Search

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]

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]

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]