Conventional Transition State Theory Research Articles

Theoretical studies on the kinetics and mechanism of hydroxyl radical reaction with quinclorac and quinmerac herbicides in aqueous media

AbstractThe kinetic and mechanistic studies for the reaction of hydroxyl radical with two quinoline based herbicides, namely, quinclorac and quinmerac has been performed using various computational methods in aqueous media. Geometry optimizations were performed using Density Functional Theory (DFT) methods including water as the solvent. Local reactivity parameters of these herbicides towards the •OH radical are predicted using condensed Fukui function. Single point energies of various species were calculated using double hybrid method, namely, B2PLYP–D for better accuracy. The pKa values for these acid based herbicides allow them to exist in deprotonated form in aqueous condition. Hence, the calculations are also performed for the deprotonated or the anionic form apart from the neutral species. Individual rate coefficients for •OH radical addition reaction with each carbon atoms were evaluated using conventional transition state theory using one–dimensional tunneling corrections. The solvent effect on reaction is implemented through Collins–Kimball formulations. Both the approaches, namely, the Fukui index and individual rate constant determination confirms that the most reactive site for the •OH radical addition in these two herbicide is the carbon atom attached to the COOH group. The total rate constant for the •OH radical reaction with both neutral and anionic forms of these two herbicides are relatively high and equal to its diffusion‐limit value. Evaluation of the ecotoxicities of the parent herbicides and their OH adducts is estimated using the structure–activity relationship concept.

Theoretical Investigation on the H Atom Abstraction Reaction from C1-C4 Alkanes and Alkenes by ṄH2 Radicals.

The H atom abstraction reactions from alkanes and alkenes by ṄH2 are decisive in predicting the combustion characteristics of NH3/CxHy binary fuels. Theoretical investigation is carried out on the energy barriers of H atom abstraction reactions from C1-C4 alkanes/alkenes by ṄH2 radicals at the QCISD(T)/CBS//M06-2X/6-311++G(d,p) level of theory. Single-point energies of each species are computed using QCISD(T)/cc-pVDZ, TZ level of theories with basis set corrections from MP2/cc-pVDZ, TZ, and QZ methods. One-dimensional hindered rotor potentials are obtained by the M06-2X/cc-pVTZ method with 10° increment. Rate constants of each channel across temperatures of 298.15-2000 K are calculated by solving the RRKM/Master Equation with conventional transition state theory. For alkanes, rate constants order follows ktertiary> ksecondary> kprimary, while for alkenes the order follows kallylic> kprimary> kvinylic. Among the vinylic carbon sites within the same alkene species, the hydrogen atom sharing the same carbon with the allylic carbon on the C-C double bond is the preferred site for the H atom abstraction reaction. The branching ratio results indicate that the abstraction from tertiary or secondary carbon sites on alkanes and allylic carbon sites on alkenes are dominating during the investigated temperature range but become less important as the temperature increases. The data provided in this work are in good agreement with the literature data, but for the ṄH2+alkenes system, the literature data are scarce and further investigation is needed.

Theoretical Prediction of the Reaction Probabilities of H, O, and OH Radicals on the Polypropylene Surface.

To determine the H-abstraction reaction probabilities of H/O/OH radicals with a polypropylene (PP) surface, a first-principles calculation was performed based on the DLPNO-CCSD(T)/CBS//M06-2X-D3/def-TZVP theory level. The PP chain model used in this study was 2,4,6-trimethylheptane. The rate constants of the H/O/OH radicals with the isolated PP chain model were calculated based on the conventional transition-state theory. By comparing the experimental values and considering the error factors and their compensation, it was concluded that the orders of magnitude of the predicted rate constants were accurate. The resulting rate constants were converted to reaction probabilities between the H/O/OH radicals and the PP surface. The method used in this study is applicable for obtaining theoretical values of surface reaction probabilities based on first-principles calculations. The calculation at the DLPNO-CCSD(T)/CBS theory level has high accuracy but consumes a large amount of computational resources. The study also demonstrated that the double-hybrid functionals, wB97x-2-D3(BJ) and rev-DSD-PBEP86-D3(BJ), with a 3-ζ or 4-ζ basis set, could reproduce the electronic energy values obtained from DLPNO-CCSD(T)/CBS while using only approximately 1/100 of the computational resources required by the latter under our computer configuration.

Rapid Atmospheric Reactions between Criegee Intermediates and Hypochlorous Acid.

Hypochlorous acid (HOCl) is a paramount compound in the atmosphere due to its significant contribution to both tropospheric oxidation capacity and ozone depletion. The main removal routes for HOCl are photolysis and the reaction with OH during the daytime, while these processes are unimportant during the nighttime. Here, we report the rapid reactions of Criegee intermediates (CH2OO and anti/syn-CH3CHOO) with HOCl by using high-level quantum chemical methods as the benchmark; their accuracy is close to coupled cluster theory with single, double, and triple excitations and quasiperturbative connected quadruple excitations with a complete basis limit by extrapolation [CCSDT(Q)/CBS]. Their rate constants have been calculated by using a dual-level strategy; this combines conventional transition state theory calculated at the benchmark level with variational transition state theory with small-curvature tunneling by a validated density functional method. We find that the introduction of the methyl group into Criegee intermediates not only affects their reactivities but also exerts a remarkable influence on anharmonicity. The calculated results uncover that anharmonicity increases the rate constants of CH2OO + HOCl by a factor of 18-5, while it is of minor importance in the anti/syn-CH3CHOO + HOCl reaction at 190-350 K. The present findings reveal that the loose transition state for anti-CH3CHOO and HOCl is a rate-determining step at 190-350 K. We also find that the reaction of Criegee intermediates with HOCl contributes significantly to the sink of HOCl during the nighttime in the atmosphere.

Theoretical Study of the NO Reduction Mechanism on Biochar Surfaces Modified by Li and Na Single Adsorption and OH Co-Adsorption

Theoretical and experimental investigations have shown that biochar, following KOH activation, enhances the efficiency of NO removal. Similarly, NaOH activation also improves NO removal efficiency, although the underlying mechanism remains unclear. In this study, zigzag configurations were employed as biochar models. Density functional theory (DFT) was utilized to examine how Li and Na single adsorption and OH co-adsorption affect the reaction pathways of NO reduction on the biochar surface. The rate constants for all reaction-determining steps (RDSs) within a temperature range of 200 to 1000 K were calculated using conventional transition state theory (TST). The results indicate a decrease in the activation energy for NO reduction reactions on biochar when activated by Li and Na adsorption, thus highlighting their beneficial role in NO reduction. Compared to the case with Na activation, Li-activated biochar exhibited superior performance in terms of the NO elimination rate. Furthermore, upon the adsorption of the OH functional group onto the Li-decorated and Na-decorated biochar models (LiOH-decorated and NaOH-decorated chars), the RDS energy barriers were higher than those of Li and Na single adsorption but easily overcome, suggesting effective NO reduction. In conclusion, Li-decorated biochar showed the highest reactivity due to its low RDS barrier and exothermic reaction on the surface.

Theoretical kinetic study of key reactions between ammonia and fuel molecules, part I: Hydrogen atom abstraction from alkanes by ṄH2 radical

Cross reactions between ṄH2 radical and fuel molecules are essential in determining the reactivity for ammonia/fuel blends. A systematic theoretical chemical kinetic investigation of hydrogen atom abstraction by ṄH2 radical from C1–C6 alkanes, n-heptane, and isooctane has been performed in this study. The geometry optimization, frequency, and zero-point energy calculations for all species related to the title reactions were calculated at the M06-2X/6-311++G(d,p) level of theory. One-dimensional hindered rotor treatment has also been performed at the same level of theory to obtain the potential energy surface for the low frequency torsional modes. Single-point energy was obtained at the QCISD(T)/cc-pVDZ, QCISD(T)/cc-pVTZ, MP2/cc-pVDZ, MP2/cc-pVTZ and MP2/cc-pVQZ levels of theory and then extrapolated to the complete basis set. Benchmark calculations for single point energy have also been carried out at the CCSD(T)-F12/cc-pVTZ, CCSD(T)-F12/cc-pVQZ, CCSD(T)-F12/aug-cc-pVTZ and CCSD(T)-F12/aug-cc-pVQZ levels of theory based on the geometries obtained at the CCSD(T)/cc-pVDZ level of theory for the reaction systems of methane + ṄH2 and ethane + ṄH2. The rate constants of 41 reactions in the temperature range 298.15–2000 K were calculated by using the Master Equation System Solver (MESS) with conventional transition state theory. Good agreement has been gained between our calculated results and the available theoretical and experimental data in the literature. Rate rules for different reaction sites have also been derived, and the average rate constants for primary, secondary, and tertiary sites were also reported on a per hydrogen atom basis. The impact of calculation data for the ammonia/alkane blends model has also been investigated. The calculated rate constants are valuable for the combustion kinetic model development for ammonia blending fuels.

Theoretical study on the H-atom abstraction reactions of pentanol + HȮ2, part I: five branched pentanol isomers.

The chemical kinetic studies of hydrogen atom (H-atom) abstraction reactions by hydroperoxyl (HȮ2) radicals from five branched pentanol isomers, including 3-methyl-1-butanol, 2-methyl-1-butanol, 1,1-dimethyl-1-propanol, 1,2-dimethyl-1-propanol, and 2,2-dimethyl-1-propanol were investigated systematically through high-level ab initio calculations. Geometry optimization, frequency analysis, and zero-point energy (ZPE) corrections were performed for six reactants, twenty-three transition states (TSs), and twenty-four products at the M06-2X/6-311++G(d,p) level of theory. The intrinsic reaction coordinate calculation was performed at the same level of theory to confirm the transition state connection. The one-dimensional hindered rotor treatment for low-frequency torsional modes was also treated at the M06-2X/6-311++G(d,p) level of theory. The QCISD(T)/CBS level of theory was used to calculate the single-point energies for the species whose T1 diagnostic value was lower than 0.035. At the same time, the CASPT2/CBS level of theory was used to calculate the single-point energies for the channel in which the T1 diagnostic value of transition states was greater than 0.035. Rate constants for the H-atom abstraction reactions from the five branched pentanol isomers by HȮ2 radicals were calculated by using conventional transition state theory with asymmetric Eckart tunneling corrections in the temperature range of 500-2000 K. Rate constants and branching ratios for the title reactions and the rate rules for ten different H-atom abstraction types were investigated. Temperature-dependent thermochemistry properties for all reactants and products were calculated by the composite methods of G3/G4/CBS-QB3/CBS-APNO, which were in good agreement with the data available in the literature. Rate constants for the H-atom abstraction reactions by HȮ2 radical from branched pentanol isomers were investigated in this work as part I, and those for linear pentanol isomers will be analyzed in part II. All the calculated kinetics and thermochemistry data can be utilized in the model development for branched pentanol isomers oxidation.

Oxidation pathways and kinetics of the 1,1,2,3-tetrafluoropropene (CF2CF-CH2F) reaction with Cl-atoms and subsequent aerial degradation of its product radicals in the presence of NO.

To give a comprehensive account of the environmental acceptability of 1,1,2,3-tetrafluoropropene (CF2CF-CH2F) in the troposphere, we have examined the oxidation reaction pathways and kinetics of CF2CF-CH2F initiated by Cl-atoms using the second-order Møller-Plesset perturbation (MP2) theory along with the 6-31+G(d,p) basis set. We also performed single-point energy calculations to further refine the energies at the CCSD(T) level along with the basis sets 6-31+G(d,p) and 6-311++G(d,p). The estimation of the relative energies and thermodynamic parameters of the CF2CF-CH2F + Cl reaction clearly shows that Cl-atom addition reaction pathways are more dominant compared to H-abstraction reaction pathways. The value of the rate coefficient for each reaction channel is calculated using the conventional transition state theory (TST) over the temperature range of 200-1000 K at 1 atm. The estimated overall rate coefficients for the title reaction are found to be 1.10 × 10-12, 1.21 × 10-10, and 1.13 × 10-8 cm3 per molecule per s via the respective calculation methods viz. MP2/6-31+G(d,p), CCSD(T)//MP2/6-31+G(d,p), and CCSD(T)/6-311++G(d,p)//MP2/6-31+G(d,p), at 298.15 K. Moreover, the calculated rate coefficients and percentage branching ratio values suggest that the Cl-atom addition reaction at the β-carbon atom is more preferable to that of the α-carbon addition to CF2CF-CH2F. Based on the rate coefficient values calculated by the three different methods, the atmospheric lifetime for the title reaction at 298.15 K is estimated. The radiative efficiency (RE) and Global Warming Potential (GWP) results of the title molecule show that its GWP would be negligible. Further, we have explored the degradation of its product radicals in the presence of O2 and NO. From the degradation results, we have found that CF2(Cl)COF, FCOCH2F, FCFO and FCOCl are formed as stable end products along with various radicals such as ˙CF2Cl and ˙CH2F. Therefore, these findings of kinetic and mechanistic data can be applied to the development and implementation of a novel CFC replacement.

Experimental and kinetic study on 2-methoxyethanol part I: Molecular chemistry and pyrolysis investigation

Glycol ethers have been widely recognized as an alternative biofuels. 2-Methoxyethanol (ME2, HOCH2CH2OCH3) is the simplest and most representative glycol ether. Hence, the combustion chemistry of ME2 has aroused extensive interest. The ME2 pyrolysis has been investigated experimentally in a jet-stirred reactor (JSR) at atmospheric pressure within the temperature range of 650–1075 K for the first time. FTIR, GC–MS, and GC-FID were used to identify and quantify species profiles in ME2 pyrolysis. Sixteen intermediates were newly detected by the present work including acetaldehyde, hydroxyacetaldehyde, and methyl vinyl ether which are essential for mechanism development. The H abstraction reactions of ME2 by H, CH3, HO2, OH, O2, and the unimolecular reactions of four fuel-related radicals were first studied via high level ab initio calculations. The geometries optimization, vibrational frequencies, and intrinsic reaction coordinate (IRC) calculations were carried out at the M06–2X/6–311++G(d,p) level of theory. Conventional transition state theory (TST) and Rice-Ramsperger-Kassel-Marcus/Master Equation (RRKM/ME) were used to calculate the temperature- and pressure-dependent rate constants from 300 to 2000 K and 0.01 to 100 atm for the first time. It is found that the H abstraction reactions at the two middle carbon atoms are the most favored pathways, and b-scissions for fuel-related radicals have the lowest energy barriers. The thermochemical properties of ME2 and its derived radicals have been obtained via the combination of composite methods. A mechanism involving 596 species and 3123 reactions was developed for the first time to predict the ME2 pyrolysis characteristics based on experimental and calculated results obtained by the present work. Good agreements were achieved between the measured and simulated results. Rate of production (ROP), as well as sensitivity analysis, were performed to get a deeper insight into the ME2 pyrolysis. It has been revealed that the major ME2 decomposition channels are the H abstractions by H, CH3, and OH radicals, and unimolecular reactions have scarce contributions to fuel decay. The results, in terms of the measured data and of the calculated results, and the newly developed kinetic model as well as findings obtained within this study will assist and contribute to a better understanding of combustion characteristics of ME2 and of glycol ethers as well.

Insight into the fate of nitrogen during char thermal conversion and the influence mechanism of potassium: A theoretical research

Nitrogen oxides (NOₓ) are primary pollutants produced during biomass combustion. During the devolatilization stage, char nitrogen (char(N)) is formed. In the subsequent char combustion stage, char(N) can decompose directly into NOx precursors or engage in heterogeneous reactions with O2 or NO to form NO and N2. Nonetheless, a comprehensive understanding of the reaction mechanisms and competitiveness of char(N) migration, especially the influence of the alkali metal potassium (K) present in biomass, remains incomplete. Building on the Zigzag char(N) models, the present study delves into the migration reactions of char(N), assessing their competitive dynamics through the integration of density functional theory, electronic structure analysis, and conventional transition state theory. Furthermore, it examines the impact of K on char(N) conversion. The competitiveness of the heterogeneous reactions follows the sequence: heterogeneous reduction of NO to N2 > heterogeneous oxidation of char(N) to NO > decomposition of char(N) to NOx precursors. Moreover, the formation of HCN is more favorable than NH₃ production. The successive conversion from char(N) to NO and then to N2 is the predominant migration route for char(N), with NO generation as the pivotal step. The less preferred char(N) migration route involves decomposition to NH3/HCN, followed by oxidation to NOx within the main combustion zone, which cannot be mitigated by char. K can accelerate NO generation and sustain the primacy of the heterogeneous NO reduction, consequently enhancing the oxidation-reduction process of char(N). As a result, K plays a constructive role in managing NOx emissions during the thermal conversion of char.

Utility of (MgO)12 nanocage as a chemical sensor for recognition of amphetamine drug: A computational inspection

DFT calculations on sensor-drug interactions are necessary for understanding binding mechanisms, predicting sensor performance, evaluating stability and reactivity, and rational design of sensor materials. We scrutinized the adsorption of amphetamine (AFE) on the pure magnesium oxide nano-cage (MgONC) by applying density functional theory. All geometries and single point energy computations were optimized at M06–2X/6–311 G (d, p). Furthermore, we performed an analysis of the natural bond orbital (NBO) and evaluated the values of partial natural charges. Additionally, we investigated donor-acceptor (D-A) interactions and examined the Wiberg bond index (WBI) in greater depth. The MgONC was capable of adsorbing AFE with greater strength with the energy of adsorption (Eads) of −48.19 kcal/mol (for stable configurations). Moreover, the NBO method demonstrated more effective D-A interactions between AFE and the MgONC. Based on the computations, for the most stable configuration, there was a substantial alteration in the HOMO-LUMO gap of the MgONC following the drug adsorption, thus increasing the electrical conductance (EC) of the MgONC. The sensing mechanism is related to the gap difference, which depends on the change in the EC. We adopted the conventional transition state theory for the prediction of recovery time. The computations indicated that the MgONC+ AFE configuration had a short recovery time for the desorption of AFE. Finally, based on our findings, we could conclude that the MgONC is an appropriate choice for the improvement of effective AFE sensors. DFT study of drug sensors will focus on enhancing sensitivity, selectivity, and stability while exploring novel materials and optimizing performance through theoretical simulations and analysis.

Theoretical study of H-atom abstraction by CH3OȮ radicals from aldehydes and alcohols: Ab initio and comprehensive kinetic modeling

Reaction kinetics of H-atom abstraction from C1C3 alcohols, including CH3OH, C2H5OH, NC3H7OH, IC3H7OH, and C1C2 aldehydes, including CH2O, CH3CHO, by CH3OȮ radicals is investigated in this work through high-level ab initio calculations. Electronic structure optimizations are performed for all the stationary points with M06–2X/6−311++g(d,p) method. Quadratic configuration interaction method QCISD(T)/cc-pVXZ (where X = D and T) and Møller–Plesset perturbation theory MP2/cc-pVXZ (where X = D, T, and Q) are used to calculate single point energies. Subsequently, rate coefficients for all H-atom abstraction channels are determined using conventional transition state theory with unsymmetric tunneling corrections. These calculated results are updated to a recently updated model and then compared against expansive experimental datasets to investigate their influence on the model prediction performance. The updated model shows obvious discrepancies with the original model, where the updated model is less reactive for CH3CHO, C2H5OH, NC3H7OH, and IC3H7OH, while showing negligible differences for CH3OH and CH2O due to the lack of CH3OȮ radical formation. Sensitivity and flux analyses are further conducted, through which the difference between the studied species in their oxidation pathways and the relative importance of H-atom abstraction by CH3OȮ radicals is highlighted. With the updated rate parameters, the branching ratios of H-atom abstractions from CH2O and CH3CHO are significantly altered for CH3CHO, C2H5OH, NC3H7OH and IC3H7OH, with an obvious shift away from the abstraction channel by CH3OȮ. Further analyses highlight the critical roles of CH2O and CH3CHO chemistry as core chemistries for large hydrocarbons (e.g., carbon number > 1), which have been inadequately described in existing chemistry models. Systematic efforts are urgently needed to improve the existing CH2O and CH3CHO chemistries.

Polyrate 2023: A computer program for the calculation of chemical reaction rates for polyatomics. New version announcement

Polyrate 2023: A computer program for the calculation of chemical reaction rates for polyatomics. New version announcement

TUMME 2023: Tsinghua University Minnesota Master Equation program. New version announcement

This is an updated version of TUMME (Tsinghua University Minnesota Master Equation program), which is a computer program for setting up and solving master equations for chemical kinetics of unimolecular and bimolecular reactions. The master equation is discretized in terms of energy bins, and rate constants are calculated from chemically significant eigenmodes. TUMME has interfaces to Gaussian 16, Polyrate 2023, and MSTor 2023 output files that allow the master equation code to obtain the microcanonical flux coefficients needed for the kernel of the master equation as calculated by conventional transition state theory (TST), variational transition state theory (VTST) with various tunneling methods, or multi-structural or multi-path VTST (MS-VTST or MP-VTST) with various tunneling methods. The tunneling methods supported include zero-curvature tunneling (ZCT), small-curvature tunneling (SCT), large-curvature tunneling (LCT), and microcanonically optimized multidimensional tunneling (μOMT). For mechanisms involving only unimolecular isomerization (no bimolecular pairs), TUMME can set up and solve a conservative master equation for both rate constants and time-dependent energy-bin populations. For mechanisms involving bimolecular pairs, TUMME 2023 can set up and solve two kinds of master equation: (i) a nonconservative master equation for calculating rate constants of bimolecular reactions and (ii) a conservative master equation that includes bimolecular association in the transition matrix and that can be used for calculating the time evolution of the concentration of a pseudo-first-order bimolecular reactant. TUMME is written in double precision with Python 3; quadruple and octuple precision are also available for some subtasks in C++. The Python code can run in serial or parallel (MP or MPI), and C++ code can run on a single processor or on multiple processors with OpenMP. The program includes a manual and a tutorial. New version program summaryProgram Title: TUMME 2023CPC Library link to program files:https://doi.org/10.17632/whcnvm2mc9.2Developer's repository link:https://doi.org/10.5281/zenodo.7943283Licensing Provisions: Apache-2.0 for the program and CC-BY-4.0 for the manualProgramming languages: Python 3 and C++Does the new version supersede the previous version?: YesJournal reference of previous version: Comput. Phys. Commun. 270 (2022) 108140 [1]External libraries: Numpy, Scipy, Numba, mpi4py (optional), modified mpack (optional), qd (optional), omp (optional)Interfaces to other programs: Gaussian 16 [2], Polyrate 2023 [3], and/or MSTor 2023 [4]Nature of problem: Characterize and calculate rate constants for a temperature-dependent and pressure-dependent complex reaction systemSolution method: Solve the energy-grained master equation based on the chemically significant eigenmodes to get phenomenological rate constants and time evolutions of the populations [1,5–7].Reason for new version: The new version increases the functionality and accuracy, and it offers an easier way to run the program. In addition, some bugs have been corrected.Summary of revisions:1.Added an option (available in high precision) to treat pseudo-first-order bimolecular reactions to obtain the time evolution of the reactants and intermediates. [8]2.Added an inverse-Laplace-transform option in the REACTION block.3.Abandoned rounding the sizes of energy bins to an integer wavenumber (ESOT).4.Changed the way to run TUMME by providing an executable bash script.5.Slightly adjusted the strategy for eigenmode mergers to deal with some unexpected cases.6.Adjusted the output for the partition functions.7.Added options for large-curvature tunneling (LCT) and microcanonically optimized multidimensional tunneling (μOMT).8.Added the keyword pyrfile to support a Polyrate 2023 [3] file that ends with a suffix .fu100.9.Fixed some bugs.10.Updated the manual and added a tutorial to the program distribution, so the distribution now has both a manual and a tutorial.

A Theoretical Study of NH2 Radical Reactions with Propane and Its Kinetic Implications in NH3-Propane Blends’ Oxidation

The reaction of NH2 radicals with C3H8 is crucial for understanding the combustion behavior of NH3/C3H8 blends. In this study, we investigated the temperature dependence of the rate coefficients for the hydrogen abstraction reactions of C3H8 by NH2 radicals using high-level theoretical approaches. The potential energy surface was constructed at the CCSD(T)/cc-pV(T, Q)//M06-2X/aug-cc-pVTZ level of theory, and the rate coefficients were computed using conventional transition state theory, incorporating the corrections for quantum tunneling and hindered internal rotors (HIR). The computed rate coefficients showed a strong curvature in the Arrhenius behavior, capturing the experimental literature data well at low temperatures. However, at T > 1500 K, the theory severely overpredicted the experimental data. The available theoretical studies did not align with the experiment at high temperatures, and the possible reasons for this discrepancy are discussed. At 300 K, the reaction of NH2 with C3H8 predominantly occurs at the secondary C-H site, which accounts for approximately 95% of the total reaction flux. However, the hydrogen abstraction reaction at the primary C-H site becomes the dominant reaction above 1700 K. A composite kinetic model was built, which incorporated the computed rate coefficients for NH2 + C3H8 reactions. The importance of NH2 + C3H8 reactions in predicting the combustion behavior of NH3/C3H8 blends was demonstrated by kinetic modeling.

Isopropylcyclohexane pyrolysis at high pressure and temperature: Part 1- theoretical study

The thermal decomposition of isopropylcyclohexane (IPCH) has been investigated at the CBS-QB3 and CASSCF/MRCI levels of theory. The pyrolysis of IPCH follows a radical chain mechanism, which mainly includes the C–C bond scission, H-atom abstraction, dissociation of alkene and alkyl-cyclohexene, decomposition, and isomerization of alkyl-cyclohexane radicals, decomposition and isomerization of alkenyl radicals, and stepwise dehydrogenation of cyclic intermediates. The rate constants for all elementary reactions have been evaluated with conventional transition state theory (TST/VTST) in the temperature range of 800–2000 K. The energy barriers and rate constants have been compared with previous theoretical reports of other cyclohexanes (like cyclohexane, methylcyclohexane, and ethylcyclohexane) to see the effect of isopropyl group in the cyclohexane dissociation. The final products of IPCH thermal decomposition are methane (CH4), ethylene(C2H4), propylene (C3H6), 1,3-butadiene (1,3-C4H6), and 1,3-pentadiene (1,3-C5H8), benzene, cyclopentadiene etc. The main goal of this work has been to come up with a detailed description of the IPCH thermal decomposition using high-level quantum chemical methods and provide reliable kinetic information to help in understanding the results from shock tube investigations discussed in part 2 of this work.

Pyrolytic elimination of ethylene from ethoxyquinolines and ethoxyisoquinolines: a computational study

This work reports a thermo-kinetic study on unimolecular thermal decomposition of some ethoxyquinolines and ethoxyisoquinolines derivatives (1-ethoxyisoquinoline (1-EisoQ), 2-ethoxyquinoline (2-EQ), 3-ethoxyquinoline (3-EQ), 3-ethoxyisoquinoline (3-EisoQ), 4-ethoxyquinoline (4-EQ), 4-ethoxyisoquinoline (4-EisoQ), 5-ethoxyquinoline (5-EQ), 5-ethoxyisoquinoline (5-EisoQ), 8-ethoxyquinoline (8-EQ) and 8-ethoxyisoquinoline (8-EisoQ)) using density functional theory DFT (BMK, MPW1B95, M06-2X) and ab initio complete basis set-quadratic Becke3 (CBS-QB3) calculations. In the course of the decomposition of the investigated systems, ethylene is eliminated with the production of either keto or enol tautomer. The six-membered transition state structure encountered in the path of keto formation is much lower in energy than the four-membered transition state required to give enol form. Rate constants and activation energies for the decomposition of 1-EisoQ, 2-EQ, 3-EQ, 3-EisoQ, 4-EQ, 4-EisoQ, 5-EQ, 5-EisoQ, 8-EQ, and 8-EisoQ have been estimated at different temperatures and pressures using conventional transition state theory combined with Eckart tunneling and the unimolecular statistical Rice–Ramsperger–Kassel–Marcus theories. The tunneling correction is significant at temperatures up to 1000 K. Rate constants results reveal that ethylene elimination and keto production are favored kinetically and thermodynamically over the whole temperature range of 400–1200 K and the rates of the processes under study increase with the rising of pressure up to 1 atm.

The molecular level study of the fate of the CH3CH2C(O)OCH(O)CH3 radical derived from ethyl propionate

ABSTRACT A quantum chemical study has been presented on the decomposition of CH3CH2C(O)OCH(O • )CH3 which is derived from ethyl propionate (CH3CH2C(O)OCH2CH3) using the M06-2X/6-31 + G(d,p) level of theory. The thermal decomposition of CH3CH2C(O)OCH(O • )CH3 is essential to understand the chemistry of combustion of ethyl propionate. Here, the decomposition and oxidative pathways, bond fission and alpha ester rearrangement of CH3CH2C(O)OCH(O • )CH3 have been explored. We have obtained transition states for the different pathways and further verified each transition state by performing intrinsic reaction coordinate (IRC) calculations. The density of state spectra (DOS) was calculated using GaussSum software. The single-point energy calculations are performed at the same function but using larger basis sets such as 6-311++G(d,p) and 6-311++G(3df,2p). We have shown all stationary points and transition states involved in reaction pathways on the potential energy surface (PES) diagram and also discussed the thermochemistry of each reaction pathway. The thermal rate constants of various channels in the decomposition of CH3CH2C(O)OCH(O • )CH3 are determined using the Conventional Transition State Theory (CTST) in the temperature range of 250–450 K and 1atm. From PES, thermochemistry and kinetics analysis, we found that α-ester rearrangement produces propionic acid, which is dominant over the oxidative pathway for the decomposition of CH3CH2C(O)OCH(O • )CH3.

From electronic structure to model application of key reactions for gasoline/alcohol combustion: Hydrogen-atom abstractions by CH3Ȯ radical

H-atom abstraction by methoxy radical (CH3Ȯ) plays an important role in capturing the kinetics of reactions between gasoline components and alcohols. This study focuses on determining the reaction rates and thermodynamic properties of methoxy radical reactions with five gasoline fuel components: n-heptane, iso-octane, 1-hexene, cyclopentane and toluene. Electronic structure calculations were performed for all the stationary points with M06-2X/6−311++g(d,p) method. G3 composite method with atomization method is used for determining ΔfH0 of all the closed shell and radical species, using which the necessary thermodynamic data of all the species was determined. Coupled cluster theory QCISD(T)/cc-pVXZ (where X = D and T) and Møller–Plesset perturbation theory MP2/cc-pVXZ (where X = D, T and Q) were used to calculate single point energies. Subsequently, rate constants for all hydrogen atom abstraction channels have been performed using conventional transition state theory with unsymmetric tunneling corrections. A systematic comparison of rates for abstraction from different sites within the same species and same site from different species is done in order to get insights into this reaction class. The computed thermodynamic properties and rate constants were incorporated in to a recent gasoline mechanism to investigate the impact of the calculations performed in this work. A shift in predicted NTC (negative temperature coefficient) behavior and significant reduction in model reactivity is observed upon incorporating the rates calculated herein.

Radical–Radical Reactions in Molecular Weight Growth: The Phenyl + Propargyl Reaction

The mechanism for hydrocarbon ring growth in sooting environments is still the subject of considerable debate. The reaction of phenyl radical (C6H5) with propargyl radical (H2CCCH) provides an important prototype for radical-radical ring-growth pathways. We studied this reaction experimentally over the temperature range of 300-1000 K and pressure range of 4-10 Torr using time-resolved multiplexed photoionization mass spectrometry. We detect both the C9H8 and C9H7 + H product channels and report experimental isomer-resolved product branching fractions for the C9H8 product. We compare these experiments to theoretical kinetics predictions from a recently published study augmented by new calculations. These ab initio transition state theory-based master equation calculations employ high-quality potential energy surfaces, conventional transition state theory for the tight transition states, and direct CASPT2-based variable reaction coordinate transition state theory (VRC-TST) for the barrierless channels. At 300 K only the direct adducts from radical-radical addition are observed, with good agreement between experimental and theoretical branching fractions, supporting the VRC-TST calculations of the barrierless entrance channel. As the temperature is increased to 1000 K we observe two additional isomers, including indene, a two-ring polycyclic aromatic hydrocarbon, and a small amount of bimolecular products C9H7 + H. Our calculated branching fractions for the phenyl + propargyl reaction predict significantly less indene than observed experimentally. We present further calculations and experimental evidence that the most likely cause of this discrepancy is the contribution of H atom reactions, both H + indenyl (C9H7) recombination to indene and H-assisted isomerization that converts less stable C9H8 isomers into indene. Especially at low pressures typical of laboratory investigations, H-atom-assisted isomerization needs to be considered. Regardless, the experimental observation of indene demonstrates that the title reaction leads, either directly or indirectly, to the formation of the second ring in polycyclic aromatic hydrocarbons.