Dual-level Direct Dynamics Research Articles

Mechanisms and kinetics of hydrogen abstraction of methylhydrazine and deuterated methylhydrazine with H/D atoms

The mechanisms of multi-channel reactions (1) CH3NHNH2 + H → products (R1) and (2) (CH3)2NNH2 + H → products (R2) have been investigated, and the dynamic properties are studied by using the dual-level direct dynamics method. The electronic structures and energies of the stationary points are calculated by the combination of various methods and basis sets. The rate constants of reactions are evaluated in a temperature range of 200–2,000 K by using the canonical variational transition state theory with a small-curvature tunneling correction at the MCG3-MPWPW91//MPW1K/6-311G(d,p) level. The effect of treating torsional anharmonicity with different models on the rate constants is compared. Theoretical and experimental studies indicate that substitution of H by –CH3 group on NH2NH2 leads to a significant rate coefficients increase. The kinetic isotope effects are studied for both the methylhydrazine with hydrogen/deuterium and the methylhydrazine/deuterated methylhydrazine with hydrogen atom.

Theoretical studies on mechanism and kinetics of the hydrogen-abstraction reaction of CF3CH2CH2OH with OH radical

The hydrogen abstraction reaction of CF3CH2CH2OH+OH has been studied theoretically by dual-level direct dynamics method. The required potential energy surface information for the kinetic calculation was obtained at the MCG3-MPWB//M06-2X/aug-cc-pVDZ level. Five stable conformers of CF3CH2CH2OH have been located. For each conformer, there are three potential H-abstraction sites (Cα, Cβ and –OH), and some of the H atoms can be abstracted by more than one abstraction channel due to the different attack orientations of the incoming OH radical. As a result, 31 distinct Habstraction channels have been identified for the reaction. The individual rate constants for each Habstraction channel were calculated by the improved canonical transition-state theory with smallcurvature tunneling correction (ICVT/SCT), and the overall rate constant was evaluated by the Boltzmann distribution function. It is shown that the calculated rate constant is in good agreement with the available experimental data at 298K, and exhibits negative temperature dependence with 200–350K. H-abstraction from the α site dominates the reaction at low temperatures, while the contributions from the β and OH abstractions should be taken into account as temperature increases. The fitted four-parameter expressions within 200–1000K for the overall rate constants as well as the rate constants from the α, β and OH abstractions were given to provide good estimation for future laboratory investigations. In addition, because of the lack of available experimental data for the product radicals involved in the reactions, their enthalpies of the formation (ΔHf,298°) were predicted via isodesmic reaction at the MCG3-MPWB//M06-2X/aug-cc-pVDZ level.

Thermochemistry and kinetics of the [formula omitted] reaction

Thermochemical and kinetics properties of the hydrogen abstraction and addition processes of the trans-N2H2+N reaction were computed using high-level ab initio and DFT approximation methods with aug-cc-pVXZ (X=T,Q) basis set. The CCSD (T)/CBS//BB1K/aug-cc-pVTZ results for classical barrier height are 13.1 and 15.0kcal/mol for the abstraction and addition reactions, respectively. The thermal rate constants were calculated using the dual-level direct dynamics by variational transition state theory with the BB1K potential energy surface and thermochemical properties corrected with the CCSD (T)/CBS//BB1K/aug-cc-pVTZ results. The rate constants calculated show that the variational and tunneling effects play a relevant role only for the abstraction reaction.

Mechanistic study and kinetic properties of the CF3CHO + Cl reaction

Theoretical investigations have been carried out on the mechanism and kinetics for the reaction of CF3CHO + Cl using duallevel direct dynamics method. The potential energy surface information was obtained at the MC-QCISD/3//MP2/cc-pVDZ level and the kinetic calculations were done using variational transition state theory with interpolated single-point energy (VTST-ISPE) approach. The calculated results show that the reaction proceeds primarily via the H-abstraction channel, while the Cl-addition channel is unfavorable due to the higher barriers. The improved canonical variational transition-state theory (ICVT) with the small-curvature tunneling correction (SCT) was used to calculate the rate constants. The theoretical rate constants at room temperature are in general agreement with the experimental values. A three-parameter rate constant expression was fitted over a wide temperature range of 200–2000 K.

Dual-Level Direct Dynamics Studies on the Hydrogen Abstraction Reactions of CH2CH3 - n Xn+HBr (X=Cl, Br and n=1, 2)

The reactions of the HBr molecule with CH2CH2Cl (reaction R1), CH2CHCl2 (R2), CH2CH2Br (R3) and CH2CHBr2 (R4) are investigated by a dual-level direct dynamics method. The optimized geometries and frequencies of the stationary points were calculated at the MPW1K/6–311+G(d,p) and BMK/6–311+G(d,p) levels. To refine the reaction enthalpy and energy barrier height of each reaction, single-point energy calculations were carried out by the G2M(RCC5) method based on the geometries optimized at the above-mentioned two levels. Using the canonical variational transition state theory or the canonical variational transition state theory with the small-curvature tunneling correction, the rate constants of HBr with CH2CH2Cl (R1), CH2CHCl2 (R2), CH2CH2Br (R3), and CH2CHBr2 (R4) were calculated over a wide temperature range of 200–2000 K at the G2M(RCC5)//MPW1K/6–311+G(d,p) level. The effect of chlorine or bromine substitution on the ethyl radical reactivity is discussed. Finally, the total rate constants are fitted by two models, i.e. three-parameter and four-parameter expressions.

Dual-level direct dynamics study on the hydrogen abstraction reaction of fluorine atom with 1,1-difluoro-1-chloroethane

The kinetic properties of the reaction of F atoms with CH2H′CF2Cl are investigated by a dual-level direct dynamics method. Optimized geometries and frequencies of all the stationary points and extra points along the minimum-energy path (MEP) are obtained at the MPW1K/6–311+G(d,p) level of theory. Two complexes with energy less than that of the reactants are located in the two reactant paths, respectively. The energy profiles of two reactions are refined with the interpolated single-point energies (ISPE) method at the G3(MP2)/MPW1K level. The rate constants are evaluated using the canonical variational transition state theory (CVT) with a small-curvature tunneling correction (SCT) over a wide range of temperature 200–2000 K. Agreement between the calculated CVT/SCT rate constant and the experimental value is good at 295 K. Our calculations show that the reaction path CH2H′CF2Cl + F → CH2CF2Cl + H′F (Ra) is the major reaction path below 400 K. Moreover, the contribution of CH2H′CF2Cl + F → CHH′CF2Cl + HF (Rb) to the whole reaction increases with the temperature increasing and exceeds path Ra to be the major reaction path.

MECHANISM AND KINETICS OF THE CH3CH2C(O)OCH2CH3 + OH REACTION: A THEORETICAL STUDY

The hydrogen abstraction reaction of CH 3 CH 2 C(O)OCH 2 CH 3 + OH has been studied theoretically by dual-level direct dynamics method. Six H-abstraction channels were found for this reaction. The required potential energy surface information for the kinetic calculations was obtained at the MCG3-MPWB//M06-2X/aug-cc-pVDZ level. The rate constants were calculated by the improved canonical variational transition-state theory with small-curvature tunneling correction (ICVT/SCT) approach in the temperature range of 200–2000 K. It is shown that the "methylene H-abstraction" from the alkoxy end of the ester CH 3 CH 2 C(O)OCH 2 CH 3 is the dominant channel at lower temperature (< 400 K), while the other channels from the acetyl end should be taken into account as the temperature increases and become the competitive ones at higher temperature. The calculated global rate constants are in good agreement with the experimental ones in the measured temperature range and exhibit a negative temperature dependence below 500 K. A four-parameter rate constant expression was fitted from the calculated kinetic data between 200–2000 K.

Hydrogen abstraction reactions of OH radicals with CH3CH2CH2Cl and CH3CHClCH3: A mechanistic and kinetic study

The hydrogen abstraction reactions of OH radicals with CH₃CH₂CH₂Cl and CH₃CHClCH₃ (R2) have been investigated theoretically by a dual-level direct dynamics method. The optimized geometries and frequencies of the stationary points are calculated at the B3LYP/6-311G(d,p) level. To improve the reaction enthalpy and potential barrier of each reaction channel, the single point energy calculation is performed by the BMC-CCSD method. Using canonical variational transition-state theory (CVT) with the small-curvature tunneling correction, the rate constants are evaluated over a wide temperature range of 200-2000 K at the BMC-CCSD//B3LYP/6-311G(d,p) level. For the reaction channels with the negative barrier heights, the rate constants are calculated by using the CVT. The calculated total rate constants are consistent with available experimental data. The results show that at lower temperatures, the tunneling correction has an important contribution in the calculation of rate constants for all the reaction channels with the positive barrier heights, while the variational effect is found negligible for some reaction channels. For reactions OH radicals with CH₃CH₂CH₂Cl (R1) and CH₃CHClCH₃ (R2), the channels of H-abstraction from -CH₂ - and -CHCl groups are the major reaction channels, respectively, at lower temperatures. With temperature increasing, contributions from other channels should be taken into account. Finally, the total rate constants are fitted by two models, i.e., three-parameter and four-parameter expressions. The enthalpies of formation of the species CH₃CHClCH₂, CH₃CHCH₂Cl, and CH₃CH₂CH₂Cl are evaluated by isodesmic reactions.

Theoretical Studies on Mechanism and Kinetics of the Hydrogen-Abstraction Reaction of CF3CH2CHO with OH Radicals

The hydrogen abstraction reaction of + OH has been studied theoretically by dual-level direct dynamics method. Two stable conformers, trans- and cis-, have been located, and there are four distinct OH hydrogen-abstraction channels from t- and two channels from c-. The required potential energy surface information for the kinetic calculation was obtained at the MCG3-MPWB//M06-2X/aug-cc-pVDZ level. The rate constants, which were calculated using improved canonical transitionstate theory with small-curvature tunneling correction (ICVT/SCT) were fitted by a four-parameter Arrhenius equation. It is shown that the reaction proceeds predominantly via the H-abstraction from the -CHO group over the temperature range 200-2000 K. The calculated rate constants were in good agreement with the experimental data between 263 and 358 K.

Direct dynamics studies for the reactions of CF3CHFCF3 and CF3CF2CHF2 with H atoms

The rate constants of the hydrogen abstraction reactions of CF3CHFCF3+H (R1) and CF3CF2CHF2+H (R2) have been calculated by means of the dual-level direct dynamics method. Optimized geometries and frequencies of stationary points and extra points along the minimum-energy path (MEP) are obtained at the MPW1K/6-311+G(d,p) level, and the classical energetic information is further corrected with the interpolated single-point energy (ISPE) approach by the G3(MP2) level of theory. Using the canonical variational transition state theory (CVT) with small-curvature tunneling corrections (SCT), the rate constants are evaluated over a wide temperature range of 200–2000K. The calculated CVT/SCT rate constants are in good agreement with available experimental values. It is found that the variational effect is very small and almost negligible over the whole temperature region. However, the small-curvature tunneling correction plays an important role in the lower temperature range. Furthermore, the heats of formation of species CF3CF2CHF2 (SC1 or SC2) and CF3CF2CF2 are studied using isodesmic reactions to further elucidate the thermodynamic properties.

Hydrogen abstraction reactions of OH radicals with CF2ClCClXH (X = F, Cl) and CFCl2CClXH (X = F, Cl): a mechanistic and kinetic study

The reactions of OH (OD) radicals with CF2ClCClFH (R1), CF2ClCCl2H (R2), CFCl2CClFH (R3), and CFCl2CCl2H (R4) have been investigated theoretically by a dual-level direct dynamics method. The optimized geometries and frequencies of the stationary points are calculated at the MPW1K/6-311+G(d,p) level. To improve the reaction enthalpy and potential barrier of each reaction channel, the single-point energy calculation is made by the MC-QCISD method. The enthalpies of formation of the species CF2ClCClFH, CF2ClCCl2H, CFCl2CClFH, CFCl2CCl2H, CF2ClCClF, CF2ClCCl2, CFCl2CClF, and CFCl2CCl2 are evaluated by two sets of isodesmic reactions. Using canonical variational transition state theory (CVT) with the small-curvature tunneling correction (SCT) method, the rate constants of OH and OD radicals with CF2ClCClXH (X = F, Cl) and CFCl2CClXH (X = F, Cl) are evaluated over a wide temperature range of 100–2,000 K at the MC-QCISD//MPW1K/6-311+G(d,p) level. The calculated CVT/SCT rate constants are consistent with available experimental data. The results show that the tunneling correction has an important contribution in the calculation of rate constants at lower temperatures. For the above-mentioned four reactions, the kinetic isotope effects are also calculated. Finally, the effect of fluorine or chlorine substitution on reactivity of the C–H bond is discussed.

On the kinetic mechanism of the hydrogen and oxygen abstraction reactions of CH3S with HOO: A dual-level direct dynamics study

The hydrogen and oxygen abstraction mechanism for the radical–radical reaction of CH3S with HOO has been investigated at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-311+G(d,p) level of theory on both the singlet and triplet potential energy profiles. Four hydrogen abstraction channels and one oxygen abstraction process are found for the title reaction. The calculations indicate that the triplet potential energy profile is much lower than the singlet energetically. Two hydrogen abstraction channels Rc, Rd and one oxygen abstraction process Re are identified on the triplet potential energy profile. Channel Rc, formation of CH3SH+3O2, is the most favorable. Furthermore, the rate constants of channel Rc in a temperature range 200–800K are calculated by means of the canonical variational transition state theory with small-curvature tunneling correction. The fitted three-parameter expression for kCVT/SCT of the major pathway is kCVT/SCT=2.80×10−39T7.49exp(6071/T)cm3molecule−1s−1 and it shows a negative dependence in the calculated temperature range.

Dual-level direct dynamics studies on the hydrogen abstraction reactions of fluorine atoms with CF3CH2X(X=F, Cl)

The kinetic properties of the hydrogen abstraction reactions of CF3CH2F + F → CF3CHF + HF (R1) and CF3CH2Cl + F → CF3CHCl + HF (R2) have been studied by dual-level direct dynamics method. Optimized geometries and frequencies of all the stationary points and extra points along the minimum-energy path (MEP) were obtained at the B3LYP/6-311 + G(2d,2p) level. Two complexes with energies less than that of the reactants were located in the reactant side of each reaction. The energy profiles were further refined with the interpolated single-point energies (ISPE) method at the G3(MP2) level of theory. Using canonical variational transition state theory (CVT) with the small-curvature tunneling correction (SCT) method, the rate constants were evaluated over a wide temperature range of 200–2,000 K. Our calculations have shown that C–H bond activity decreases when one hydrogen atom of CF3CH3 is substituted by a fluorine atom, than when substituted with a chlorine atom. This is in good agreement with the experimental results.

Theoretical study and rate constants calculation of hydrogen abstraction reactions CF 3CHCl 2 + F and CF 3CHClF + F

The hydrogen abstraction reactions of CF 3CHCl 2 + F (R1) and CF 3CHClF + F (R2) are investigated by dual-level direct dynamics method. The optimized geometries and frequencies of the stationary points are calculated at the B3LYP/6-311G(d,p) and MP2/6-311G(d,p) levels. Higher-level energies are obtained at the G3(MP2) method using the B3LYP and MP2-optimized geometries, respectively. Complexes with energies lower than those of the reactants are located at the entrance of these two reactions at the B3LYP level, respectively. Using the variational transition-state theory (VTST) with the inclusion of the small-curvature tunneling correction, the rate constants are calculated over a wide temperature range of 200–2000 K. The agreement between theoretical and experimental rate constants is good. In addition, the effect of fluorine substitution on reactivity of the C–H bond is discussed. Our calculations show that the fluorine substitution deactivates the C–H bond reactivity.

DIRECT AB INITIO DYNAMICS STUDIES ON THE REACTION OF METHANETHIOL WITH CHLORINE ATOMS

The reaction of CH3SH with Cl atoms has been theoretically studied using dual-level direct dynamics method. Three reaction channels, two H-abstraction (from the –SH and –CH3 groups) and one substitution channel, have been found. The optimized geometries and frequencies of all the stationary points are calculated at the MP2/6-311+G(d,p) level, and the potential-energy profile along each reaction path is refined by the single-point calculation at the CCSD(T)/6-311+G(d,p) level. The rate constants for the two H-abstraction channels are evaluated by the improved canonical variational transition-state theory (ICVT) with the small-curvature tunneling (SCT) correction over a wide temperature range of 193–1000 K. The calculated ICVT/SCT rate constants are in good agreement with available experimental values and exhibit negative temperature dependence below 500 K. The present calculations indicate that the H-abstraction from the –SH group is the major channel for the title reaction, while the H-abstraction from the –CH3 group should be taken into account only at high temperatures.

Reaction of Cl with CF 3CH 2OCHO: A mechanistic and kinetic study

The mechanism and kinetics of the reaction CF 3CH 2OCHO + Cl are investigated theoretically by using a dual-level direct dynamics method. The geometries and frequencies are calculated at the B3LYP/6-311G(d,p), B3LYP/6-311+G(d,p), and MP2/6-31G(d,p) levels, and high-level single-point energy calculations are performed at the BMC-CCSD level. It is shown that the reaction proceeds exclusively via two hydrogen abstraction channels, while the Cl addition–elimination channel is unfavorable. Furthermore, the rate constants of the two H-abstraction channels are evaluated by means of canonical variational transition-state theory (CVT) with the small-curvature tunneling (SCT) correction over a temperature range of 200–1000 K. The overall rate constant at room temperature is in good agreement with the experimental value. This study show that the formyl-H-abstraction channel is the major reaction pathway at low temperatures, while as the temperature increases, methylene-H-abstraction channel becomes more important and competes with the former. The three parameter rate-temperature expression is fitted to be k = 0.17 × 10 −20 T 3.13exp(−23.9/ T) cm 3 molecule −1 s −1 (200–1000 K). The present study may assist in further laboratory work.

Reactions of chlorine atoms and hydroxyl radicals with trichloroethanol: a mechanistic and kinetic study

The multi-channel reactions (1) CCl3CH2OH+Cl→ products and (2) CCl3CH2OH+OH→ products have been investigated by using the dual-level direct dynamics method. Two reaction channels, i.e., methylene- and hydroxyl-hydrogen abstraction, are identified for each reaction. The optimized geometries and frequencies of the stationary points are calculated at the B3LYP/6-311G(d,p) and MP2/6-311G(d,p) levels. Higher-level energies are obtained at the MC-QCISD and G3(MP2) levels based on the B3LYP and MP2 geometries, respectively, as well as by the CCSD(T)/6-31G(d)+CF method using the B3LYP geometries. Complexes with energies lower than those of the reactants are located at the entrance of each reaction channel. The rate constants for each reaction channel are evaluated by using the canonical variational transition state theory (CVT) incorporating the small-curvature tunneling (SCT) correction in a temperature range of 200–2000 K at the MC-QCISD//B3LYP/6-311G(d,p) level. The agreement of the calculated rate constants and experimental values for two reactions is seen to be remarkably good. Theoretical results indicate that in a low temperature range, the branching ratio to the hydroxyl-H-abstraction channel for both reactions is found negligible. The reactions proceed practically via methylene-H-abstraction yielding the products of CCl3CHOH+HCl and CCl3CHOH+H2O, respectively; while for reaction of CCl3CH2OH+Cl, hydroxyl-H-abstraction channel appears to be probable with the increase of temperature. The enthalpies of formation for the CCl3CH2OH, CCl3CHOH, and CCl3CH2O species are evaluated via isodesmic reactions at several levels.

Dual-level direct dynamics studies on the reactions of tetramethylsilane with chlorine and bromine atoms

The multiple-channel reactions Cl + Si(CH3)4 and Br + Si(CH3)4 are investigated by direct dynamics method. The minimum energy path is calculated at the MP2/6-31+G(d,p) level, and energetic information is further refined by the MC-QCISD (single-point) method. The rate constants for individual reaction channel are calculated by the improved canonical variational transition state theory with small-curvature tunneling correction over the temperature range 200–3,000 K. The theoretical three-parameter expression k1(T) = 9.97 × 10−13T0.54exp(613.22/T) and k2(T) = 1.16 × 10−17T2.30exp(−3525.88/T) (in unit of cm3 molecule−1 s−1) are given. Our calculations indicate that hydrogen abstraction channel is the major channel due to the smaller barrier height among feasible channels considered.

Theoretical investigation of the hydrogen abstraction from CF 3CH 2CF 3 by OH radicals, F, and Cl atoms: A dual-level direct dynamics study

The hydrogen abstraction reactions of the CF 3CH 2CF 3 with the OH radicals (R1), F (R2), and Cl (R3) atoms have been studied theoretically over a wide temperature range 200–1000 K. In this study, the recently developed hybrid density functional theory BB1K is used to obtain reaction path information including the geometries and frequencies of the stationary points. To refine the energies, single-point energy calculations were performed at the BMC-CCSD level using the BB1K geometries. The rate constants are carried out by the canonical variational transition state theory (CVT) with the small-curvature tunneling correction method (SCT). It is found that the activation energies for the title reactions are on the order of R3 > R1 > R2 and the rate constants exhibit just the opposite order, i.e., k 2 > k 1 > k 3. The calculated CVT/SCT rate constants are compared with the available experimental values in the temperature region 269–413 K. The temperature dependence of the rate constants can be expressed by three-parameter Arrhenius expressions (cm 3 mol −1 s −1): k 1 = 1.18 × 10 −19 T 2.51 exp(−1761.16/ T), k 2 = 2.47 × 10 −15 T 1.36 exp(−689.34/ T), and k 3 = 4.24 × 10 −21 T 3.22 exp(−2819.55/ T) cm 3 mol −1 s −1, respectively. Furthermore, in order to further reveal the thermodynamics properties, the enthalpies of formation of CF 3CH 2CF 3 and CF 3CHCF 3 are studied using isodesmic reactions.

Direct Dynamics Study of Hydrogen-Transfer Isomerization of 1-Pentyl and 1-Hexyl Radicals

The rate constants of three intramolecular hydrogen-transfer isomerization reactions, namely, 1-4 isomerization of the 1-pentyl radical and 1-4 and 1-5 isomerizations of the 1-hexyl radical, are calculated using variational transition state theory with multidimensional tunneling, in particular by using canonical variational theory (CVT, which is the version of variational transition state theory in which the transition state dividing surface is optimized for a canonical ensemble) with small-curvature tunneling (SCT) for the transmission coefficient. The required potential energy surfaces were obtained implicitly by direct dynamics employing interpolated variational transition state theory with mapping (IVTST-M) and variational transition state theory with interpolated single-point energies (VTST-ISPE). Single-level direct dynamics calculations were performed for all of the reactions by IVTST-M using M06-2X/MG3S or M08-HX/cc-pVTZ+ potential energy surfaces or both. The stationary points of 1-4 isomerization of 1-pentyl and the stationary points for the forward reactions of 1-4 and 1-5 isomerizations of 1-hexyl were also optimized by BMC-CCSD, and for all three reactions we also performed dual-level direct dynamics calculations using VTST-ISPE in which MCG3-MPW single-point energies served as the higher level. The calculated MCG3-MPW//M06-2X/MG3S rate constants agree well with experimental values for 1-4 isomerization of the 1-pentyl radical at high temperature, and this validates the accuracy of this theoretical method for 1-4 isomerization. The MCG3-MPW//M06-2X/MG3S method was therefore used to make a reliable prediction for the rata constants of 1-4 isomerization of the 1-hexyl radical for which a direct experimental measurement is not available. The calculated CVT/SCT/M08-HX/cc-pVTZ+ rate constants agree well with experimental values for 1-5 isomerization of the 1-hexyl radical, and they show that the tunneling effect for these reactions was underestimated in previous work.