/6-311++G(d,p) and CBS-QB3 methods, respectively. The subsequent isomerization and β-scission reactions of the produced primary radicals were also investigated using these methods. The phenomenological rate coefficients of these reactions were predicted over 500 to 1800 K and 0.001 to 100 atm via solving the time-dependent master equations based on Rice–Ramsperger–Kassel–Marcus theory. The rate coefficients of the two dominant H-abstraction channels at 1 atm were expressed as the sum of two modified Arrhenius equations (units: cm³ per molecule per s and cal mol⁻¹): k1 (MC + OH → PC1 + H2O) = 2.29 × 10⁻⁴T−2.02 exp(−10 204/RT) + 7.31 × 10⁻¹³T0.1 exp(−819/RT), k2 (MC + OH → PC2 + H2O) = 1.83 × 10⁻²T−2.39 exp(−10 360/RT) + 1.27 × 10⁻⁹T−0.57 exp(−2858/RT). The sub-mechanisms of MC + OH and its primary radicals were validated using a previously described combustion model; the updated model well reproduced the previous experimental results. The present rate coefficients provide valuable kinetic data to improve our understanding of the combustion mechanism of MC and can be used as a reliable reference for further investigation of practical fuels.
Oxygenates with carbonyl and hydroperoxy functional groups are important intermediates that are generated during the autooxidation of organic compounds in the atmosphere and during the autoignition of transport fuels. In the troposphere, the degradation of carbonyl hydroperoxides leads to low-vapor-pressure polyfunctional species that be taken into in cloud and fog droplets or to the formation of secondary organic aerosols (SOAs). In combustion, the fate of carbonyl hydroperoxides is important for the performance of advanced combustion engines, especially for autoignition. A key fate of the carbonyl hydroperoxides is reac-tion with OH radicals, for which kinetics data are experimentally unavailable. Here, we study 4-hydroperoxy-2-pentanone (CH3C(=O)CH2CH(OOH)CH3) as a model compound to clarify the kinetics of OH reactions with carbonyl hydroperoxides, in par-ticular H-atom abstraction and OH addition reactions. With a combination of electronic structure calculations, we determine previ-ously missing thermochemical data, and with multipath variational transition state theory (MP-VTST), a multidimensional tunnel-ing (MT) approximation, multiple-structure anharmonicity, and torsional potential anharmonicity we obtained much more accurate rate constants than the ones that can computed by conventional single-structure harmonic transition state theory (TST) and than the empirically estimated rate constants that are currently used in atmospheric and combustion modeling. The roles of various factors in determining the rates are elucidated. The pressure-dependent rate constants for the addition reaction are computed using system-specific quantum RRK theory. The calculated temperature range is 298-2400 K, and the pressure range is 0.01–100 atm. The accu-rate thermodynamic and kinetics data determined in this work are indispensable in the global modeling of SOAs in atmospheric science and in the detailed understanding and prediction of ignition properties of hydrocarbons and alternative fuels.
The reaction of CH2OO, the smallest carbonyl oxide (Criegee intermediate, CI), with several acids was investigated using the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ quantum chemical method, as well as microvariational transition state theory and RRKM master equation theoretical kinetic methodologies. For oxoacids HNO3 and HCOOH, a 1,4-insertion mechanism allows for barrierless reactions with high rate coefficients, in agreement with literature experimental data. This mechanism relies on the presence of a double bond in α-position to the acidic OH group. We predict that reactions of CI with enols will likewise have high rate coefficients, reacting through a similar mechanism. The hydracid HCl was found to react through a less favorable 1,2-insertion reaction, leading to lower rate coefficients, again in good agreement with literature. We conclude that the reaction mechanism is the main indicator for the reaction rate for CH2OO + acid reactions, with acidity only of secondary influence. At room temperature and 1 atm the main product for all reactions was found to be the thermalized hydroperoxide initial adduct, with minor yields of fragmentation products. One of the product channels characterized is a novel reaction path involving intramolecular H-abstraction after a roaming reaction in the OH + product radical complex formed from dissociation of the hydroperoxide adduct; this channel is the lowest fragmentation route for some of the reaction studied.
Criegee Sighting
Standard mechanistic models for the reaction of ozone with unsaturated hydrocarbons implicate a transient carbonyl oxide compound, termed the “Criegee intermediate,” which has largely eluded detection. Welz et al. (p. 204 ; see the Perspective by Marston ) have now detected the compound by using mass spectrometry, following the low-pressure photolytic reaction of oxygen with diiodomethane, and measured its decay kinetics in the presence of nitric oxide, nitrogen dioxide, and sulfur dioxide. Reaction rates were higher than expected, suggesting that the intermediate may play a more prominent role in atmospheric chemistry than previously assumed.
This undergraduate level textbook was updated with a mixture of both SI and English units. Additional material concerning numerical methods in solving transport problems is included in this edition, but emphasis is on the understanding of fundamental transport processes. The organization of the text is such that the vertical arrangement of momentum transfer, energy transfer and mass transfer may be followed, as well as a 'horizontal' approach covering similar subject matters for all three types of transfer. (A.J.)
Recent experiments on low-temperature oxidation of dimethyl ether (DME) at atmospheric pressure reveal much lower fuel conversion than the predictions of all published models. Consistent with previous measurements, the present work on DME oxidation in a laminar flow reactor between 400 and 1160 K at atmospheric pressure also confirmed this behavior. To reduce the gap between model predictions and experimental results, both regarding key oxygenated species and fuel conversion, an extended and updated kinetic model of DME oxidation was developed from the widely used model of Z. Zhao, M. Chaos, A. Kazakov, F.L. Dryer, Int. J. Chem. Kinet. 40 (2008) 1–18. The development of the model focused on the reaction sequences that affect the low-temperature oxidation reactivity. In particular, the reactivity of DME oxidation in the low-temperature regime and at atmospheric pressure can now be predicted. This is possible upon the inclusion of an additional, chemically-activated pathway of CH3OCH2 reaction with O2, as well as of the new decomposition chemistry of OCH2OCHO radical, investigated theoretically in this work. The present model was examined against species concentrations in DME oxidation from low to intermediate temperatures and shock tube ignition delay times. It shows satisfactory performance in reproducing the respective literature data.
As two of the most important species that characterize hydrocarbon low temperature ignition, HO2 and H2O2 formation during dimethyl ether (DME) oxidation was quantified using the same experimental conditions, for the first time, in an atmospheric flow reactor at low and intermediate temperature range. Dual-Modulation Faraday Rotation Spectroscopy (DM-FRS) and Molecular Beam Mass Spectrometry (MBMS) were used to measure HO2 and H2O2 respectively. DME and other important intermediate species such as CH2O and CO are also measured by MBMS between 400 and 1150 K at different fuel concentrations. Species profiles in the reactor were calculated by using both zero- and two-dimensional computations with different detailed kinetics for cross-validation and comparison with experimental results. The models predict adequately the low and intermediate oxidation temperature windows near 600 and 1000 K, respectively. However, both models over-predicted the DME consumption as well as CO, HO2 and H2O2 formations at the low temperature oxidation window by more than a factor of four. Moreover, although the model predicted reasonably well the formation of CH2O and CO/CO2 at the intermediate temperature oxidation window, the concentration of H2O2 was also over-predicted, suggesting the large uncertainties existing in the DME low temperature chemistry and in H2O2 chemistry at intermediate temperature. Furthermore, to analyze the uncertainty of the low temperature chemistry, a branching ratio of QOOH decomposition to CH2O was derived using measured DME, CH2O and CO concentrations. The large difference between the modeled and measured branching ratios of QOOH decomposition suggests an underestimated QOOH decomposition rate to form CH2O in the current DME models.
The development of accurate chemical kinetic models capable of predicting the combustion of methane
and dimethyl ether in common combustion environments such as compression ignition engines and gas
turbines is important as it provides valuable data and understanding of these fuels under conditions that
are difficult and expensive to study in the real combustors. In this work, both experimental and chemical
kinetic model-predicted ignition delay time data are provided covering a range of conditions relevant to
gas turbine environments (T = 600–1600 K, p = 7–41 atm, / = 0.3, 0.5, 1.0, and 2.0 in ‘air’ mixtures). The
detailed chemical kinetic model (Mech_56.54) is capable of accurately predicting this wide range of data,
and it is the first mechanism to incorporate high-level rate constant measurements and calculations
where available for the reactions of DME. This mechanism is also the first to apply a pressure-dependent
treatment to the low-temperature reactions of DME. It has been validated using available literature data
including flow reactor, jet-stirred reactor, shock-tube ignition delay times, shock-tube speciation, flame
speed, and flame speciation data. New ignition delay time measurements are presented for methane,
dimethyl ether, and their mixtures; these data were obtained using three different shock tubes and a
rapid compression machine. In addition to the DME/CH4 blends, high-pressure data for pure DME and
pure methane were also obtained. Where possible, the new data were compared with existing data from
the literature, with good agreement.
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 1500K, 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.
A detailed chemical kinetic mechanism has been developed to describe the oxidation of small hydrocarbon and oxygenated hydrocarbon species. The reactivity of these small fuels and intermediates is of critical importance in understanding and accurately describing the combustion characteristics, such as ignition delay time, flame speed, and emissions of practical fuels. The chosen rate expressions have been assembled through critical evaluation of the literature, with minimum optimization performed. The mechanism has been validated over a wide range of initial conditions and experimental devices, including flow reactor, shock tube, jet-stirred reactor, and flame studies. The current mechanism contains accurate kinetic descriptions for saturated and unsaturated hydrocarbons, namely methane, ethane, ethylene, and acetylene, and oxygenated species; formaldehyde, methanol, acetaldehyde, and ethanol.
The inhibition mechanism of diethyl ether oxidation was investigated both at the experimental and theoretical (density functional theory, DFT) level to find the best inhibitor for such a process. First, a detailed theoretical analysis was conducted by DFT calculations on reactions with 12 potentially effective antioxidants from different chemical classes (phenols, amines, and phosphines). Results of this study show that phenolic antioxidants are the best performers for inhibiting the process. In particular, the efficacy of butylated hydroxytoluene, an additive already commonly used in diethyl ether (DEE) storage, was clearly explained from a kinetic (low activation barrier) and a thermodynamic (product stabilization) point of view. The order of effectiveness of the antioxidants obtained from DFT calculations was confirmed by the complementary experimental work on DEE oxidation, which were conducted under accelerated conditions in autoclaves.
A potential barrier of the kind studied by Fowler and others may be represented by the analytic function V (Eq. (1)). The Schrödinger equation associated to this potential is soluble in terms of hypergeometric functions, and the coefficient of reflection for electrons approaching the barrier with energy W is calculable (Eq. (15)). The approximate formula, 1-ρ=exp{-∫4πh(2m(V-W))12dx} is shown to agree very well with the exact formula when the width of the barrier is great compared to the de Broglie wave-length of the incident electron, and W<Vmax.
The flame structure and kinetics of dimethyl ether (DME) flames with and without CO2 dilution at reduced and elevated pressures were studied experimentally and computationally. The species distributions of DME oxidation in low-pressure premixed flat flames were measured by using electron-ionization molecular-beam mass spectrometry (EI-MBMS) at an equivalence ratio of 1.63 and 50 mbar. High-pressure flame speeds of lean and rich DME flames with and without CO2 dilution were measured in a nearly-constant-pressure vessel between about 1 and 20 bar. The experimental results were compared with predictions from four kinetic models: the first was published by Zhao et al. (2008) [9], the second developed by the Lawrence Livermore National Laboratory (LLNL) (Kaiser et al., 2000) [13], and the third has been made available to us as the Aramco mechanism (Metcalfe et al., 2013) [14]; as the fourth, we have used an updated model developed in this study. Good agreement was found between measurements and predictions from all four models for all major and most typical intermediate species with and without CO2 addition in low-pressure flat flame experiments. However, none of the models was able to reliably predict high-pressure flame speeds. Although the updated model improved the prediction of flame speeds for lean mixtures, errors remained for rich conditions at elevated pressure, likely due to uncertainty in the rates of CH3 + H(+M) = CH4(+M) and the branching and termination reaction pair of CH3 + HO2 = CH3O + OH and CH3 + HO2 = CH4 + O2. CO2 addition considerably decreased the flame speed. Kinetic comparisons between inert and chemically active CO2 in DME flames showed that CO2 addition affects rich and lean DME flame kinetics differently. For lean flames, both the inert third-body effect and the kinetic effect of CO2 reduce H-atom production. However, for rich flames, the inert third-body effect increases H-atom production via HCO(+M) = H + CO(+M) and suppression of the kinetic effect of CO2 by shifting the equilibrium of CO + OH = CO2 + H.
Low-temperature combustion is a major strategy today to reduce both soot and NOx emission. Kinetic reaction models for low-temperature combustion which are validated against a wide range of experimental data are necessary e.g. for control purposes or as a basis for subsequent model reduction.In this study, the low-temperature oxidation of dimethyl ether in a highly diluted gas mixture was investigated experimentally in an atmospheric laminar flow reactor. The respective gas composition was analyzed by a time-of-flight mass spectrometer. This technique allows detection of all species simultaneously within the investigated temperature regime. Stoichiometries of ϕ = 0.8, 1.0, and 1.2 were studied with high temperature resolution in the range of 400–1200 K, and quantitative species mole fraction profiles have been determined.This wide temperature range comprises the different kinetic regimes occurring during the DME oxidation, which have been clearly resolved. The distinct negative temperature coefficient (NTC) region of the system was observed and extensive speciation is available. Special attention is given to species only occurring in the low-temperature region including formic acid and methyl formate.
The reaction of OH with dimethyl ether (CH3OCH3) has been studied from 195 - 850 K using laser flash photolysis coupled to laser induced fluorescence detection of OH radicals. The rate coefficient from this work can be parameterised by the modified Arrhenius expression k = (1.23 ± 0.46) × 10-12 (T/298)2.05±0.23 exp((257 ± 107)/T) cm3 molecule-1 s-1. Including other recent literature data (923 - 1423 K) gives a modified Arrhenius expression of k1 = (1.54 ± 0.48) × 10-12 (T/298K)1.89±0.16 exp((184 ± 112)/T) cm3 molecule-1 s-1 over the range 195 - 1423 K. Various isotopomeric combinations of the reaction have also been investigated with deuteration of dimethyl ether leading to a normal isotope effect. Deuteration of the hydroxyl group leads to a small inverse isotope effect. To gain an insight into the reaction mechanisms and to support the experimental work, theoretical studies have also been undertaken calculating the energies and structures of the transition states and complexes using high level ab initio methods. The calculations also identify pre and post-reaction complexes. The pre reaction complex (binding energy of ~22 kJ mol-1) may contribute to the kinetics of the reaction, especially at low temperatures, but there is no direct evidence of this occurring under our experimental conditions, although the Arrhenius plot for the rate coefficient shows considerable curvature. The experimental data have been modelled using the recently developed MESMER (Master Equation Solver for Multi Energy Well Reactions) code giving good agreement. The calculations qualitatively reproduce the observed isotope effects closely above ~600 K but overestimate them at low temperatures, which may derive from an inadequate treatment of tunnelling and of an enhanced role of an outer transition state leading to the pre-reaction complex.
We consider an alternative formulation of the master equation for complex-forming chemical reactions with multiple wells and bimolecular products. Within this formulation the dynamical phase space consists of only the microscopic populations of the various isomers making up the reactive complex, while the bimolecular reactants and products are treated equally as sources and sinks. This reformulation yields compact expressions for the phenomenological rate coefficients describing all chemical processes, i.e., internal isomerization reactions, bimolecular-to-bimolecular reactions, isomer-to-bimolecular reactions, and bimolecular-to-isomer reactions. The applicability of the detailed balance condition is discussed and confirmed. We also consider the situation where some of the chemical eigenvalues approach the energy relaxation time scale and show how to modify the phenomenological rate coefficients so that they retain their validity.
H2O2 is one of the most important species in dimethyl ether (DME) oxidation, acting not only as a marker for low temperature kinetic activity but also responsible for the “hot ignition” transition. This study reports, for the first time, direct measurements of H2O2 and CH3OCHO, among other intermediate species concentrations in helium-diluted DME oxidation in an atmospheric pressure flow reactor from 490 to 750 K, using molecular beam electron-ionization mass spectrometry (MBMS). H2O2 measurements were directly calibrated, while a number of other species were quantified by both MBMS and micro gas chromatography to achieve cross-validation of the measurements. Experimental results were compared to two different DME kinetic models with an updated rate coefficient for the H + DME reaction, under both zero-dimensional and two-dimensional physical model assumptions. The results confirm that low and intermediate temperature DME oxidation produces significant amounts of H2O2. Peroxide, as well as O2, DME, CO, and CH3OCHO profiles are reasonably well predicted, though profile predictions for H2/CO2 and CH2O are poor above and below ∼625 K, respectively. The effect of the collisional efficiencies for the H + O2 + M = HO2 + M reaction on DME oxidation was investigated by replacing 20% He with 20% CO2. Observed changes in measured H2O2 concentrations agree well with model predictions. The new experimental characterizations of important intermediate species including H2O2, CH2O and CH3OCHO, and a path flux analysis of the oxidation pathways of DME support that kinetic parameters for decomposition reactions of HOCH2OCO and HCOOH directly to CO2 may be responsible for model under-prediction of CO2. The H abstraction reactions for DME and/or CH2O and the unimolecular decomposition of HOCH2O merit further scrutiny towards improving the prediction of H2 formation.
We present new reaction pathways relevant to low-temperature oxidation in gaseous and condensed phases. The new pathways originate from γ-ketohydroperoxides (KHP), which are well-known products in low-temperature oxidation and are assumed to react only via homolytic O-O dissociation in existing kinetic models. Our ab initio calculations identify new exothermic reactions of KHP forming a cyclic peroxide isomer, which decomposes via novel concerted reactions into carbonyl and carboxylic acid products. Geometries and frequencies of all stationary points are obtained using the M06-2X/MG3S DFT model chemistry, and energies are refined using RCCSD(T)-F12a/cc-pVTZ-F12 single-point calculations. Thermal rate coefficients are computed using variational transition-state theory (VTST) calculations with multidimensional tunneling contributions based on small-curvature tunneling (SCT). These are combined with multistructural partition functions (Q(MS-T)) to obtain direct dynamics multipath (MP-VTST/SCT) gas-phase rate coefficients. For comparison with liquid-phase measurements, solvent effects are included using continuum dielectric solvation models. The predicted rate coefficients are found to be in excellent agreement with experiment when due consideration is made for acid-catalyzed isomerization. This work provides theoretical confirmation of the 30-year-old hypothesis of Korcek and co-workers that KHPs are precursors to carboxylic acid formation, resolving an open problem in the kinetics of liquid-phase autoxidation. The significance of the new pathways in atmospheric chemistry, low-temperature combustion, and oxidation of biological lipids are discussed.
Spin-polarised density functional theory (DFT-B3LYP) energies, harmonic vibrational frequencies, and moments of inertia are used to construct modified Arrhenius rate expressions for elementary steps in chain-propagation and chain-branching pathways for dimethyl ether combustion. Barrierless reactions were treated with variational RRKM theory, and global kinetics were modeled using master equation and perfectly stirred reactor simulations. Our kinetics analysis suggests that the bottleneck along the chain propagation path is the isomerisation of CH(3)OCH(2)OO, contrary to earlier interpretations. Comparing the rate constants for competing decomposition pathways of the key chain-branching intermediate hydroperoxymethyl formate (HPMF), we find that formation of formic acid and the atmospherically relevant Criegee intermediate (CH(2)OO) via a H-bonded adduct may be more favourable than O-O bond scission. Since the latter forms a source of a second OH radical beyond that supplied in chain propagation, which is necessary for explosive combustion, the more favourable pathway to formic acid may inhibit autoignition of the fuel. We predict that the HPMF O-O scission product, OCH(2)OC(=O)H, most likely directly dissociates to HCO + HC(=O)OH. This implies an overabundance of CO at 550-700 K, since HC(=O)OH is a fairly stable product in this temperature range and facile H abstraction from HCO leads to CO. We find that CO(2) product yields are sensitive to the creation of CH(2)OO and that creation of CH(2)OO is competitive with the O-O scission reaction.
This paper reviews the properties and application of di-methyl ether (DME) as a candidate fuel for compression-ignition engines. DME is produced by the conversion of various feedstock such as natural gas, coal, oil residues and bio-mass. To determine the technical feasibility of DME, the review compares its key properties with those of diesel fuel that are relevant to this application. DME’s diesel engine-compatible properties are its high cetane number and low auto-ignition temperature. In addition, its simple chemical structure and high oxygen content result in soot-free combustion in engines. Fuel injection of DME can be achieved through both conventional mechanical and current common-rail systems but requires slight modification of the standard system to prevent corrosion and overcome low lubricity. The spray characteristics of DME enable its application to compression-ignition engines despite some differences in its properties such as easier evaporation and lower density. Overall, the low particulate matter production of DME provides adequate justification for its consideration as a candidate fuel in compression-ignition engines. Recent research and development shows comparable output performance to a diesel fuel led engine but with lower particulate emissions. NOx emissions from DME-fuelled engines can meet future regulations with high exhaust gas recirculation in combination with a lean NOx trap. Although more development work has focused on medium or heavy-duty engines, this paper provides a comprehensive review of the technical feasibility of DME as a candidate fuel for environmentally-friendly compression-ignition engines independent of size or application.
The reduced equation of state for compressed gases and liquids is computed according to the theory of Lennard‐Jones and Devonshire for a large number of temperatures and densities. The corrections due to gas imperfection of internal energy, specific heat, and entropy as well as the compressibility factor (pv/RT) are expressed in terms of reduced variables. The calculations were made by punched‐card methods. A comparison is made between experiment and theory. It is shown that the theory of Lennard‐Jones and Devonshire is unsatisfactory at densities near the critical point and lower but improves at higher densities becoming better for normal liquids. Such results were to be expected from the nature of the cell or free‐volume method.
We presented a direct ab initio and density-functional theory dynamics study of the thermal rate constants of the unimolecular decomposition reaction of CH3OCH2 → CH2O + CH3 at a wide temperature range of 200−2500 K. MPW1K/6-31+G(d,p), MP2/6-31+G(d,p), and QCISD/6-31+G(d,p) methods were employed to optimize the geometries of all stationary points and to calculate the minimum energy path (MEP). The energies of all the stationary points were refined at the QCISD(T)/aug-cc-pVTZ level of theory. The rate constants were evaluated based on the energetics from the QCISD(T)/aug-cc-pVTZ//MPW1K/6-31+G(d,p) level of theory using both microcanonical variational transition state theory (μVT) and canonical variational transition state theory (CVT) in the temperature range of 200−2500 K. The calculated rate constants at the QCISD(T)/aug-cc-pVTZ//MPW1K/6-31+G(d,p) level of theory are in good agreement with experimental data. The fitted three-parameter Arrhenius expression from the μVT rate constants in the temperature range 200−2500 K is k = 4.45 × 1014T -0.22 e(-1.37×104/T) s-1.
In part I, we discussed the chain-propagating and possible competing mechanisms of low-temperature (300−1000 K) dimethyl ether (DME) combustion. Here we consider the chain-branching mechanism that results in explosive combustion, initiated by O2 addition to the ·CH2OCH2OOH intermediate formed in the earlier chain-propagation step. Ideally, chain-branching leads to the formation of two highly reactive ·OH radicals from the ·OOCH2OCH2OOH precursor. Each of these two ·OH radicals can initiate a chain-reaction “branch” with another DME molecule, which, ideally, leads to the formation of four more ·OH, and so on. This exponential increase in ·OH concentration causes an exponential increase in the DME oxidation rate, leading to explosive combustion. Here we show that although the pathway to create the first ·OH from ·OOCH2OCH2OOH in a hydrogen-transfer isomerization step is unambiguous, the formation of the second ·OH from the remaining hydroperoxyformate (HPMF or HOOCH2OC(O)H) fragment is potentially very complicated. HPMF has many possible fates, including HĊO + formic acid (HC(O)OH) + ·OH; H2O + formic acid anhydride (HC(O)OC(O)H); the Criegee intermediate (·CH2OO·) + formic acid; peroxyformic acid (HC(O)OOH) + H2 + CO; dihydroxymethylformate ((HO)2HCOC(O)H); ·OCH2OC(O)H + ·OH; and quite possibly others. The first and last of these products derived from HPMF directly produce ·OH and thus can complete the chain-branching step. Activation energies of 42−44 kcal/mol are needed to overcome barriers to form these two sets of products from HPMF. While these pathways directly form ·OH, they may not be the most favorable. The formation of a Criegee intermediate (·CH2OO·)−formic acid hydrogen-bonded adduct requires 15 kcal/mol less enthalpy than paths directly producing ·OH. Formation of the Criegee intermediate has never been considered as an intermediate in DME combustion before, but its formation (along with formic acid) appears to be the most favorable unimolecular path for HPMF decomposition. In atmospheric chemistry, decomposition of vibrationally excited ·CH2OO· can potentially lead to ·OH formation. Thus, we propose ·CH2OO· as a new intermediate that may significantly contribute to dimethyl ether's chain-branching mechanism.
High-accuracy ab initio thermochemistry is presented for 219 small molecules relevant in combustion chemistry, including many radical, biradical, and triplet species. These values are critical for accurate kinetic modeling. The RQCISD(T)/cc-PV∞QZ//B3LYP/6-311++G(d,p) method was used to compute the electronic energies. A bond additivity correction for this method has been developed to remove systematic errors in the enthalpy calculations, using the Active Thermochemical Tables as reference values. On the basis of comparison with the benchmark data, the 3σ uncertainty in the standard-state heat of formation is 0.9 kcal/mol, or within chemical accuracy. An uncertainty analysis is presented for the entropy and heat capacity. In many cases, the present values are the most accurate and comprehensive numbers available. The present work is compared to several published databases. In some cases, there are large discrepancies and errors in published databases; the present work helps to resolve these problems.
The unimolecular decomposition reaction of dimethyl ether (DME) was studied theoretically using RRKM/master equation calculations. The calculated decomposition rate is significantly different from that utilized in prior work (Fischer et al., Int J Chem Kinet 2000, 32, 713–740; Curran et al., Int J Chem Kinet 2000, 32, 741–759). DME pyrolysis experiments were performed at 980 K in a variable-pressure flow reactor at a pressure of 10 atm, a considerably higher pressure than previous validation data. Both unimolecular decomposition and radical abstraction are significant in describing DME pyrolysis, and hierarchical methodology was applied to produce a comprehensive high-temperature model for pyrolysis and oxidation that includes the new decomposition parameters and more recent small molecule/radical kinetic and thermochemical data. The high-temperature model shows improved agreement against the new pyrolysis data and the wide range of high-temperature oxidation data modeled in prior work, as well as new low-pressure burner-stabilized species profiles (Cool et al., Proc Combust Inst 2007, 31, 285–294) and laminar flame data for DME/methane mixtures (Chen et al., Proc Combust Inst 2007, 31, 1215–1222). The high-temperature model was combined with low-temperature oxidation chemistry (adopted from Fischer et al., Int J Chem Kinet 2000, 32, 713–740), with some modifications to several important reactions. The revised construct shows good agreement against high- as well as low-temperature flow reactor and jet-stirred reactor data, shock tube ignition delays, and laminar flame species as well as flame speed measurements. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 40: 1–18, 2008
A new group contribution method for the estimation of properties of pure organic compounds is presented. Estimation is performed at two levels: the basic level uses contributions from first-order groups, while the next higher level uses a small set of second-order groups having the first-order groups as building blocks. Thus, the method provides both a first-order approximation (first-order group contributions) and a more accurate second-order prediction (first- and second-order group contributions). This article discusses methods for prediction of normal boiling point, normal melting point, critical pressure, critical temperature, critical volume, standard enthalpy of vaporization at 298 K, standard Gibbs energy, and standard enthalpy of formation at 298 K. The predictions are based exclusively on the molecular structure of the compound, and the method is able to distinguish among isomers. Compared to the currently-used methods, this technique demonstrates significant improvements in accuracy and applicability.
Time-resolved measurements of HO2 and OH have been conducted in 355nm photolysis of dimethyl ether/Cl2/O2 mixture at elevated temperature, using near-infrared frequency modulation spectroscopy. A part of OH was found to be produced at a timescale of several microseconds by the methoxymethyl with O2 reaction, while HO2 is formed mainly in milliseconds with the yield increasing up to 60% between 500 and 600K. It was rationalized that HO2 is not a direct product of the O2 adduct decomposition, but a secondary product through HCHO+OH reaction. Another pathway through HCO formation from the adduct is also discussed.
For quantitative description of a molecular charge distribution it is convenient to dissect the molecule into well-defined atomic fragments. A general and natural choice is to share the charge density at each point among the several atoms in proportion to their free-atom densities at the corresponding distances from the nuclei. This prescription yields well-localized bonded-atom distributions each of which closely resembles the molecular density in its vicinity. Integration of the atomic deformation densities — bonded minus free atoms — defines net atomic charges and multipole moments which concisely summarize the molecular charge reorganization. They permit calculation of the external electrostatic potential and of the interaction energy between molecules or between parts of the same molecule. Sample results for several molecules indicate a high transferability of net atomic charges and moments.
Criegee biradicals, i.e., carbonyl oxides, are critical intermediates in ozonolysis and have been implicated in autoignition chemistry and other hydrocarbon oxidation systems, but until recently the direct measurement of their gas-phase kinetics has not been feasible. Indirect determinations of Criegee intermediate kinetics often rely on the introduction of a scavenger molecule into an ozonolysis system and analysis of the effects of the scavenger on yields of products associated with Criegee intermediate reactions. Carbonyl species, in particular hexafluoroacetone (CF(3)COCF(3)), have often been used as scavengers. In this work, the reactions of the simplest Criegee intermediate, CH(2)OO (formaldehyde oxide), with three carbonyl species have been measured by laser photolysis/tunable synchrotron photoionization mass spectrometry. Diiodomethane photolysis produces CH(2)I radicals, which react with O(2) to yield CH(2)OO + I. The formaldehyde oxide is reacted with a large excess of a carbonyl reactant and both the disappearance of CH(2)OO and the formation of reaction products are monitored. The rate coefficient for CH(2)OO + hexafluoroacetone is k(1) = (3.0 ± 0.3) × 10(-11) cm(3) molecule(-1) s(-1), supporting the use of hexafluoroacetone as a Criegee-intermediate scavenger. The reactions with acetaldehyde, k(2) = (9.5 ± 0.7) × 10(-13) cm(3) molecule(-1) s(-1), and with acetone, k(3) = (2.3 ± 0.3) × 10(-13) cm(3) molecule(-1) s(-1), are substantially slower. Secondary ozonides and products of ozonide isomerization are observed from the reactions of CH(2)OO with acetone and hexafluoroacetone. Their photoionization spectra are interpreted with the aid of quantum-chemical and Franck-Condon-factor calculations. No secondary ozonide was observable in the reaction of CH(2)OO with acetaldehyde, but acetic acid was identified as a product under the conditions used (4 Torr and 293 K).
The thermal dissociation of dimethyl ether has been studied with a combination of reflected shock tube experiments and ab initio dynamics simulations coupled with transition state theory based master equation calculations. The experiments use the extraordinary sensitivity provided by H-atom ARAS detection with an unreversed light source to measure both the total decomposition rate and the branching to radical products versus molecular products, with the molecular products arising predominantly through roaming according to the theoretical analysis. The experimental observations also provide a measure of the rate coefficient for H + CH3OCH3. An evaluation of the available experimental results for H + CH3OCH3 can be expressed by a three parameter Arrhenius expression as
The oxidation of dimethylether (DME) has been studied in a fused silica jet-stirred reactor (JSR) at 10 atm, 0.2≤φ≤1, 550–1100 K. Concentration profiles of reactants, intermediates, and products of the oxidation were measured by low-pressure sonic probe sampling and off-line gas chromatography analyses. The results obtained in the cool flame regime are the first to be reported. The ignition delays of DME/O2/Ar mixtures have been measured in a shock tube at 1200 to 1600 K, at 3.5 atm and 0.5≤φ≤2. A numerical model consisting of a detailed kinetic reaction mechanism with 331 reactions (most of them reversible) among 55 species is proposed to describe both the low and high-temperature oxidation of DME in the JSR (550–1275 K, 1–10 atm) and the ignition of DME in shock tubes from low to high temperature (650–1600 K, 3.5–40 bar). A general good agreement between the data and the model was observed. A kinetic analysis involving sensitivity and reaction path analysis is used to interpret the results.
The ignition temperature of nitrogen-diluted dimethyl ether (DME) by heated air in counterflow was experimentally determined for DME concentration from 5.9% to 30%, system pressure from 1.5 to 3.0 atm, and pressure-weighted strain rate from 110 to 170 s−1. These experimental data were compared with two mechanisms that were, respectively, available in 1998 and 2003, with the latter being a substantially updated version of the former. The comparison showed that while the 1998-mechanism uniformly over-predicted the ignition temperature, the 2003-mechanism yielded a surprisingly close agreement for all experimental data. Sensitivity analysis for the near-ignition state based on both mechanisms identified the deficiencies of the 1998-mechanism, in particular, the specifics of the low-temperature cool flame chemistry in effecting ignition at higher temperatures, as the fuel stream is being progressively heated from its cold boundary to the high-temperature ignition region around the hot-stream boundary. The 2003-mechanism, consisting of 79 species and 398 elementary reactions, was then systematically simplified by using the directed relation graph method to a skeletal mechanism of 49 species and 251 elementary reactions, which in turn was simplified further by using computational singular perturbation method and quasi-steady-state species assumption to a reduced mechanism consisting of 33 species and 28 lumped reactions. It was demonstrated that both the skeletal and reduced mechanisms mimicked the performance of the detailed mechanism with high accuracy.
Using sequences of Dunning's correlation consistent basis sets, cc-pVnZ (n = 3,4,5), convergence of molecular properties is much slower for second-row compounds than for their first-row counterparts, both at the Hartree-Fock and at correlated levels, due to core polarization effects. By adding a single high-exponent d function to the standard cc-pVnZ basis sets, convergence is greatly accelerated. After correcting for core correlation, computed D-0, r(e), and omega(e) values for a number of diatomics generally agree with experiment to better than 0.02 eV, 0.001 Angstrom, and 5 cm(-1), respectively. (C) 1998 Elsevier Science B.V.
A large number of organic compounds, such as ethers, spontaneously form unstable peroxides through a self-propagating process of autoxidation (peroxidation). Although the hazards of organic peroxides are well known, the oxidation mechanisms of peroxidizable compounds like ethers reported in the literature are vague and often based on old experiments, carried out in very different conditions (e.g. atmospheric, combustion). With the aim to (partially) fill the lack of information, in this paper we present an extensive Density Functional Theory (DFT) study of autoxidation reaction of diethyl ether (DEE), a chemical that is largely used as solvent in laboratories, and which is considered to be responsible for various accidents. The aim of the work is to investigate the most probable reaction paths involved in the autoxidation process and to identify all potential hazardous intermediates, such as peroxides. Beyond the determination of a complex oxidation mechanism for such a simple molecule, our results suggest that the two main reaction channels open in solution are the direct decomposition (β-scission) of DEE radical issued of the initiation step and the isomerization of the peroxy radical formed upon oxygen attack (DEEOO˙). A simple kinetic evaluation of these two competing reaction channels hints that radical isomerization may play an unexpectedly important role in the global DEE oxidation process. Finally industrial hazards could be related to the hydroperoxide formation and accumulation during the chain propagation step. The resulting information may contribute to the understanding of the accidental risks associated with the use of diethyl ether.
Alkyl hydroperoxides are found to be important intermediates in the combustion and oxidation processes of hydrocarbons. However, studies of ethyl hydroperoxide (CH(3)CH(2)OOH) are limited. In this work, kinetics and mechanisms for unimolecular decomposition of CH(3)CH(2)OOH have been investigated. The potential energy surface of decomposition reactions have first been predicted at the CCSD(T)/6-311+G(3df,2p)//B3LYP/6-311G(d,p) level. The results show that the formation of CH(3)CH(2)O + OH via O-O direct bond dissociation is dominant, the branching ratio of which is over 99% in the whole temperature range from 300 to 1000 K, and its rate constant can be expressed as k1 = 9.26 × 10(52)T(-11.91)exp(-26879/T) s(-1) at 1 atm. The rate constants of the reaction CH(3)CH(2)OOH → CH(3)CH(2)O + OH at different temperatures and pressures have been calculated, which can help us to comprehend the reactions of CH(3)CH(2)OOH at experimental conditions.
Biofuels, such as bio-ethanol, bio-butanol, and biodiesel, are of increasing interest as alternatives to petroleum-based transportation fuels because they offer the long-term promise of fuel-source regenerability and reduced climatic impact. Current discussions emphasize the processes to make such alternative fuels and fuel additives, the compatibility of these substances with current fuel-delivery infrastructure and engine performance, and the competition between biofuel and food production. However, the combustion chemistry of the compounds that constitute typical biofuels, including alcohols, ethers, and esters, has not received similar public attention. Herein we highlight some characteristic aspects of the chemical pathways in the combustion of prototypical representatives of potential biofuels. The discussion focuses on the decomposition and oxidation mechanisms and the formation of undesired, harmful, or toxic emissions, with an emphasis on transportation fuels. New insights into the vastly diverse and complex chemical reaction networks of biofuel combustion are enabled by recent experimental investigations and complementary combustion modeling. Understanding key elements of this chemistry is an important step towards the intelligent selection of next-generation alternative fuels.
The unimolecular dissociation of CH3OOH is investigated by exciting the molecule in the region of its 5nu(OH) band and probing the resulting OH fragments using laser-induced fluorescence. The measured OH fragment rotational and translational energies are used to determine the CH3O-OH bond dissociation energy, which we estimate to be approximately 42.6+/-1 kcal/mol. Combining this value with the known heats of formation of the fragments also gives an estimate for the heat of formation of CH3OOH which at 0 K we determine to be deltaH(f)0=-27+/-1 kcal/mol. This experimental value is in good agreement with the results of ab initio calculations carried out at the CCSD(T)/complete basis set limit which finds the heat of formation of CH3OOH at 0 K to be deltaH(f)0=-27.3 kcal/mol.
Dimethyl ether (DME) has been proposed for use as an alternative fuel or additive in diesel engines and as a potential fuel in solid oxide fuel cells. The oxidation chemistry of DME is a key element in understanding its role in these applications. The reaction between methoxymethyl radicals and O(2) has been examined over the temperature range 295-600 K and at pressures of 20-200 Torr. This reaction has two product pathways. The first produces methoxymethyl peroxy radicals, while the second produces OH radicals and formaldehyde molecules. Real-time kinetic measurements are made by transient infrared spectroscopy to monitor the yield of three main products-formaldehyde, methyl formate, and formic acid-to determine the branching ratio for the CH(3)OCH(2) + O(2) reaction pathways. The temperature and pressure dependence of this reaction is described by a Lindemann and Arrhenius mechanism. The branching ratio is described by f = 1/(1 + A(T)[M]), where A(T) = (1.6(+2.4)(-1.0) x 10(-20)) exp((1800 +/- 400)/T) cm(3) molecule(-1). The temperature dependent rate constant of the methoxymethyl peroxy radical self-reaction is calculated from the kinetics of the formaldehyde and methyl formate product yields, k(4) = (3.0 +/- 2.1) x 10(-13) exp((700 +/- 250)/T) cm(3) molecule(-1) s(-1). The experimental and kinetics modeling results support a strong preference for the thermal decomposition of alkoxy radicals versus their reaction with O(2) under our laboratory conditions. These characteristics of DME oxidation with respect to temperature and pressure might provide insight into optimizing solid oxide fuel cell operating conditions with DME in the presence of O(2) to maximize power outputs.
The reaction of HO2 with C2H5O2 has been studied using the density functional theory (B3LYP) and the coupled-cluster theory [CCSD(T)]. The reaction proceeds on the triplet potential energy surface via hydrogen abstraction to form ethyl hydroperoxide and oxygen. On the singlet potential energy surface, the addition-elimination mechanism is revealed. Variational transition state theory is used to calculate the temperature-dependent rate constants in the range 200-1000 K. At low temperatures (e.g., below 300 K), the reaction takes place predominantly on the triplet surface. The calculated low-temperature rate constants are in good agreement with the experimental data. As the temperature increases, the singlet reaction mechanism plays more and more important role, with the formation of OH radical predominantly. The isotope effect of the reaction (DO2 + C2D5O2 vs HO2 + C2H5O2) is negligible. In addition, the triplet abstraction energetic routes for the reactions of HO2 with 11 alkylperoxy radicals (CnHmO2) are studied. It is shown that the room-temperature rate constants have good linear correlation with the activation energies for the hydrogen abstraction.
The CH3 + OH bimolecular reaction and the dissociation of methanol are studied theoretically at conditions relevant to combustion chemistry. Kinetics for the CH3 + OH barrierless association reaction and for the H + CH2OH and H + CH3O product channels are determined in the high-pressure limit using variable reaction coordinate transition state theory and multireference electronic structure calculations to evaluate the fragment interaction energies. The CH3 + OH --> 3CH2 + H2O abstraction reaction and the H2 + HCOH and H2 + H2CO product channels feature localized dynamical bottlenecks and are treated using variational transition state theory and QCISD(T) energies extrapolated to the complete basis set limit. The 1CH2 + H2O product channel has two dynamical regimes, featuring both an inner saddle point and an outer barrierless region, and it is shown that a microcanonical two-state model is necessary to properly describe the association rate for this reaction over a broad temperature range. Experimental channel energies for the methanol system are reevaluated using the Active Thermochemical Tables (ATcT) approach. Pressure dependent, phenomenological rate coefficients for the CH3 + OH bimolecular reaction and for methanol decomposition are determined via master equation simulations. The predicted results agree well with experimental results, including those from a companion high-temperature shock tube determination for the decomposition of methanol.
Product formation pathways in the photolytically initiated oxidation of CH3OCH3 have been investigated as a function of temperature (298-600 K) and pressure (20-90 Torr) through the detection of HO2 and OH using Near-infrared frequency modulation spectroscopy, as well as the detection of CH3OCH2O2 using UV absorption spectroscopy. The reaction was initiated by pulsed photolysis with a mixture of Cl2, O2, and CH3OCH3. The HO2 and OH yield is obtained by comparison with an established reference mixture, including CH3OH. The CH3OCH2O2 yield is also obtained through the procedure of estimating the CH3OCH2O2/HO2 ratio from their UV absorption. A notable finding is that the OH yield is 1 order of magnitude larger than those known in C2 and C3 alkanes, increasing from 10% to 40% with increasing temperature. The HO2 yield increases gradually until 500 K and sharply up to 40% over 500 K. The CH3OCH2O2 profile has a prompt rise, followed by a gradual decay whose time constant is consistent with slow HO2 formation. To predict species profiles and yields, simple chlorine-initiated oxidation model of DME under low-pressure condition was constructed based on the existing model and the new reaction pathways, which were derived from this study. To model rapid OH formation, OH direct formation from CH3OCH2 + O2 was required. We have also proposed that a new HCO formation pathway via QOOH isomerization to HOQO species and OH + CH3OCH2O2 --> HO2 + CH3OCH2O are to be considered, to account for the fast and slow HO2 formations, as well as the total yield. The constructed model including these new pathways has successfully predicted experimental results throughout the entire temperature and pressure ranges investigated. It was revealed that the HO2 formation mechanism changes at 500 K, i.e., HCO + O2 via HCHO + OH and the above proposed direct HCO formation dominates over 500 K, while a series of reactions following CH3OCH2O2 self-reaction and OH + CH3OCH2O2 reaction mainly contribute below 500 K. The pressure dependent rate constant of the CH3OCH2 thermal decomposition reaction has been separately measured since it has large negative sensitivity for HO2 formation and is essential to eliminate the ambiguity in the CH3OCH2 + O2 mechanism at higher temperature.
Case studies of ten reactions using a variety of standard electronic structure methods are presented. These case studies are used to illustrate the usefulness and shortcomings of these standard methods for various classes of reactions. Limited comparisons with experiment are made. The reactions studied include four radical-radical combinations, H + CH(3)--> CH(4), CH(3) + CH(3)--> C(2)H(6), H + HCO --> H(2)CO and CH(3) + HCO --> CH(3)CHO, three abstraction reactions, H + HO(2)--> H(2) + O(2), H + HCO --> H(2) + CO and CH(3) + HCO --> CH(4) + CO, a radical-molecule addition, H + HCCH --> C(2)H(3), and two molecular decompositions, H(2)CO --> H(2) + CO and CH(3)CHO --> CH(4) + CO. The electronic structure methods used are DFT, MP2, CCSD(T), QCISD(T), CASSCF, CASPT2, and CAS+1+2+QC.
We present the Voronoi Deformation Density (VDD) method for computing atomic charges. The VDD method does not explicitly use the basis functions but calculates the amount of electronic density that flows to or from a certain atom due to bond formation by spatial integration of the deformation density over the atomic Voronoi cell. We compare our method to the well-known Mulliken, Hirshfeld, Bader, and Weinhold [Natural Population Analysis (NPA)] charges for a variety of biological, organic, and inorganic molecules. The Mulliken charges are (again) shown to be useless due to heavy basis set dependency, and the Bader charges (and often also the NPA charges) are not realistic, yielding too extreme values that suggest much ionic character even in the case of covalent bonds. The Hirshfeld and VDD charges, which prove to be numerically very similar, are to be recommended because they yield chemically meaningful charges. We stress the need to use spatial integration over an atomic domain to get rid of basis set dependency, and the need to integrate the deformation density in order to obtain a realistic picture of the charge rearrangement upon bonding. An asset of the VDD charges is the transparency of the approach owing to the simple geometric partitioning of space. The deformation density based charges prove to conform to chemical experience.
- T Yamada
- J W Bozzelli
- T H Lay
- S L Fischer
- F L Dryer
- H J Curran