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To uncover the fundamental chemistry of branched alkyl radical reaction with O 2 , especially the second oxidation process (O 2 + QOOH), iso-butyl is chosen as a combustion archetype and reactions on the potential energy surfaces of C 4 H 9 O 2 and C 4 H 9 O 4 are investigated by using quantum chemical calculations coupled with RRKM/master-equation method. In the present study we calculate the thermodynamics properties, the rate constants and branching ratios for major reaction pathways in a wide temperature range of 298−2400 K and pressure range of 0.01−10 atm. Our study indicates that the treatment of estimating the fate of OOQOOH in the second oxidation process by analogy with similar reaction type and molecular size for the fate of RO 2 in the first oxidation process could cause significant deviations. The accurate calculations conform that the rate constants for the second O 2 barrierless addition (QOOH + O 2) is half of the first O 2 barrierless addition (R + O 2). Morever, the accurate thermodynamics and kinetics data determined in the present work are indispensable for the detailed understanding and prediction of ignition properties of larger hydrocarbons. [ABSTRACT FROM AUTHOR]

• High precision schematic energetics calculation. • Site-specific reactivity. • Revealing the competing relationship between OH-addition and H-abstraction pathways. • Providing valuable kinetic data in a wide temperature and pressure condition. Oxidation of furan-based biofuels in the atmosphere and combustion are mainly initiated by reactions with small radicals (e.g. OH radical), the kinetics of which are crucial to obtain a comprehensive understanding of combustion processes, especially in engine operation. However, the structure- and site-dependent chemistry for OH-initiated reactions of furans with oxygenated substituents is unknown. In the present study, 2-furfuryl alcohol (2FFOH) was selected as a model fuel, and a full schematic energy diagram for 2FFOH with OH was calculated by a high-level quantum chemical method (CCSD(T)/CBS//M06-2X/def2-TZVP). The temperature- and pressure-dependent behavior of the rate coefficients k (T , P) for 2FFOH with OH was calculated at 298–2500 K and 0.01–10 atm by using the Rice–Ramsperger–Kassel–Marcus/Master equation method. Our calculations reveal that the rate coefficients of H-abstraction from the side chain are faster than those from the ring at T ≤ 2300 K. The observed temperature dependence behaviors of the total rate coefficients are consistent with prior experimental studies of other furans (e.g., furan, 25DMF and 2MF) with OH. The total rate coefficients of 2FFOH with OH exhibit a slightly positive pressure dependence only in the temperature range of 500–1000 K. We also found that the OH-addition pathways for 2FFOH with OH system dominate at T ≤ 1300 K. Moreover, the thermodynamic and kinetic data determined in the present work would be valuable for the future development of the fundamental chemical reaction mechanism of furan-based fuels in the atmosphere and combustion. [ABSTRACT FROM AUTHOR]

The rate constants for H-initiated reactions of 2-furfuryl alcohol (2FFOH) were calculated for the first time by a high-level quantum chemical method combined with the Rice–Ramsperger–Kassel–Marcus (RRKM) theory/Master equation method. A detailed potential energy surface was constructed by the CCSD(T)/CBS//M06–2X/def2-TZVP method, involving H-abstraction, initial H-addition, adduct-subsequent pathways and interconnected conversion pathways. The H-addition reaction on the β -carbon and δ -carbon sites to form bimolecular products 2-methylene-2,3-dihydrofuran plus OH and 2-methylene-2,5-dihydrofuran plus OH were found to be the major product channels at high temperatures. No significant pressure dependence is observed for the total rate constants of 2FFOH + H. However, the overall kinetics behavior does not reflect the site characteristics, i.e., the site-specific rate constants for each reaction channel show different temperature and pressure dependences. The total rate constants via the H-addition mechanism are higher by factors of 3.3–10 than those via H-abstraction channels under all studied conditions. The present updated kinetic model obtained by applying these new calculated rate constants can better predict 2FFOH decomposition, especially at 30 Torr, and it can also lead to good predictions of the speciation profiles (e.g., vinyl ketene) during 2FFOH pyrolysis. The data calculated in the present work can provide valuable information for the development of combustion models of furan-based biofuels. [ABSTRACT FROM AUTHOR]

Isopentanol (3-methyl-1-butanol) is a fuel additive and a second-generation biofuel; H-abstraction reactions from isopentanol by H atoms and CH 3 radicals are the basic starting responses in the combustion reaction mechanism of isopentanol. This work utilizes high quantum chemical theory and kinetic methods to describe the H-abstraction reaction from isopentanol by the H atom and CH 3 radical, as well as the isomerization and β -dissociation of isopentanol radicals. The potential energy surfaces (PESs) were calculated using the CCSD(T)/CBS//M06–2X-D3(0)/def2-TZVP method. The second-order Møller-Plesset perturbation theory (MP2), combined with a coupled cluster method CCSD(T), fully exploited the consistency of the Dunning basis set, the set of cc-pVQZ, cc-pVTZ, and cc-pVDZ basis sets, which were used and extrapolated to the complete basis set (CBS) limit. From the conventional transition state theory (CTST), the rate constants of the title reactions were calculated at temperatures ranging from 300 to 2000 K. Compared with the H-abstraction reaction by CH 3 radicals, the H-abstraction reaction by H atoms follows the Evans-Polanyi principle. It was found that the α -site was the most favorable H-abstraction site, while the O-site was relatively difficult. In the reaction of isopentanol radicals, the isomerization reaction pathways usually dominate at low temperatures, especially the 1,4-H and 1,5-H shift isomerization reactions. β -C-C bond dissociation dominates at high temperatures. This study extends the kinetic data for the H-abstraction of isopentanol and subsequent β -dissociation and isomerization of isopentanol radicals over a wide range of pressures and temperatures. [ABSTRACT FROM AUTHOR]

[Display omitted] • High precision theoretical calculations. • Revealed the conformation-dependent reactivity. • Concerted elimination path dominated the oxidation process. • Provide valuable kinetic data to low temperature combustion. Methylcyclohexane is one of the important components of transportation alternative fuels, and its reaction kinetics of low temperature combustion are crucial for the performance of advanced combustion engines, especially for autoignition. In the present work, we study 3-methylcyclohexyl radical (3MCHyl) as a model compound to clarify the kinetics of methylcyclohexyl radical with O 2 and the subsequent isomerization and dissociation of cis 3-methylcyclohexylperoxyl (cis -3MCHylOO). Combined the electronic structure calculations with Rice–Ramsperger–Kassel–Marcus/Master Equation (RRKM/ME) method, we determine previously missing kinetics data in a wide temperature and pressure range. The similarities and difference of kinetics between various methylcyclohexyl conformation with O 2 are clarified, which is helpful to better understand the formation mechanism of highly oxygenated molecules (HOM) and combustion pollutants. The accurate calculated kinetics data in the present work are crucial and valuable for the prediction of ignition properties of alternative fuels. [ABSTRACT FROM AUTHOR]

[Display omitted] • Dealing with the multi-structural torsional anharmonicity. • Using the multi-structure variational transition state theory. • Intramolecular hydrogen abstraction. • Providing accurate kinetics and thermodynamics data. The kinetics of ROO and OOQOOH radicals play an important role in simulating the ignition behavior of fuel during low-temperature combustion. Diethyl ether (DEE) was chosen in the present work because of its typical and representative character. In this work, we use the multi-structural torsional (MS-T) and multi-structural local quansi-harmonic (MS-LQH) methods to deal with multiple conformations and coupled torsion for structures of reactant, transition state and products, and use the multi-structure variational transition state theory (MS-VTST) to perform dynamic calculations for hydrogen shift isomerizations. Our results indicate that the multistructural anharmonicity effect plays a crucial role in determining the accuracy of the thermodynamic and kinetic quantities. Morever, the kinetics calculations indicate that rate constants for intramolecular hydrogen abstraction in the first oxidation mechanism (ROO → QOOH) and the second oxidation mechanism (OOQOOH → Products) exhibits different feature. This work will be helpful for understanding the oxidation mechanism of diethyl ether combustion. [ABSTRACT FROM AUTHOR]

• The overall rate constant was aggravated due to the presence of armchair site. • OH attacking should preferentially occur at the free-edge site. • Subsequent step of O 2 attacking should favor the armchair-edge site. • Providing new kinetics and thermodynamics data for OH-initiated oxidative degradation mechanism. To get a better understanding of oxidative degradation of polycyclic aromatic hydrocarbons (PAH) in the atmospheric and combustion chemistry, benzo[ ghi ]perylene (BP) was selected as their prototype to be investigated by combining ab initio electronic structure theory calculations (M06-2X/6-31G(d,p)) and the conventional transition state theory. For BP-OH adducts, O 2 addition favored the armchair edge comparing with the free and zigzag ones. The overall rate constant was increased due to the presence of armchair site; and thereby it has a prompt effect on the oxidation reactivity. This implies that given an arbitrary shaped PAH, OH attack preferentially occurs at the free-edge sites and subsequent O 2 attack favors the armchair-edge sites. Morever, the current study provides new kinetics and thermodynamics data for OH-initiated oxidative degradation mechanism of benzo[ ghi ]perylene in a wider temperature range, which can be used for modeling the fate of large PAHs during combustion and in the atmosphere. [ABSTRACT FROM AUTHOR]

Pyrolysis experiments of dimethyl methylphosphonate (DMMP) were carried out in a jet stirred reactor (JSR) coupled to synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) at 1 atm, from 900 to 1200 K, to quantitatively measure the mole fraction profiles of DMMP and its pyrolysis products. Furthermore, theoretical calculations were carried out. Geometry optimization and vibrational frequency analysis were conducted for DMMP unimolecular and isomerization reactions at the MN15/6-311+G(2df,2p) level of theory, and kinetic calculations were performed using the kinetic code of MESS. The results show that DMMP mainly undergoes isomerization channels to form its isomers before decomposing into pyrolysis products. This is different from the DMMP decomposition channels adopted in the literature models. The kinetic model of DMMP in the literature was modified and improved by adding newly calculated reactions and their rate constants and updating the rate constants of unimolecular reactions for DMMP as well based on computations. Some rate tuning of reactions by 5 or 10 factors has been tried in model development to better describe the experimental measurements. Finally, the model developed by this work showed much better prediction of DMMP consumption and major species production than the prediction by literature models. In addition, the updated model was validated against various previous experimental data (CO mole fraction distributions in the shock tube, the flame speed and species mole fraction distributions in the premixed flame), and the satisfactory validation results indicate the reliability of the DMMP model that was improved in this work. This preliminary work laid a foundation to further study the combustion properties and to develop the kinetic model of organophosphorus compounds. [ABSTRACT FROM AUTHOR]

Isopentanol (3-methyl-1-butanol, 3M1B) is a prospective bioderived fuel with advantages over ethanol that make it a suitable substitute for gasoline. Although there have been studies on the combustion properties of 3M1B, reaction kinetics for its thermal decomposition is scarce. This study explores the reaction pathways for the thermal decomposition of 3M1B through theoretical calculations. Six reactions with tight transition states, and nine bond dissociation reactions (i.e., barrierless reactions) were considered. For reactions with tight transition states, the CCSD(T)/CBS//M06–2X-D3(0)/def2-TZVP method was used, and the MRCISD(+Q)/CBS//CASPT2/cc-pVDZ method was adopted for the bond dissociation reactions. The conventional transition state theory (CTST), including multi-structural torsional anharmonicity (MS-T) correction, addressed those reactions with tight transition states. The MS-T method thoroughly examined the co-effect of the multiple structures and torsional anharmonicity. Direct bond-breaking reactions were studied using the variational transition state theory (VTST). Subsequently, system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory and Rice-Ramsperger-Kassel-Marcus/Master Equation (RRKM/ME) were employed to obtain the rate constants for reactions with tight transition states and direct bond-breaking reactions at different temperatures and pressures (T/P) regimes (T = 298–2400 K, P = 0.01–100 atm), respectively. The results showed that water elimination and C-C bond dissociation play a significant role for 3M1B unimolecular decomposition, other molecular elimination and bond dissociation channels can be ignored. Combining the reaction pathways and the corresponding pressure dependent rate constants into the kinetic model resulted in good predictions of the speciation profiles during 3M1B pyrolysis. These data are valuable for the development of the combustion models of 3M1B. [ABSTRACT FROM AUTHOR]

• Pyrolysis of di- n -propyl ether was studied using SVUV-PIMS at 0.04 and 1 atm. • A kinetic model for di- n -propyl ether pyrolysis was developed and validated. • Free radical chain mechanism plays a vital role in fuel pyrolysis. • Pyrolysis characteristics of di- n -propyl ether and n -propanol were compared. The pyrolysis of di- n -propyl ether (DPE), a potential biofuel candidate, was studied under a temperature range of 850–1350 K in a flow reactor at low and atmospheric pressures. Synchrotron vacuum ultraviolet photoionization mass spectrometry was used for isomeric identification and mole fraction measurements of the pyrolysis products, especially the free radicals. Specific products were observed and measured for the unimolecular decomposition reactions of DPE. A detailed kinetic model of DPE pyrolysis was developed based on the n -propanol combustion model. Modeling analyses including the rate of production (ROP) and sensitivity analyses were performed to reveal the consumption pathways of DPE, as well as the formation and consumption pathways of intermediates and products. ROP and sensitivity analyses indicated that the consumption reaction of DPE was mainly controlled by the unimolecular decomposition reactions and H-abstractions at both low and atmospheric pressures. The pericyclic reaction, leading to n -propanol + propylene, was the most important unimolecular decomposition reactions, especially at low pressure. The H-abstraction reactions of DPE played crucial roles in its consumption. Further β -C–C and β -C–O scission reactions of the three primary radicals can produce some oxygenated and hydrocarbon products. Relatively high mole fractions of C1–C3 aldehydes and C1–C3 alkane, alkene, and alkyl radicals were observed, which demonstrated the structural feature of DPE. Comparison of the product distribution between DPE and n -propanol were also analyzed. [ABSTRACT FROM AUTHOR]