Small-curvature Tunneling Correction Research Articles

Kinetic Study on the H-Abstraction Reactions from 1-Nitropropane by H and OH Radicals.

To improve the comprehension of the combustion kinetics of 1-nitropropane (1-NP), the H-abstraction reactions from 1-NP by H and OH radicals are theoretically explored using the canonical variational transition-state theory combined with the multistructural torsional anharmonicity and small-curvature tunneling corrections (MS-CVT/SCT). The M08-HX/cc-pVTZ method is adopted for geometry optimizations and frequency calculations due to its effective performance in describing the current reaction systems with an average mean unsigned deviation of 0.95 kcal mol-1 against the benchmark by the high-level DLPNO-CCSD(T)/CBS(T-Q) method. The rate constants for the investigated reactions are calculated using the MS-CVT/SCT method at 200-2000 K, and a good agreement is achieved by comparing our calculations with the available literature data. The rate constant calculations show that the multistructural torsional anharmonicity, variational effect, and tunneling effect have different influences on the H-abstraction reactions 1-NP + H/OH, and the reaction channel at the Cβ position is the most important above the room temperature. With our calculations, a literature combustion kinetic model of 1-NP is revised, and the new model demonstrates improved performance across most conditions. Sensitivity analysis demonstrates that the H-abstraction reaction channel, 1-NP + OH = CH3CHCH2NO2 + H2O, alters the combustion kinetics of 1-NP and plays an important role in controlling its ignition process.

Theoretical Investigation on Water-Free, Water- and Self-Assisted H-Abstraction Reactions from Dimethylamine by Hydroxy Radicals.

Accurate branching ratios of the H-abstraction reactions from dimethylamine (DMA) by OH radicals are important in understanding the atmospheric fate of DMA. In this work, the reaction kinetics of the water-free, water-assisted, and self-assisted H-abstraction reactions between DMA and OH radicals are accurately determined using the multipath canonical variational theory with the small-curvature tunneling correction, to explore the catalytic effects of the reactant (DMA) and product (water). To choose a suitable method that well describes the current reaction systems, various combinations with seven DFT methods and six basis sets are first evaluated, and the M08-HX/ma-TZVP method is identified as the most appropriate, with a mean unsigned deviation of 0.9 kcal mol-1 against the gold-standard CCSD(T)/CBS(T-Q) method. Based on the determined potential energy surfaces with the considerations of ground-state structures and specific-reaction parameters of zero-point energies, rate constants and branching ratios are calculated in a wide temperature range. The calculations show that the participation of water and DMA can lead to three-body complexes with a lower energy and influence the energy barriers, but neither of them shows the catalytic effect on the H-abstraction reactions in terms of kinetics. Additionally, the branching ratio analysis demonstrates that the product distribution is significantly altered in the presence of DMA and water.

Predicting pressure-dependent rate constants for the furan + OH reactions and their impact under tropospheric conditions.

In this work, we have evaluated the influence of temperature and pressure on the mechanism of furan oxidation by the OH radical. The stationary points on the potential energy surface were described at the M06-2X/aug-cc-pVTZ level of theory. In the kinetic treatment at the high-pressure limit (HPL), we have combined the multistructural canonical variational theory with multidimensional small-curvature tunneling corrections and long-range transition state theory. The system-specific quantum Rice-Ramsperger-Kassel theory was employed to estimate the pressure-dependent rate. In the HPL, the OH addition on the α carbon is the dominant pathway in the mechanism, producing a product via the ring-opening process, also confirmed by the product branching ratio calculations. The overall rate constant, obtained by a kinetic Monte Carlo simulation, reads the form koverall=5.22×10-13T/3001.10⁡exp1247(K/T) and indicates that the furan oxidation by OH radicals is a pressure-independent reaction under tropospheric conditions.

Theoretical study on iso‐pentanol oxidation chemistry: Fuel radical isomerization and decomposition kinetics and mechanism development

AbstractThis study undertakes a detailed theoretical investigation into the iso‐pentanol radical isomerization and decomposition kinetics and the mechanism development of the iso‐pentanol oxidation. The CCSD(T)/CBS//M08‐HX/6‐311+G(2df,2p) method was adopted to calculate the reaction potential energy surface. The reaction rate coefficients were calculated by variational transition state theory (VTST) with multistructural torsional (MS‐T) partition function and small curvature tunneling (SCT) correction. Moreover, the pressure‐dependent rate coefficients were determined using the system‐specific quantum Rice‐Ramsperger‐Kassel theory (SS‐QRRK). The variational and tunneling effects were discussed, and the dominant reaction channels were identified. It reveals that the isomerization reactions play a significant role at low temperatures, while the decomposition reactions dominate the high‐temperature regime. Notably, the quantitative rate expressions for iso‐pentanol radical decomposition reactions were also obtained. Furthermore, a new kinetic model incorporating the calculated rate coefficients was constructed, exhibiting satisfactory prediction performance on ignition delay times and improved predictive accuracy of species mole fractions. This work provides accurate rate data of isomerization and decomposition kinetics and contributes to a more comprehensive understanding of the iso‐pentanol oxidation mechanism.

Reactions of a hydrogen atom with haloacetates in aqueous solutions: Computational evidence for proton-coupled electron transfer and competing mechanisms.

A computational study of the mechanisms and kinetics of the aqueous reactions of a hydrogen atom with haloacetates is presented. Several mechanisms in the close competition are observed, such as proton-coupled electron transfer (PCET), hydrogen atom transfer (HAT), and halogen abstraction (XA). Computations predict that dechlorination takes place via PCET mechanisms and not via XA, as stated earlier, while XA is the fastest mechanism for . The reaction rate constants are reasonably well predicted within the theoretically most reliable canonical variational transition state theory with small curvature tunneling corrections and compared with the experimental ones. To reproduce the experimental rate constants of the debromination process it is necessary to include the PCET and XA cumulative values. Small curvature tunneling corrections to the rate constants are the highest for HAT and PCET mechanisms, up to 70 times larger than the Wigner, while variational effects for XA mechanisms are very small.

Kinetics of the reaction CH2CO + O (3P): Are the CH2 and CO2 the most favorable products?

Overall thermal rate coefficients for the CH2CO + O (3P) ⟶ CH2 + CO2 reaction were estimated by combining the canonical variational theory with multidimensional small-curvature tunneling corrections (CVT/SCT) and multifaceted variable-reaction-coordinate transition state theory (VRC-TST) results. The energies, equilibrium geometries, and harmonic vibrational frequencies were calculated at M06-2X/aug-cc-pVTZ level of theory. Our thermal rate constants show excellent agreement with experimental data confirming the CH2 + CO2 as the most favorable channel. Fitted total thermal rate coefficients into a modified Arrhenius equation shows a temperature-dependent activation energy.

Advanced kinetic calculations with multi-path variational transition state theory for reactions between dimethylamine and nitrogen dioxide in atmospheric and combustion temperature ranges.

Rate constants of the reactions between dimethyl amine (DMA) and NO2 are accurately determined using the advanced multi-path canonical variational theory with a small-curvature tunneling correction. To select a suitable method for the direct kinetic calculations, various combinations with nine DFT methods and seven basis sets are evaluated, and the M08-HX/ma-TZVP method is identified as the best one for the current reaction system, with a mean unsigned deviation of 1.1 kcal mol-1 in comparison with the benchmark CCSD(T)-F12/jun-cc-pVTZ. A total of 13 elementary reactions are found, but only the H-abstraction reactions are kinetically favorable and included in the kinetic calculations. The recrossing and tunneling effects are different among the various H-abstraction reaction channels as well as different reaction paths. The recrossing effects are comparably more significant for reactions at the N-site, and tunneling coefficients of the reaction channels producing a trans-HONO are the largest. The higher-energy reaction paths have much higher tunneling coefficients, which should be considered in the rate constant calculation, especially at low temperatures. Branching ratio analysis demonstrates that the most important products are CH3NCH3 + cis-HONO at 200-2000 K. Additionally, by comparing our calculations with the available literature data, the importance of the investigated reactions is discussed at combustion and atmospheric temperatures.

Theoretical investigation on the reaction kinetics of NO2 with CH3OH and HCHO under combustion conditions

Methanol (CH3OH) and formaldehyde (HCHO) reacting with nitrogen dioxide (NO2) contribute to the largest uncertainty for the CH3OH/NOx low temperature combustion mechanism. CH3OH and NO2 only undergo H-abstraction reactions, while HCHO + NO2 involves multiple reaction channels, among which H-abstraction dominates. In the present work, a high level quantum chemical method, CCSD(T)/aug-cc-pVQZ//M06–2X-D3/6-311++G(d,p), was employed to investigate the reaction pathways. The reaction kinetics were explored by RRKM/master equation simulations with multidimensional small-curvature tunneling (SCT) corrections and hindered rotor approximations. The H-abstraction reactions with barriers higher than 20 kcal/mol indicate a nonnegligible quantum tunneling effect even under combustion conditions. Our computations predict the tunneling factors to be 3–4 for the studied reactions at 500 K. A significant tunneling effect is also expected for H-abstraction of large alcohols and aldehydes by NO2. The computed total rate coefficients show good agreement with previous experimental measurements over narrow ranges of temperature and pressure, ensuring the accuracy of the reported branching ratios covering a wide T, P range for the two reactions. The results of CH3OH + NO2 reveal the dominant role of HONOcis + CH2OH. It's also uncovered the dominance of HONOcis + CHO pathway in HCHO + NO2 under the studied conditions. The detailed reaction kinetics information reported in this work is useful for building rate rules for the mechanisms of other nitrogen-containing alcohol-based fuels.

Accurate rate constants for elementary reactions of molecular hydrogen and carbon monoxide mixtures and the role of the H2 rich environment.

This investigation provides accurate rate constant values for a set of elementary reactions relevant to mixtures between molecular hydrogen (H2) and carbon monoxide (CO) such as syngas. We considered intermediates and products including formaldehyde (H2CO), hydroxymethylene (c-HCOH and t-HCOH) and methanol (CH3OH). The calculations were performed employing the improved canonical variational transition state theory with small-curvature tunneling corrections based on high-level electronic structure results. This study demonstrates for the first time that H2 can act as an effective catalyst to the reaction from t-HCOH to H2CO. In this case, the adiabatic barrier height for the reaction decreases from 30.6 kcal⋅mol- 1 to 18.1 kcal⋅mol- 1 in the presence of H2. The results obtained here can improve the comprehension regarding processes such as the combustion of hydrogen-rich syngas.

Diethyl ether oxidation: Revisiting the kinetics of the intramolecular hydrogen abstraction reactions of its primary and secondary peroxy radicals

Diethyl ether is an attractive alternative to diesel fuel for its high autoignition propensity, high energy density, and potential to reduce pollutant emissions. Low-temperature oxidation kinetics play an important role governing fuel autoignition in engines. Peroxy radical intramolecular hydrogen atom abstraction reactions are key isomerization steps in the chain branching reaction sequence at low temperatures. In this study, the kinetics of intramolecular hydrogen atom abstraction reactions of the primary and secondary peroxy radicals were revisited using the multistructural torsional variational transition state theory with small curvature tunneling corrections. High-pressure limit rate constants within a broad temperature range were obtained, which differ from those previously reported for the investigated reactive systems due to the differences in the estimated barrier heights, the role played by multistructural torsional anharmonicity, and the effects of tunneling. We also observed that some higher energy conformers of the saddle point species, some of which with diastereomers, play a surprisingly large role. Similarly, tunneling is pronounced at low temperature combustion conditions, claiming for robust methods to estimate this effect. Different kinetic models for diethyl ether were used to test our calculated rate constants and NASA polynomials for both diethyl ether peroxy radicals, and the results showed an overall slightly better performance in the prediction of ignition delay times at high pressures.

Mechanistic studies of the reactions of nitrogen dioxide with dimethyl ether and methyl ethyl ether

Ab initio and direct kinetics study of the CH3OCH3 + NO2 and CH3OC2H5 + NO2 reactions was performed over the temperature range of 300–1500 K. The geometries of all stationary points were optimized at the M06-2X/cc-pVTZ level of theory and the electronic energies were refined by the CCSD(T)/CBS method. The rate coefficients calculated by CVT theory including the small curvature tunneling (SCT) correction agreed well with the experimental data. Note that the H-abstraction from ethers to produce cis-HONO was the dominant channel at temperatures lower than 1000 K and the three channels to produce HNO2, trans-HONO and cis-HONO proceeded simultaneously.

On-The-Fly Kinetics of the Hydrogen Abstraction by Hydroperoxyl Radical: An Application of the Reaction Class Transition State Theory.

A Reaction Class Transition State Theory (RC-TST) is applied to calculate thermal rate constants for hydrogen abstraction by OOH radical from alkanes in the temperature range of 300–2500 K. The rate constants for the reference reaction C2H6 + ∙OOH → ∙C2H5 + H2O2, is obtained with the Canonical Variational Transition State Theory (CVT) augmented with the Small Curvature Tunneling (SCT) correction. The necessary parameters were obtained from M06-2X/aug-cc-pVTZ data for a training set of 24 reactions. Depending on the approximation employed, only the reaction energy or no additional parameters are needed to predict the RC-TST rates for other class representatives. Although each of the reactions can in principle be investigated at higher levels of theory, the approach provides a nearly equally reliable rate constant at a fraction of the cost needed for larger and higher level calculations. The systematic error is smaller than 50% in comparison with high level computations. Satisfactory agreement with literature data, augmented by the lack of necessity of tedious and time consuming transition state calculations, facilitated the seamless application of the proposed methodology to the Automated Reaction Mechanism Generators (ARMGs) programs.

Theoretical studies of the reactions of 1,2-Ethanediol with triplet oxygen and hydrogen atoms

An direct kinetics study of the (CH2OH)2 + O(3P) and (CH2OH)2 + H reactions was performed over the temperature range of 300–1500 K. The geometries of all stationary points were optimised at the M06-2X/cc-pVQZ level of theory. The electronic energies were refined by CCSD(T)/CBS and QCISD(T)/cc-pVTZ methods. The rate coefficients were calculated using canonical variational transition state theory (CVT) with small-curvature tunneling (SCT) correction. The results showed that the channels of hydrogen abstraction from the CH2 group were most kinetically favourable at the temperature less than 1300 K and have positive temperature dependence. The OH-transfer channels were ignorable.

Theoretical study of the reactions of triplet oxygen atom with 1-propanol and 2-propanol

The reactions of triplet oxygen atom (O(3P)) with 1-Propanol and 2--Propanol were characterized at CCSD(T)/CBS//M06-2X/cc-pVTZ level of theory. The rate coefficients were calculated by the canonical variational transition state theory with the small curvature tunneling correction. The calculated total rate coefficients agreed with the available reference values. The dominant channel in the 1-C3H7OH + O(3P) reaction is hydrogen-abstraction from the α-CH2 group at temperature less 450 K or hydrogen-abstraction from β-CH2 groups at temperatures higher than 500 K; the dominant channel in the 2-C3H7OH + O(3P) reaction is hydrogen-abstraction from the CH group at temperature less than 1200 K.

Kinetics of the Reactions of Ozone with Halogen Atoms in the Stratosphere

It is well established that reaction cycles involving inorganic halogens contribute to the depletion of ozone in the atmosphere. Here, the kinetics of O3 with halogen atoms (Cl, Br, and I) were investigated between 180 and 400 K, expanding the temperature range relative to prior studies. Canonical variational transition state theory including small curvature tunneling correction (CVT/SCT) were considered, following the construction of the potential energy surfaces. MRCI + Q/aug-ano-pVTZ//MP2/aug-cc-pV(T + d)Z and MRCI + Q/aug-ano-RCC-VTZP//MP2/aug-cc-pV(T + d)Z levels of theory were used to calculate the kinetic parameters. Calculated rate coefficients were used to fit the Arrhenius equations, which are obtained to be k1 = (3.48 ± 0.4) × 10−11 exp[(−301 ± 64)/T] cm3 molecule−1 s−1, k2 = (3.54 ± 0.2) × 10−11 exp[(−990 ± 35)/T] cm3 molecule−1 s−1 and k3 = (1.47 ± 0.1) × 10−11 exp[(−720 ± 42)/T] cm3 molecule−1 s−1 for the reactions of O3 with Cl, Br, and I atoms, respectively. The obtained rate coefficients for the reactions of O3 with halogen atoms using CVT/SCT are compared to the latest recommended rate coefficients by the NASA/JPL and IUPAC evaluations. The reactivity trends and pathways of these reactions are discussed.

Kinetics study on the reactions of dimethyl ether with triplet oxygen and hydrogen atoms

The reaction kinetics of dimethyl ether (CH3OCH3) with O(3P) and hydrogen atom have been studied. Electronic energies were calculated by single-point energy calculations at the CCSD(T)/cc-PVnZ (n = D, T, Q) levels. Rate coefficients of the reactions were calculated by the canonical variational transition state theory (CVT) with the small curvature tunneling (SCT) correction. The hydrogen-abstraction channels involve the formation of reactant complexes in the entrance channels and show positive temperature dependence. Their branching rates are almost 1.0 at the temperature less than 1000 K.

Atmospheric fate of formyl chloride and mechanisms of the gas-phase reactions with OH radicals and Cl atoms

Formyl chloride (HCOCl) is a major product in the atmospheric degradation of several chlorinated hydrocarbons, but its atmospheric fate is not well known. Here, the kinetics of HCOCl with OH radicals and Cl atoms were studied over the temperature range of 180 to 400 K using Canonical Variational Transition state theory including Small Curvature Tunneling correction (CVT/SCT). The obtained temperature-dependent rate coefficients for the reactions of HCOCl with OH and Cl are (1.86 ± 0.7) × 10−11 exp[(−1217 ± 19)/T] cm3molecule−1s−1 and (9.87 ± 0.6) × 10−12 exp[(−747 ± 33)/T] cm3molecule−1s−1, respectively. Reaction mechanisms and thermodynamic parameters for the title reactions were studied, and the atmospheric lifetime of HCOCl was determined.

Theoretical study of the reactions of hydrogen atom with methyl and ethyl hydroperoxides

An ab initio dynamic study of the reactions of H atom with CH3OOH and CH3CH2OOH was performed over the temperature range of 250–1500 K. The geometries of all stationary points were optimised at the MP2, B3LYP and M06-2X theories. The final electronic energies for all species were refined by the CCSD(T) and QCISD(T) methods. The rate coefficients were calculated using canonical variational transition state theory (CVT) with small-curvature tunnelling (SCT) correction or the multichannel Rice−Ramsperger−Kassel−Marcus (RRKM) and CVT methods. The results showed that the channel of H-abstraction from OH group is dominant channel only at temperature less than 350 K. With the temperature higher than 350 K, the OH-transfer channel becomes dominant channel. For the CH3OOH + H reaction, the channel to produce H2 + CH2O + OH becomes the dominant channel at temperature higher than 800 K. For the CH3CH2OOH + H reaction, the channel to produce H2 + CH3CH2O + OH is important at temperature less than 600 K and has a maximum branching ratio of 0.3 around 350 K.

Theoretical study of the reactions of nitrogen dioxide with hydrogen and methyl peroxides

Four channels for HOOH + NO2 system and eight channels for CH3OOH + NO2 system were explored by the MP2 level of theory. Energies for all species were evaluated at the CCSD(T) theory. The rate coefficients calculated using CVT theory including the small curvature tunneling (SCT) correction agreed well with the available value. The results showed that the three channels to produce HNO2, trans-HONO and cis-HONO proceeded simultaneously for the same H-abstraction from HOOH or CH3OOH by NO2, and the H-abstraction from hydroperoxyl to produce cis-HONO was the dominant channel at the temperature less than 1000 K.

Quantum chemical insights into the atmospheric reactions of CH2FCH2OH with OH radical, fate of CH2FC•HOH radical and ozone formation potential

Quantum chemical calculations on the gas-phase H-abstraction reactions of CH2FCH2OH with OH radical were performed using the CCSD(T)//M06-2X/Aug-cc-pVTZ method. The rate coefficients were evaluated using the VTST−ISPE method incorporating small curvature tunnelling (SCT) correction. The orientation of F and –OH groups plays a major role in the reactivity of the CH2FCH2OH compound. The calculated rate coefficient k(CH2FCH2OH + OH) = 6.73 × 10−13 cm3 molecule−1 s−1 at 298K is in close agreement with the reported experimental result. Atmospheric lifetime and Global Warming Potential at a 100-year time horizon of CH2FCH2OH are reported. Atmospheric loss processes of CH2FC•HOH radical through several competing reaction channels are discussed. Rate coefficient values of the primary oxidation products: CH2FCOOH + OH = 5.63 × 10−14 and CH2FCHO + OH = 2.49 × 10−12 cm3 molecule−1 s−1 were reported for the first time. Regeneration of hydroxyl radical (OH) from the degradation reaction of CH2FC•HOH along with the ozone formation potential in the troposphere are deliberated.