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1.
Direct-dynamics canonical variational transition-state theory (CVT) and quasi-classical trajectory (QCT) calculations have been performed to study the dynamics of the initiation steps in the methanol combustion at high oxygen concentration. The initiation steps in combustion of methanol is hydrogen abstraction from carbon or oxygen in methanol to produce hydroxymethyl radical (CH2OH) or methoxy radical (CH3O), respectively, and hydroperoxyl radical (HO2). A new analytical potential energy function driven from our DFT calculations is constructed to study the dynamics of the title reactions. Reactive cross sections and reaction probabilities at various relative translational energies and initial vibrational and rotational reactant excitation were obtained to calculate the rate constants. The calculated rate constants from CVT and QCT calculations are compared.  相似文献   

2.
We present a theoretical study of the hydrogen abstraction reactions from CH(3)F and CH(2)F(2) by an ozone molecule. The geometries and harmonic vibrational frequencies of all stationary points are calculated at the MPW1K, BHandHLYP, and MPWB1K levels of theory. The energies of all of the stationary points were refined by using both higher-level (denoted as HL) energy calculations and QCISD(T)/6-311++G(2df,2pd) calculations based on the optimized geometries at the MPW1K/6-31+G(d,p) level of theory. The minimum energy paths (MEPs) were obtained by the MPW1K/6-31+G(d,p) level of theory. Energetic information of the points along the MEPs is further refined by the HL method. The rate constants were evaluated on the basis of the MEPs from the HL level of theory in the temperature range 200-2500 K by using the conventional transition-state theory (TST), the canonical variational transition-state theory (CVT), the microcanonical variational transition-state theory (microVT), the CVT coupled with the small-curvature tunneling (SCT) correction (CVT/SCT), and the microVT coupled with the Eckart tunneling correction (microVT/Eckart) based on the ab initio calculations. A general agreement was found among the TST, CVT, and microVT theories. The fitted three-parameter Arrhenius expressions of the calculated forward CVT/SCT and microVT/Eckart rate constants of the ozonolysis of fluoromethane are k(CVT/SCT)(T) = 2.76 x 10(-34)T(5.81)e((-13975/)(T)) and k(microVT/Eckart)(T) = 1.15 x 10(-34)T(5.97)e((-14530.7/)(T)), respectively. The fitted three-parameter Arrhenius expressions of the calculated forward CVT/SCT and microVT/Eckart rate constants of the ozonolysis of difluoromethane are k(CVT/SCT)(T) = 2.29 x 10(-36)T(6.42)e((-15451.6/)(T)) and k(microVT/Eckart)(T) = 1.31 x 10(-36)T(6.45)e((-15465.8/)(T)), respectively.  相似文献   

3.
A direct ab initio dynamics method is used to investigate the hydrogen‐abstraction reaction CH3CHF2+Cl. One transition state is located for α‐H abstraction, and two are identified for β‐H abstraction. The potential‐energy surface (PES) is obtained at the G3(MP2)//MP2/6‐311G(d, p) level. Furthermore, the rate constants of the three channels are evaluated by using canonical variational transition‐state theory (CVT) with small‐curvature tunneling (SCT) contributions over a wide temperature range of 200–2500 K. The dynamic calculations show that the reaction proceeds mainly by α‐H abstraction over the whole temperature range. The calculated rate constants and branching ratios are both in good agreement with the available experimental values.  相似文献   

4.
The kinetics of the hydrogen abstraction at alkanes by formyl radicals is investigated using the reaction class transition state theory (RC-TST) approach combined with the linear energy relationship (LER) or the barrier height grouping (BHG). The rate constants of a reaction in this class can be estimated through those of the reference reaction, CHO + C2H6, which are obtained from rate constants of the reaction that involves the smallest species, namely CHO + CH4, using the explicit RC-TST scaling. The thermal rate constants of this smallest reaction are evaluated at the canonical variational transition state theory (CVT) with the corrections from the small-curvature tunneling (SCT) and hindered rotation (HR) treatments. Our analyses indicate that less than 40% systematic errors, on the average, exist in the predicted rate constants using both the LER approach, where only reaction energy is needed, and the BHG approach, where no additional information is needed; while comparing to explicit rate calculations the differences are less than 60%. Contribution to Mark S. Gordon 65th Birthday Festschrift Issue.  相似文献   

5.
We present a new parametrization (based on ab initio calculations) of the bending potentials for the two lowest potential energy surfaces of the reaction O(3P) + H2, and we use it for rate constant calculations by variational transition-state theory with multidimensional semiclassical tunneling corrections. We present results for the temperature range 250–2400 K for both the rate constants and the intermolecular kinetic isotope effects for the reactions of O(3P) with D2 and HD. In general, the calculated rate constants for the thermal reactions are in excellent agreement with available experiments. We also calculate the enhancement effect for exciting H2 to the first excited vibrational state. The calculations also provide information on which aspects of the potential energy surfaces are important for determining the predicted rate constants.  相似文献   

6.
This paper presents the theoretical studies of the reactions of Cl atoms with CF3CH2OCH3, CF3CH2OCH2F and CF3CH2OCHF2 using an ab initio direct dynamics theory. The geometries and vibrational frequencies of the reactants, complexes, transition states and products are calculated at the MP2/6-31+(d,p) level. The minimum energy path is also calculated at same level. The MC-QCISD method is carried out for further refining the energetic information. The rate constants are evaluated with the canonical variational transition state theory (CVT) and CVT with small curvature tunneling contributions in the temperature range 200–1,500 K. The results are in good agreement with experimental values.  相似文献   

7.
The hydrogen abstraction reactions of OH radicals with CH3CH2CH2Cl (R1) 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 CH2CH2CH2Cl are evaluated by isodesmic reactions. © 2011 Wiley Periodicals, Inc. J Comput Chem, 2011  相似文献   

8.
Quantum tunneling paths are important in reactions when there is a significant component of hydrogenic motion along the potential energy surface. In this study, variational transition state with multidimensional tunneling corrections are employed in the calculations of the thermal rate constants for hydrogen abstraction from the cis‐CH3OCHO by O (3P) giving CH3OCO + OH (R1) and CH2OCHO + OH (R2). The structures and electronic energies are computed with the M06‐2X method. Benchmark calculations with the CBSD–T approach give an enthalpy of reaction at 0 K for R1 (−2.8 kcal/mol) and R2 (−2.5 kcal/mol) which are in good agreement with the experiment, i.e. −2.61 and −1.81 kcal/mol. At the low and intermediate values of temperatures, small‐ and large‐curvature tunneling dominate the kinetics of R1, which is the dominant path over the range of temperature from 250 to 1200 K. This study shows the importance of multidimensional tunneling corrections for both R1 and R2, for which the total rate constant at 298 K calculated with the CVT/μOMT method is 8.2 × 10−15 cm3 molecule−1 s−1 which agrees well with experiment value of 9.3 × 10−15 cm3 molecule−1 s−1 (Mori, Bull. Inst. Chem. Res. 1981, 59, 116). © 2018 Wiley Periodicals, Inc.  相似文献   

9.
The potential energy surfaces of the reactions CHF2CH3 − n F n (n = 1–3) + OH were investigated by MPWB1K and BMC-CCSD (single-point) methods. Furthermore, with the aid of canonical variational transition state theory including the small-curvature tunneling correction, the rate constants of the title reactions were calculated over a wide temperature range of 220–1,500 K. Agreement between the CVT/SCT rate constants and the experimental values is good. Our results show that the order of rate constants is CHF2CH2F + OH > CHF2CHF2 + OH > CHF2CF3 + OH. For reaction CHF2CH2F + OH, the 1-H-abstraction channel dominates the reaction at the whole temperature, while 2-H-abstraction channel appears to be competitive with the increase of temperature.  相似文献   

10.
王文亮  刘艳  王渭娜  罗琼  李前树 《化学学报》2005,63(17):1554-1560
采用密度泛函方法(MPW1PW91)在6-311G(d,p)基组水平上研究了CH3S自由基H迁移反应CH3S→CH2SH (R1), 脱H2反应CH3S→HCS+H2 (R2)以及脱H2产物HCS异构化反应HCS→CSH (R3)的微观动力学机理. 在QCISD(t)/6- 311++G(d,p)//MPW1PW91/6-311G(d,p)+ZPE水平上进行了单点能校正. 利用经典过渡态理论(TST)与变分过渡态理论(CVT)分别计算了各反应在200~2000 K温度区间内的速率常数kTSTkCVT, 同时获得了经小曲率隧道效应模型(SCT)校正后的速率常数kCVT/SCT. 结果表明, 反应 R1, R2 和R3的势垒△E分别为160.69, 266.61和241.63 kJ/mol, R1为反应的主通道. 低温下CH3S比CH2SH稳定, 高温时CH2SH比CH3S更稳定. 另外, 速率常数计算结果显示, 量子力学隧道效应在低温段对速率常数的计算有显著影响, 而变分效应在计算温度段内对速率常数的影响可以忽略.  相似文献   

11.
The rate constants and H/D kinetic isotope effect for hydrogen abstraction reactions involving isotopomers of methyl formate by methyl radical are computed employing methods of the variational transition state theory (VTST) with multidimensional tunneling corrections. The energy paths were built with a dual-level method using the moller plesset second-order perturbation theory (MP2) method as the low-level and complete basis set (CBS) extrapolation as the high-level energy method. Benchmark calculations with the CBSD-T approach give an enthalpy of reaction at 0 K for R1 (−4.5 kcal/mol) and R2 (−4.2 kcal/mol) which are in good agreement with the experiment, that is, −4.0 and − 4.8 kcal/mol. For the reactional paths involving the isotopomers CH3 + CH3OCOH → CH4 + CH3OCO and CH3 + CH3OCOD → CH3D + CH3OCO, the value of kH/kD (T = 455 K) using the canonical VTST/small-curvature tunneling approximation method is 6.7 in close agreement with experimental value (6.2). © 2019 Wiley Periodicals, Inc.  相似文献   

12.
 Variational transition state theory including tunneling corrections (as implemented in Polyrate 8.7) and using multilevel energy calculations at the MCCM-CCSD(T)-1sc level for the CH4 + OH reaction and at the MCCM-CCSD(T)-2m level for the CD4 + OH process, reproduces very well the experimental rate constants. However, no single methodology was found that reproduces equally well the experimental rate constants for both title reactions. Received: 24 March 2002 / Accepted: 11 April 2002 / Published online: 4 July 2002  相似文献   

13.
Faced with the contradictory results of two recent experimental studies [Jara‐Toro et al., Angew. Chem. Int. Ed. 2017 , 56, 2166 and Chao et al., Angew. Chem. Int. Ed. 2019 , 58, 5013] of the possible catalytic effect of water vapor on CH3OH + OH reaction, we report calculations that corroborate the conclusion made by Chao et al. and extend the rate constant evaluation down to 200 K. The rate constants of the CH3OH + OH reaction catalyzed by a water molecule are computed as functions of temperature and relative humidity using high‐level electronic structure and kinetics calculations. The Wuhan–Minnesota Scaling (WMS) method is used to provide accurate energetics to benchmark a density functional for direct dynamics. Both high‐frequency and low‐frequency anharmonicities are included. Variational and tunneling effects are treated by canonical variational transition state theory with multidimensional small‐curvature tunneling. And, most significantly, we include multistructural effects in the rate constant calculations. Our calculations show that the catalytic effect of water vapor is not observable at 200–400 K.  相似文献   

14.
The mechanism of the multichannel reaction CH3NHNH2 (SC1 and SC2) + O → products is investigated theoretically using ab initio and density functional theory, and dynamics properties are explored by a dual‐level direct dynamics method. The calculation of the potential energy surface is carried out at the BMC‐CCSD//MPW1K/6‐311G(d,p) level. Using canonical variational transition state theory with a small‐curvature tunneling correction, the rate constants of each channel are evaluated over a wide temperature range of 200–2000 K on the basis of obtained electronic structures and energy information. The total rate constants are calculated from the sum of the individual rate constants taking into account the Boltzmann distribution of two conformers. The reactivity of the H atom located in different groups is compared. © 2013 Wiley Periodicals, Inc.  相似文献   

15.
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.  相似文献   

16.
It is believed that the dehydrogenation of LiNH2BH3 (LAB) proceeds through a combination of the decomposition of the LiBH2NH3 (LBA) and LAB isomers. The dehydrogenation of LBA, an isomer of LAB, is discussed in this article. It is demonstrated that the loss of H2 from LBA takes place in a two‐step reaction. Studies of the dehydrogenation process were performed using Møller–Plesset second‐order perturbation theory with a 6‐311++G(3df,2pd) basis set. The intrinsic reaction coordinate was calculated to determine the minimum energy paths. Finally, the rate constants were obtained using the transition‐state theory (TST), TST/Eckart, canonical variational transition‐state theory (CVT), CVT/small‐curvature tunneling correction, and CVT/zero‐curvature tunneling correction methods from 200 to 2500 K. This is the first report on a different dehydrogenation mechanism for an alkali‐metal amidoborane, and the energy barrier of LBA is much lower than that of the traditionally studied LAB. © 2012 Wiley Periodicals, Inc.  相似文献   

17.
齐斌  晁余涛 《化学学报》2007,65(19):2117-2123
在6-311+G(2d,2p)水平下, 采用密度泛函理论(DFT)的B3LYP方法, 研究了Criegee 自由基CH2O2与H2O的反应. 结果表明反应存在三个通道: CH2O2+H2O®HOCH2OOH (R1); CH2O2+H2O®HCO+OH+H2O (R2); CH2O2+H2O®HCHO+H2O2 (R3), 各通道的势垒高度分别为43.35, 85.30和125.85 kJ/mol. 298 K下主反应通道(R1)的经典过渡态理论(TST)与变分过渡态理论(CVT)的速率常数kTSTkCVT均为2.47×10-17 cm3•molecule-1•s-1, 而经小曲率隧道效应模型(SCT)校正后的速率常数kCVT/SCT 5.22×10-17 cm3•molecule-1•s-1. 另外, 还给出了200~2000 K 温度范围内拟合得到的速率常数随温度变化的三参数Arrhenius方程.  相似文献   

18.
Phorate, an extremely hazardous organophosphorus insecticide, is still widely used in many countries and territories currently. It may be released in the atmosphere where it can undergo transport and chemical transformations. In this study, the reaction mechanism for the OH-initiated atmospheric oxidation of phorate was investigated. The geometrical parameters and vibrational frequencies of all the stationary points were calculated at the MPWB1K level with the 6-31G(d,p) basis set. Single-point energy calculations were carried out at the MPWB1K/6-311+G(3df,2p) level. Canonical variational transition-state theory with small curvature tunneling contribution was used to calculate the rate constants over the temperature range of 200–370 K. The Arrhenius formulas were fitted. The results indicate that the channel of the formation of phorate oxon resulting from OH addition to phorate, and H abstractions from the –CH2– portion of the –CH2CH3 group in phorate are energetically favorable. The main degradation products include phorate oxon, SO2.  相似文献   

19.
The reaction of C2(A3Πu) with CH4 has been investigated over a wide temperature range 200–3,000 K by direct ab initio dynamics method at the BMC‐CCSD//BB1K/6‐311+G(2d,2p) level of theory. The optimized geometries and frequencies of the stationary points are calculated at the BB1K/6‐311+G(2d,2p) level, and then the energy profiles of the reactions are refined using the BMC‐CCSD method. The activation barrier height for H‐abstraction reaction was calculated to be 4.44 kcal/mol in temperature range (337–605 K), and the electron transfer behavior was also analyzed by quasi‐restricted molecular orbital method in detail. The canonical variational transition‐state theory (CVT) with the small curvature tunneling (SCT) correction method is used to calculate the rate constants over a wide temperature range 200–3,000 K. The theoretical results shows that variational effect is to some extent large in lower temperature range, and small curvature and tunneling effect play important roles to the H‐atom abstraction only at lower temperatures. The CVT/SCT rate constants are in good agreement with the available experimental results. Our theoretical study is expected to provide a direct insight into the reaction mechanism and may be useful for estimating the kinetics of the title reaction over a wide temperature range where no experimental data are available so far. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012  相似文献   

20.
The hydrogen abstraction reaction of O(^3P) with Si2H6 has been studied theoretially. Two transition states of ^3A″ and ^3A′ symmetries have been located for this abstraction reaction. Geometries have been optimized at the UMP2 leve with 6-311G (d) basis set. G3MP2 has been used for the final single-point energy calculation. The rate constants have been calculated over a wide temperature range of 200-3000K using canonical variational transition-state sheory (CVT) with small curvature tunneling effect(SCT). The calculated CVT/SCT rate constants match well with the experimental value.  相似文献   

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