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The reaction pathways and potential energy profiles are theoretically explored for H-abstraction, addition and addition–dissociation reactions of methyl formate (MF, HC(＝O)OCH3) + NO2 using the high level quantum chemical compound method CCSD(T)/cc-pVxZ(x = T, Q)//M062X/6-311+G(2df,2p). Notably, three different HNO2 isomers (cis-HONO, trans-HONO and HNO2) are all considered in each reaction pathway. The corresponding temperature- and pressure-dependent rate constants are then computed by RRKM/ME simulations with one-dimensional hindered rotor approximation and asymmetric Eckart tunneling corrections. The calculations show that the rate constants are pressure independent. Although trans-HONO is the most stable HNO2 isomer, the results reveal that the dominant channels are cis-HONO + HC(＝O)OCH2/C(＝O)OCH3 and cis-HC(O)(ONO)OCH3 for the H-abstraction and addition, respectively. Moreover, the lowest energy barrier for the H-abstraction channel (cis-abs) is 11.2 kcal mol−1 lower than the addition channel (cis-add), and thus the addition channel is less kinetically favored. The computed rate constants for the MF + NO2 reaction are then incorporated into a kinetic model and the importance of the title reaction in predicting the ignition behavior of MF/NO2 mixtures is demonstrated by kinetic modeling. The detailed reaction kinetics in this work will be helpful for kinetic model development of other ester-based fuels. [ABSTRACT FROM AUTHOR]

Recently, methyl pentanoate (MP) was proposed as a viable biodiesel surrogate to petroleum-based fuels. To better understand the pyrolysis chemistry of MP, the unimolecular decomposition kinetics of MP is theoretically investigated on the basis of ab initio calculations; ten primary channels, including four intramolecular H-shifts and six C–C and C–O bond fissions, are identified. The geometries are optimized at the M06-2X/cc-pVTZ level of theory, and accurate barrier heights are determined using the DLPNO-CCSD(T)/CBS(T-Q) method, which shows a good performance against the CCSD(T)/CBS(T-Q) method with an uncertainty of 0.5 kcal mol−1 for small methyl esters. The atomization enthalpy method is adopted to obtain the thermodynamics of involved species. The Rice–Ramsperger–Kassel–Marcus/master equation theory coupled with one-dimensional hindered rotor approximation is employed to calculate the phenomenological rate constants at 500–2000 K and 0.01–100 atm. The branching ratio analysis indicates that two reactions, MP ↔ CH3OC(＝O)CH3 + CH2CHCH3 and MP ↔ CH3OC(＝O)CH2 + CH2CH2CH3, are the dominant channels at low and high temperatures, respectively. The model from Diévart et al. [Proc. Combust. Inst., 2013, 34(1), 821–829] is updated with our calculations, and the modified model can yield a better prediction in reproducing the ignition delay times of MP at high temperatures. This work provides a comprehensive investigation of MP unimolecular decomposition, and can serve as a prototype for understanding the pyrolysis of larger alkyl esters. [ABSTRACT FROM AUTHOR]

The reaction between CO and HO2 plays a significant role in syngas combustion. In this work, the catalytic effect of single-molecule water on this reaction is theoretically investigated at the CCSD(T)/aug-cc-pV(D,T,Q)Z and CCSD(T)-F12a/jun-cc-pVTZ levels in combination with the M062X/aug-cc-pVTZ level. Firstly, the potential energy surface (PES) of CO + HO2 (water-free) is revisited. The major products CO2 + OH are formed via a cis- or a trans-transition state (TS) channel and the formation of HCO + O2 is minor. In the presence of water, the title reaction has three different pre-reactive complexes (i.e., RC2: CO⋯HO2 + H2O, RC3: CO⋯H2O + HO2, and RC4: HO2⋯H2O + CO), depending on the initial hydrogen bond formation. Compared to the water-free process, the reaction barriers of the water-assisted process are reduced considerably, due to more stable cyclic TSs and complexes. The rate constants for the bimolecular reaction pathways CO + HO2, RC2, RC3, and RC4 are further calculated using conventional transition state theory (TST) with Eckart asymmetric tunneling correction. For reaction CO + HO2, our calculations are in good agreement with the literature. In addition, the effective rate constants for the water-assisted process decrease by 1–2 orders of magnitude compared to the water-free one at a temperature below 600 K. In particular, the effective rate constants for the water-assisted and water-free processes are 1.55 × 10−28 and 3.86 × 10−26 cm3 molecule−1 s−1 at 300 K, respectively. This implies that the contribution of a single molecule water-assisted process is small and cannot accelerate the title reaction. [ABSTRACT FROM AUTHOR]

To thoroughly understand low-temperature oxidation of dimethylamine (DMA), the reaction kinetics of triplet ground state O 2 addition to DMA radicals (CH 3 NH C ˙ H 2 and CH 3 N ˙ CH 3) are theoretically investigated. In CH 3 NH C ˙ H 2 + O 2 , the direct H-abstraction reaction is significant due to its extremely low energy barrier, and the dominant channels involving intermediates, CH 3 NHCH 2 O O ˙ = CH 3 NCH 2 + H O ˙ 2 , CH 3 NHCH 2 O O ˙ = C ˙ H 2 NHCH 2 OOH and C ˙ H 2 NHCH 2 OOH = CH 2 NH + CH 2 O + O ˙ H , also have much lower barriers than those in reaction system CH 3 CH 2 C ˙ H 2 + O 2. In CH 3 N ˙ CH 3 + O 2 , the O 2 addition to N atom has a tight transition state, hindering radical consumption at low temperatures. The second O 2 addition to C ˙ H 2 NHCH 2 OOH is further investigated because of its comparable production at low temperatures and elevated pressures. The temperature- and pressure-dependent rate constants are calculated; CH 3 NCH 2 + H O ˙ 2 and CH 2 NCH 2 OOH + H O ˙ 2 are the dominant products for the first and second O 2 addition reactions, respectively, but the formation of formamides/O-heterocycles + O ˙ H is less important. Our results reveal different reaction pathways of DMA radicals from those of alkyls, and shed light on low-temperature oxidation mechanisms of N-containing fuels. [ABSTRACT FROM AUTHOR]

The pyrolysis of nitromethane, CH 3 NO 2 , was studied at ∼3.5 atm and 1013 K–1418 K in a heated shock tube by measuring the key product nitric oxide (NO) using mid-infrared laser absorption spectroscopy. We used a quantum cascade laser (QCL) at 5.26 µm to exploit the strong NO absorption at 1900.08 cm−1. With the NO absorption cross-section data characterized at 1006 K–1789 K and 2.7 atm–3.5 atm behind reflected shock waves, we measured the NO concentration time-histories during the pyrolysis of nitromethane at two different concentrations (1.05% and 0.6%). The absorption interference from other major products such as CO and H 2 O was analyzed to be negligible, leading to an interference-free NO diagnostic in nitromethane pyrolysis. A recent kinetic model of Shang et al. (2019) was adopted to interpret the shock tube data. All the NO time-histories measured over the entire temperature range 1013 K–1418 K were well-predicted by this model in terms of the initial NO formation rate and the final plateau level. The rate-of-production, sensitivity, and reaction flux analyses were performed to identify four important reactions (CH 3 NO 2 = CH 3 + NO 2 , CH 3 NO 2 ↔ CH 3 ONO = CH 3 O + NO, CH 3 + NO 2 = CH 3 O + NO, and NO 2 + H = NO + OH) that determine the NO formation during CH 3 NO 2 pyrolysis. The satisfactory agreement between the simulation and shock tube/laser absorption measurement further validated the kinetic mechanism of nitromethane decomposition. The developed mid-infrared NO absorption sensor provides a promising diagnostic tool for studying fuel-nitrogen chemical kinetics in the shock tube experiments. [ABSTRACT FROM AUTHOR]

Methyl pentanoate (MP) was recently proposed as a potential biodiesel surrogate due to its negative temperature coefficient region. To provide a basis for constructing an accurate mechanism, chemical kinetics of H-abstraction from MP by an OH radical are investigated theoretically at 200–2000 K. M06-2X/cc-pVTZ is applied for geometry optimizations and frequency calculations. Given the long alkyl-chain in MP, the multi-structural torsional anharmonicity is characterized by using the dual-level MS-T method due to its relatively low computational cost and established accuracy. In particular, AM1 and M06-2X/cc-pVTZ are adopted as low-level and high-level methods in dual-level MS-T, respectively. The results of dual-level MS-T are further used to benchmark against the full high-level method (M06-2X/cc-pVTZ), leading to an uncertainty of less than 30% in the high temperature range. For the single-point energy calculations, the lower computational cost DLPNO-CCSD(T) method is first used to benchmark against the gold-standard method CCSD(T) for small methyl ester (C2–C4) + OH reaction systems, yielding an overestimation of less than 1.1 kcal mol−1 for barrier height; it is then used to refine the electronic energies for the present reaction system MP + OH. Phase-space theory and conventional transition state theory are used to calculate the H-abstraction rate constants. After compensating the uncertainty of barrier height, the calculated phenomenological H-abstraction rate constants agree well with the experimental data at 263–372 K. Branching ratio analysis indicates that the β-site H abstraction is the dominant channel at 200–1200 K due to its lowest barrier height. [ABSTRACT FROM AUTHOR]

Dimethyl amine (DMA) is identified as a promising nitrogen-containing fuel candidate. To better understand the atmospheric and combustion chemistry of aliphatic amines, we systematically investigate the reaction kinetics of H-abstractions from DMA by H, CH3, OH, and HO2 radicals in a broad temperature range (100–2000 K). The BHandHLYP/cc-pVTZ method is adopted to determine the optimized geometries and frequencies, and the multi-structural torsional anharmonicity method (MS-T) is employed to characterize the effects of multi-conformer and torsional coupling for the involved species. High-level methods CCSD(T) and CCSD(T)-F12 combined with cc-pVXZ (X = D, T, Q), cc-pVXZ-F12 (X = D, T), and jun-cc-pV(T+d)Z basis sets are used to refine the electronic energies. The results of the gold standard method CCSD(T)/CBS(D–T–Q) with the zero point energy correction are adopted for the kinetic calculations. For the DMA + H/CH3 reactions, the conventional transition state theory (cTST) as well as one-dimensional Eckart tunneling correction is adopted. But for the DMA + OH/HO2 reactions, the reactant-complex (RC) is formed with a deep well (−6.4 and −11.7 kcal mol−1 for RC3 and RC4, respectively), due to the strong hydrogen bonding between the reactants. Hence, the variational transition state theory (VTST) combined with cTST is used to calculate the rate constants. The Rice–Ramsperger–Kassel–Marcus/master equation method is employed to determine the pressure-dependent rate constants in the pressure range of 0.001–100 atm. Our calculations are in agreement with previous experimental measurements and show well the trend in a broad temperature range. In addition, a pronounced pressure-dependence is observed under 400 K, indicating that pressure impacts the reaction mechanisms especially at atmospheric or interstellar temperatures. [ABSTRACT FROM AUTHOR]

Entropy is one of the key thermodynamic parameters in combustion kinetic modeling. Accurate entropy prediction needs to account for the conformational torsional anharmonicity, which could be solved by the state-of-the-art multi-structural torsion (MS-T) method. However, this method is computationally expensive or even not feasible for large flexible molecules. To address this issue, we proposed a multi-structural 2-dimensional torsion (MS-2DT) method that adopts minimally coupled torsions to reduce the computational cost. In this method, a series of 2-dimensional coupled torsion combinations were used to generate an initial conformer space with a size of CN2·9 (N is the number of torsions). The standard entropy (and the heat capacity) values of 18 C6–C8 alkanes with 5–7 torsions were computed at 200–2000 K. The MS-2DT calculation is in good agreement with the benchmark MS-T method: only a small deviation of −0.19 ± 0.15 cal mol−1 K−1 in standard entropy and −0.10 ± 0.21 cal mol−1 K−1 in heat capacity. Additionally, a further application of MS-2DT to n-decane with 9 torsions implies an improved accuracy in entropy (and heat capacity) prediction compared to other conventional simplified treatments. This method provides an affordable and accurate solution to treat the conformational torsional anharmonicity of large flexible alkanes. [ABSTRACT FROM AUTHOR]

Abstract We present the first demonstration of heterodyne phase-sensitive dispersion spectroscopy (HPSDS) for in situ, non-intrusive and quantitative CO 2 concentration measurements in flames. Dispersion spectroscopy retrieves gas properties by measuring the refractive index in the vicinity of a molecular resonance. The HPSDS scheme features a significant diagnostic advantage of the intrinsic immunity to laser power fluctuations caused by beam steering, thermal radiation and soot scattering in combustion environments, and thus no extra calibration process is required. In this work, we described the spectroscopic fundamentals for measuring heterodyne phase signals in flames. As a proof of principle, we used a mid-infrared interband cascade laser (ICL) near 4183 nm to exploit the strong CO 2 transitions in the R-branch of the v 3 fundamental band. The HPSDS signals of four CO 2 lines, R(76), R(78), R(80) and R(82), were measured in CH 4 /air flames to obtain CO 2 concentrations at different equivalence ratios (Φ = 0.8–1.2), yielding a good agreement with the simultaneous laser absorption measurements using the same ICL. With its immunity to laser power fluctuations verified experimentally, the HPSDS sensor was successfully implemented to measure CO 2 concentrations in C 2 H 4 /air sooting flames (Φ = 1.78–2.38). Laser dispersion spectroscopy proves to be a promising and alternative diagnostic tool for combustion measurements. [ABSTRACT FROM AUTHOR]

We report the first theoretical characterization of the pressure-dependence of hydrogen abstraction from methyl formate (MF) by a hydroxyl radical (OH) at combustion, atmospheric and interstellar temperatures. The reaction kinetics of MF + OH over a broad temperature range of 20–2000 K were studied using Rice–Ramsperger–Kassel–Marcus/master equation (RRKM/ME) theory. The M06-2x/ma-TZVP density functional method was adopted to construct the potential energy surface. The multi-structural torsional (MS-T) method was employed to account for the multi-conformer and torsional coupling effects. The barrier-less entrance channel forming an H-bonded complex was treated by phase state theory using long-range isotropic potential. The inner channel converting the complex into products was treated by both transition state theory and variational transition state theory in conjunction with asymmetric Eckart tunneling. We calculated the rate coefficients at the high-pressure and low-pressure limits, as well as by the pre-equilibrium model (PEM). The rate coefficients at 20–2000 K and 0.001–100 bar were determined and compared with the previous experimental results. Our calculations show a fairly good agreement with the measurements at 22–1344 K: a small deviation of ＜25% at combustion temperatures and a factor of 1.5–2.2 at interstellar temperatures. Besides providing an improved rate coefficient determination at combustion temperatures, we elucidate the pressure-dependence of the rate coefficient at atmospheric and interstellar temperatures. [ABSTRACT FROM AUTHOR]

The hydrogen abstraction reactions of phenyl formate (PF) by different radicals (H/O(3P)/OH/HO2) were theoretically investigated. We calculated the reaction energetics for PF + H/O/OH using the composite method ROCBS-QB3//M06-2X/cc-pVTZ and that for PF + HO2 at the M06-2X/cc-pVTZ level of theory. The high-pressure limit rate constants were calculated using the transition state theory in conjunction with the 1-D hindered rotor approximation and tunneling correction. Three-parameter Arrhenius expressions of rate constants were provided over the temperature range of 500–2000 K. To validate the theoretical calculations, the overall rate constants of PF + OH → Products were measured in shock tube experiments at 968–1128 K and 1.16–1.25 atm using OH laser absorption. The predicted overall rate constants agree well with the shock tube data (within 15%) over the entire experimental conditions. Rate constant analysis indicates that the H-abstraction at the formic acid site dominates the PF consumption, whereas the contribution of H-abstractions at the aromatic ring increases with temperature. Additionally, comparisons of site-specific H-abstractions from PF with methyl formate, ethyl formate, benzene, and toluene were performed to understand the effects of the aromatic ring and side-chain substituent on H-abstraction rate constants. [ABSTRACT FROM AUTHOR]

A mid-infrared two-line thermometry based on H2O absorption was developed for in situ and sensitive temperature measurements of laminar flames. The H2O absorption lines inv3fundamental band were systematically investigated and a new promising line pair (4029.52 cm–1and 4030.73 cm–1) was selected to achieve strong absorption line-strength and high-temperature sensitivity over the temperature range of 1000–3000 K. This line pair was readily assessed by a single current scan of a distributed feedback laser near 2.5 μm. The scanned-wavelength laser absorption strategy was adopted for temperature measurements for both laminar sooting and non-sooting flames. This optical sensor was applied in measuring various flame conditions including CH4/air and C2H4/air premixed flames (Ф = 0.8–1.2), C2H4/air sooting flames (Ф = 1.78–2.38), and partially premixed flames. Our measurements were in good agreement with thermocouple measurements for all the flame conditions. In addition, numerical simulations using the detailed chemical kinetic mechanisms (CH4flame: GRI 3.0, C2H4flame: USC 2.0) were conducted to validate the experimental results. [ABSTRACT FROM PUBLISHER]

We report the first demonstration of heterodyne phase-sensitive dispersion spectroscopy (HPSDS) for the simultaneous temperature and H2O concentration measurements in combustion environments. Two continuous-wave distributed-feedback quantum cascade lasers (DFB-QCLs) at 5.27 and 10.53 µm were used to exploit the strong H2O transitions (1897.52 and 949.53 cm−1) at high temperatures. The injection current of each QCL was modulated at sub-GHz or GHz to generate the three-tone radiation and the dispersion signal was detected by the radio-frequency down-conversion heterodyning. The peak-to-peak ratio of the two H2O dispersion spectra exhibits a monotonic relationship with temperature over the temperature range of 1000-3000 K, indicating the capability of performing two-line thermometry using laser dispersion spectroscopy. We measured the temperatures of CH4/air flames at different equivalence ratios (Φ = 0.8-1.2), yielding a good agreement with the corresponding thermocouple measurements. In addition, one-dimensional kinetic modeling coupled with a detailed chemical kinetic mechanism (GRI 3.0) was conducted to compare with the measured H2O concentrations using HPSDS. Finally, we demonstrated HPSDS is immune to optical power fluctuations by measuring the dispersion spectra at varied incident laser powers. [ABSTRACT FROM AUTHOR]

• Uniformity of laminar premixed flames is comprehensively studied. • Thermochemical structures are obtained by LAS measurements and CFD simulations. • Almost identical radial distribution of temperature and major products are observed. • Central H 2 O/CO 2 concentrations remain constant with varied Q coflow and Re reactant. • Flame uniformity is insensitive to Φ, but more sensitive to Q coflow and Re reactant. We performed a combined experimental and numerical study of the flame uniformity for a standard McKenna burner by investigating the spatially-resolved temperature and species concentrations in laminar CH 4 /air premixed flames. In the experiment, multi-wavelength laser absorption spectroscopy (LAS) was employed for in situ , non-intrusive, and quantitative measurements of temperature, H 2 O and CO 2. In the simulation, computational fluid dynamics (CFD) simulations were performed to model the thermochemical structures of the flames. The LAS measurements and CFD simulations were compared by varying the co-flow rate, equivalence ratio (Φ = 0.8–1.2), and the Reynolds number of the reactant flow. In general, all the simulations were in relatively good agreement with the experimental data. We observed that the flame temperature and H 2 O/CO 2 concentrations have almost the identical radial distributions with a certain non-uniformity, which should be carefully considered when using the standard flame for combustion research. In particular, the flame uniformity could be improved by increasing the Reynolds number of reactant flow or reducing the co-flow rate. [ABSTRACT FROM AUTHOR]

The reaction pathways and potential energy profiles are theoretically explored for H-abstraction, addition and addition-dissociation reactions of methyl formate (MF, HC(O)OCH 3 ) + NO 2 using the high level quantum chemical compound method CCSD(T)/cc-pV x Z( x = T, Q)//M062X/6-311+G(2df,2p). Notably, three different HNO 2 isomers ( cis -HONO, trans -HONO and HNO 2 ) are all considered in each reaction pathway. The corresponding temperature- and pressure-dependent rate constants are then computed by RRKM/ME simulations with one-dimensional hindered rotor approximation and asymmetric Eckart tunneling corrections. The calculations show that the rate constants are pressure independent. Although trans -HONO is the most stable HNO 2 isomer, the results reveal that the dominant channels are cis -HONO + HC(O)OCH 2 /C(O)OCH 3 and cis -HC(O)(ONO)OCH 3 for the H-abstraction and addition, respectively. Moreover, the lowest energy barrier for the H-abstraction channel ( cis -abs) is 11.2 kcal mol -1 lower than the addition channel ( cis -add), and thus the addition channel is less kinetically favored. The computed rate constants for the MF + NO 2 reaction are then incorporated into a kinetic model and the importance of the title reaction in predicting the ignition behavior of MF/NO 2 mixtures is demonstrated by kinetic modeling. The detailed reaction kinetics in this work will be helpful for kinetic model development of other ester-based fuels.

Rate constants of the H-abstraction reactions from dimethylamine (DMA) by triplets O and O 2 are theoretically determined with the canonical variational transition-state theory (CVT). By comparing the barrier heights and reaction energies obtained from different density-functional theory methods to those computed from the gold-standard method CCSD(T)/CBS(T-Q), we identify the M08-HX/ma-TZVP method as the best with a mean unsigned deviation of 1.0 kcal mol -1 . On the basis of the optimized geometries and frequencies with the selected method, the rate constants are calculated using the CVT method combined with the multistructural torsional anharmonicity and small-curvature tunnelling (MS-CVT/SCT) options in the temperature range 200-2000 K. The calculations show that OH and HO 2 are mainly produced from the direct abstraction from the C-H bond. The multistructural torsional anharmonicity has a large contribution to the rate constants, and the effects of recrossing and tunneling at the N-site are more important than those at C-site. Additionally, given the formation of reactant complex between DMA and triplet O, the H-abstraction channel is not favored at high pressure. Our calculations with both the Polyrate and MESS codes agree with the reported data within the uncertainty.

The H-abstraction reaction kinetics of methyl acrylate (MA) + H/OH/CH 3 /HO 2 radicals have been investigated theoretically in the present work. For these reactions, the reaction energies and barrier heights are first computed using several density functionals and compared to the coupled cluster CCSD(T)-F12/jun-cc-pVTZ benchmark calculations. The M062X/maug-cc-pVTZ method shows the best performance with the smallest mean unsigned deviation (MUD) of 0.42 kcal mol -1 . Combined with the electronic structure calculations using the M062X/maug-cc-pVTZ method, the multistructural canonical variational transition-state theory (MS-CVT) with small-curvature tunneling (SCT) is employed to calculate the reaction rate constants at 500-2000 K. The variational effect is between 0.56 and 1.0, the multistructural torsional anharmonicity factor ranges from 0.004 to 4.57, and the tunneling coefficient is in the range of 0.5-4.70. Notably, given the existence of reactant complexes (RCs) between reactants and transition states for the reaction systems MA + OH/HO 2 , we further compare the rate constants under the low-pressure limit (LPL) kinetic model, which treats the reaction as a single-step process and neglects RCs, and the pre-equilibrium model, which takes RCs into account in the reaction and treats the reaction as a two-step process. The rate constants calculated by these two models are similar within the combustion temperature range, and apparent differences occur at lower temperatures. In addition, we determine the branching ratios as a function of temperature and find that the methyl site (S3) abstractions by OH and H radicals are dominant in the low- and high-temperature ranges, respectively. Moreover, we update the kinetic model with the calculated H-abstraction rate constants to simulate the ignition delay times of MA. The simulations of the updated model are in good agreement with experimental results. The accurate reaction kinetics determined in this work are useful for the understanding and prediction of consumption branching fractions and ignition properties of the unsaturated methyl esters.

The reaction between CO and HO2 plays a significant role in syngas combustion. In this work, the catalytic effect of single-molecule water on this reaction is theoretically investigated at the CCSD(T)/aug-cc-pV(D,T,Q)Z and CCSD(T)-F12a/jun-cc-pVTZ levels in combination with the M062X/aug-cc-pVTZ level. Firstly, the potential energy surface (PES) of CO + HO2 (water-free) is revisited. The major products CO2 + OH are formed via a cis- or a trans-transition state (TS) channel and the formation of HCO + O2 is minor. In the presence of water, the title reaction has three different pre-reactive complexes (i.e., RC2: COHO2 + H2O, RC3: COH2O + HO2, and RC4: HO2H2O + CO), depending on the initial hydrogen bond formation. Compared to the water-free process, the reaction barriers of the water-assisted process are reduced considerably, due to more stable cyclic TSs and complexes. The rate constants for the bimolecular reaction pathways CO + HO2, RC2, RC3, and RC4 are further calculated using conventional transition state theory (TST) with Eckart asymmetric tunneling correction. For reaction CO + HO2, our calculations are in good agreement with the literature. In addition, the effective rate constants for the water-assisted process decrease by 1-2 orders of magnitude compared to the water-free one at a temperature below 600 K. In particular, the effective rate constants for the water-assisted and water-free processes are 1.55 × 10-28 and 3.86 × 10-26 cm3 molecule-1 s-1 at 300 K, respectively. This implies that the contribution of a single molecule water-assisted process is small and cannot accelerate the title reaction.

Dimethyl amine (DMA) is identified as a promising nitrogen-containing fuel candidate. To better understand the atmospheric and combustion chemistry of aliphatic amines, we systematically investigate the reaction kinetics of H-abstractions from DMA by H, CH 3 , OH, and HO 2 radicals in a broad temperature range (100-2000 K). The BHandHLYP/cc-pVTZ method is adopted to determine the optimized geometries and frequencies, and the multi-structural torsional anharmonicity method (MS-T) is employed to characterize the effects of multi-conformer and torsional coupling for the involved species. High-level methods CCSD(T) and CCSD(T)-F12 combined with cc-pVXZ (X = D, T, Q), cc-pVXZ-F12 (X = D, T), and jun-cc-pV(T+d)Z basis sets are used to refine the electronic energies. The results of the gold standard method CCSD(T)/CBS(D-T-Q) with the zero point energy correction are adopted for the kinetic calculations. For the DMA + H/CH 3 reactions, the conventional transition state theory (cTST) as well as one-dimensional Eckart tunneling correction is adopted. But for the DMA + OH/HO 2 reactions, the reactant-complex (RC) is formed with a deep well (-6.4 and -11.7 kcal mol -1 for RC3 and RC4, respectively), due to the strong hydrogen bonding between the reactants. Hence, the variational transition state theory (VTST) combined with cTST is used to calculate the rate constants. The Rice-Ramsperger-Kassel-Marcus/master equation method is employed to determine the pressure-dependent rate constants in the pressure range of 0.001-100 atm. Our calculations are in agreement with previous experimental measurements and show well the trend in a broad temperature range. In addition, a pronounced pressure-dependence is observed under 400 K, indicating that pressure impacts the reaction mechanisms especially at atmospheric or interstellar temperatures.

The potential energy surfaces (PESs) and reaction rate constants of the unimolecular decomposition of ethyl formate (EF) were investigated using high-precision theoretical methods at the CCSD(T)/CBS(T-Q)//M06-2X/6-311++G(d,p) level of theory. The calculated PESs of EF dissociation and molecular decomposition reactions indicate that the intramolecular H-shift to produce formic acid and ethylene is the dominant decomposition pathway. A detailed chemical kinetic mechanism for EF pyrolysis was constructed by incorporating the important reactions of EF and its radicals into an existing mechanism previously developed for small methyl esters. The updated mechanism was first used to reproduce CO, CO 2 , and H 2 O concentration time histories during EF pyrolysis in the shock tube reported by Ren et al. [ Ren , W. , Mitchell Spearrin , R. , Davidson , D. F. , and Hanson , R. K. J. Phys. Chem. A 2014 , 118 , 1785 - 1798 ]. The rate of production and sensitivity analyses show that the competing dehydration and decarboxylation channels of the intermediate formic acid control the final product yields of EF pyrolysis. The EF mechanism was further validated against the shock tube data of OH, CO, CO 2 , and H 2 O time histories measured during EF oxidation (equivalence ratio Φ = 1.0) at 1331-1615 K and 1.52-1.74 atm. This revised EF mechanism captured all of the species' time histories over the entire temperature range. Such modeling capability was due to the more accurate rate constants of EF reactions determined by high-precision theoretical calculations and a high-fidelity C 0 -C 2 basis mechanism.

We report a systematic chemical kinetics study of the H atom abstractions from ethyl formate (EF) by H, O( 3 P), CH 3 , OH, and HO 2 radicals. The geometry optimization and frequency calculation of all the species were conducted using the M06 method and the cc-pVTZ basis set. The one-dimensional hindered rotor treatment of the reactants and transition states and the intrinsic reaction coordinate analysis were also performed at the M06/cc-pVTZ level of theory. The relative electronic energies were calculated at the CCSD(T)/cc-pVXZ (where X = D, T) level of theory and further extrapolated to the complete basis set limit. Rate constants for the tittle reactions were calculated over the temperature range 500-2500 K by the transition state theory (TST) in conjunction with the asymmetric Eckart tunneling effect. In addition, the rate constants of H-abstraction by hydroxyl radical were measured in shock tube experiments at 900-1321 K and 1.4-2.0 atm. Our theoretical rate constants of OH + EF → products agree well with the experimental results within 15% over the experimental temperature range of 900-1321 K. Branching ratios for the five types of H-abstraction reactions were also determined from their individual site-specific rate constants.

This study focuses on the studies of the main pressure-dependent reaction types of iso-octane (iso-C8H18) pyrolysis, including initial C-C bond fission of iso-octane, isomerization, and β-scission reactions of the alkyl radicals produced by the C-C bond fission of iso-octane. For the C-C bond fission of iso-octane, the minimum energy potentials are calculated at the CASPT2(2e,2o)/6-31+G(d,p)//CAS(2e,2o)/6-31+G(d,p) level of theory. For the isomerization and the β-scission reactions of the alkyl radicals, the optimization of the geometries and the vibrational frequencies of the reactants, transition states, and products are performed at the B3LYP/CBSB7 level, and their single point energies are calculated by using the composite CBS-QB3 method. Variable reaction coordinate transition state theory (VRC-TST) is used for the high-pressure limit rate constant calculation and Rice-Ramsperger-Kassel-Marcus/master equation (RRKM/ME) is used to calculate the pressure-dependent rate constants of these channels with pressure varying from 0.01-100 atm. The rate constants obtained in this work are in good agreement with those available from literatures. We have updated the rate constants and thermodynamic parameters for species involved in these reactions into a current chemical kinetic mechanism and also have improved the concentration profiles of main products such as C3H6 and C4H6 in the shock tube pyrolysis of iso-octane. The results of this study provide insight into the pyrolysis of iso-octane and will be helpful in the future development of branched paraffin kinetic mechanisms.