Ab initio direct dynamics studies on the reactions of chlorine atom with CH3-nFnCH2OH (n = 1-3)

The hydrogen abstraction reactions of Cl atom with a series of fluorinated alcohols, i.e., CH(3-n)F(n)CH(2)OH + Cl (n = 1-3) (R1-R3) have been studied systematically by ab initio direct dynamics method and the canonical variational transition state theory (CVT). The potential energy surface information is calculated at the MP2/6-311G(d,p) level. Energies along the minimum energy paths are improved by a series of single-point calculations at the higher modified GAUSSIAN-2 (G2M) level of theory. Theoretical analysis shows that three kinds of hydrogen atoms can be abstracted from the reactants CH(2)FCH(2)OH and CHF(2)CH(2)OH, and for CF(3)CH(2)OH, two possible pathways are found. The rate constants for each reaction channel are evaluated by CVT with the small-curvature tunneling correction (SCT) over a wide range of temperature from 200 to 2000 K. The calculated CVT/SCT rate constants are in good agreement with the available experimental values for the reactions CHF(2)CH(2)OH + Cl and CF(3)CH(2)OH + Cl. However, for the reaction CH(2)FCH(2)OH + Cl, there is negative temperature dependence below 500 K, which is different from the experimental fitted. It is shown that in the low temperature ranges, the three reactions all proceed predominantly via H-abstraction from the methylene positions, and with the increase of the temperature the H-abstraction channels from the fluorinated-methyl positions should be taken into account, while the H-abstraction channels from the hydroxyl groups are negligible over the whole temperature ranges. Also, the reactivity decreases substantially with fluorine substitution at the methyl position of alcohol.     

Theoretical studies of the reactions of CF3CHCLOCHF2/CF3CHFOCHF2 with OH radical and Cl atom and their product radicals with OH

The mechanisms and dynamics studies of the OH radical and Cl atom with CF(3)CHClOCHF(2) and CF(3)CHFOCHF(2) have been carried out theoretically. The geometries and frequencies of all the stationary points are optimized at the B3LYP/6-311G(d,p) level, and the energy profiles are further refined by interpolated single-point energies (ISPE) method at the G3(MP2) level of theory. For each reaction, two H-abstraction channels are found and four products (CF(3)CHFOCF(2), CF(3)CFOCHF(2), and CF(3)CHClOCF(2), CF(3)CClOCHF(2)) are produced during the above processes. The rate constants for the CF(3)CHClOCHF(2)/CF(3)CHFOCHF(2) + OH/Cl reactions are calculated by canonical variational transition-state theory (CVT) within 200-2000 K, and the small-curvature tunneling is included. The total rate constants calculated from the sum of the individual rate constants and the branching ratios are in good agreement with the experimental data. The Arrhenius expressions for the reactions are obtained. Our calculation shows that the substitution of Cl by F decreases the reactivity of CF(3)CHClOCHF(2) toward OH and Cl. In addition, the mechanisms of subsequent reactions of product radicals and OH radical are further investigated at the G3(MP2)//B3LYP/6-311G(d,p) level, and the main products are predicted in the this article.     

CH3NHNH2 + OH reaction: Mechanism and dynamics studies

A direct dynamics study was carried out for the multichannel reaction of CH₃NHNH₂ with OH radical. Two stable Conformers (I, II) of CH₃NHNH₂ are identified by the rotation of the ? CH₃ group. For each conformer, five hydrogen‐abstraction channels are found. The reaction mechanisms of product radicals (CH₃NNH₂ and CH₃NHNH) with OH radical are also investigated theoretically. The electronic structure information on the potential energy surface is obtained at the B3LYP/6‐311G(d,p) level and the energetics along the reaction path is refined by the BMC‐CCSD method. Hydrogen‐bonded complexes are presented at both the reactant and product sides of the five channels, indicating that the reaction may proceed via an indirect mechanism. The influence of the basis set superposition error (BSSE) on the energies of all the complexes is discussed by means of the CBS‐QB3 method. The rate constants of CH₃NHNH₂ + OH are calculated using canonical variational transition‐state theory with the small‐curvature tunneling correction (CVT/SCT) in the temperature range of 200–1000 K. Slightly negative temperature dependence of rate constant is found in the temperature range from 200 to 345 K. The agreement between the theoretical and experimental results is good. It is shown that for Conformer I, hydrogen‐abstraction from ? NH? position is the primary pathway at low temperature; the hydrogen‐abstraction from ? NH₂ is a competitive pathway as the temperature increases. A similar case can be concluded for Conformer II. The overall rate constant is evaluated by considering the weight factors of each conformer from the Boltzmann distribution function, and the three‐term Arrhenius expressions are fitted to be k_T = 1.6 × 10^?24T^4.03exp (1411.5/T) cm³ molecule^?1 s^?1 between 200–1000 K. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009     

Theoretical study and rate constant calculation for reaction of CF(3)CH(2)OH with OH

The reaction mechanism of CF(3)CH(2)OH with OH is investigated theoretically and the rate constants are calculated by direct dynamics method. The potential energy surface (PES) information, which is necessary for dynamics calculation, is obtained at the B3LYP/6-311G (d, p) level. The single-point energy calculations are performed at the MC-QCISD level using the B3LYP geometries. Complexes, with the energies being less than corresponding reactants and products, are found at the entrance and exit channels for methylene-H-abstraction channel, while for the hydroxyl-H-abstraction channel only entrance complex is located. By means of isodesmic reactions, the enthalpies of the formation for the species CF(3)CH(2)OH, CF(3)CHOH, and CF(3)CH(2)O are estimated at the MC-QCISD//B3LYP/6-311G (d, p) level of theory. The rate constants for two kinds of H-abstraction channels are evaluated by canonical variational transition state theory with the small-curvature tunneling correction (CVT/SCT) over a wide range of temperature 200-2000 K. The calculated results are in good agreement with the experimental values in the temperature region 250-430 K. The present results indicate that the two channels are competitive. Below 289 K, hydroxyl-H-abstraction channel has more contribution to the total rate constants than methylene-H-abstraction channel, while above 289 K, methylene-H-abstraction channel becomes more important and then becomes the major reaction channel.     

Dual-level direct dynamics studies for the reactions of dimethyl ether with hydrogen atom and methyl radical

The dual-level direct dynamics approach is employed to study the dynamics of the CH(3)OCH(3) + H (R1) and CH(3)OCH(3) + CH(3) (R2) reactions. Low-level calculations of the potential energy surface are carried out at the MP2/6-311+G(d,p) level of theory. High-level energetic information is obtained at the QCISD(T) level of theory with the 6-311+G(3df,3pd) basis set. The dynamics calculations are performed using variational transition state theory (VTST) with the interpolated single-point energies (ISPE) method, and small-curvature tunneling (SCT) is included. It is shown that the reaction of CH(3)OCH(3) with H (R1) may proceed much easier and with a lower barrier height than the reaction with CH(3) radical (R2). The calculated rate constants and activation energies are in good agreement with the experimental values. The calculated rate constants are fitted to k(R1) = 1.16 x 10(-19) T(3) exp(-1922/T) and k(R2) = 1.66 x 10(-28) T(5) exp(-3086/T) cm(3) mol(-1) s(-1) over a temperature range 207-2100 K. Furthermore, a small variational effect and large tunneling effect in the lower temperature range are found for the two reactions.     

Direct ab initio dynamics studies on the hydrogen-abstraction reactions of OH radicals with HOX (X = F, Cl, and Br)

The hydrogen abstract reactions of OH radicals with HOF (R1), HOCl (R2), and HOBr (R3) have been studied systematically by a dual-level direct-dynamics method. The geometries and frequencies of all the stationary points are optimized at the MP2/6-311+G(2d, 2p) level of theory. A hydrogen-bonded complex is located at the product channel for the OH + HOBr reaction. To improve the energetics information along the minimum energy path (MEP), single-point energy calculations are carried out at the CCSD(T)/6-311++G(3df, 3pd) level of theory. Interpolated single-point energy (ISPE) method is employed to correct the energy profiles for the three reactions. It is found that neither the barrier heights (DeltaE) nor the H-O bond dissociation energies [D(H-O)] exhibit any clear-cut linear correlations with the halogen electronegative. The decrease of DeltaE and D(H-O) for the three reactions are in order of HOF > HOBr > HOCl. Rate constants for each reaction are calculated by canonical variational transition-state theory (CVT) with a small-curvature tunneling correction (SCT) within 200-2000 K. The agreement of the rate constants with available experimental values for reactions R2 and R3 at 298 K is good. Our results show that the variational effect is small while the tunneling correction has an important contribution in the calculation of rate constants in the low-temperature range. Due to the lack of the kinetic data of these reactions, the present theoretical results are expected to be useful and reasonable to estimate the dynamical properties of these reactions over a wide temperature range where no experimental value is available.     

Theoretical study and rate constant calculation for the reactions of SH (SD) with Cl2, Br2, and BrCl

The mechanisms of the SH (SD) radicals with Cl2 (R1), Br2 (R2), and BrCl (R3) are investigated theoretically, and the rate constants are calculated using a dual-level direct dynamics method. The optimized geometries and frequencies of the stationary points are calculated at the MP2/6-311G(d,p) and MPW1K/6-311G(d,p) levels. Higher-level energies are obtained at the approximate QCISD(T)/6-311++G(3df, 2pd) level using the MP2 geometries as well as by the multicoefficient correlation method based on QCISD (MC-QCISD) using the MPW1K geometries. Complexes with energies less than those of the reactants or products are located at the entrance or the exit channels of these reactions, which indicate that the reactions may proceed via an indirect mechanism. The enthalpies of formation for the species XSH/XSD (X = Cl and Br) are evaluated using hydrogenation working reactions method. By canonical variational transition-state theory (CVT), the rate constants of SH and SD radicals with Cl2, Br2, and BrCl are calculated over a wide temperature range of 200-2000 K at the a-QCISD(T)/6-311++G(3df, 2pd)//MP2/6-311G(d, p) level. Good agreement between the calculated and experimental rate constants is obtained in the measured temperature range. Our calculations show that for SH (SD) + BrCl reaction bromine abstraction (R3a or R3a') leading to the formation of BrSH (BrSD) + Cl in a barrierless process dominants the reaction with the branching ratios for channels 3a and 3a' of 99% at 298 K, which is quite different from the experimental result of k3a'/k3' = 54 +/- 10%. Negative activation energies are found at the higher level for the SH + Br2 and SH + BrCl (Br-abstraction) reactions; as a result, the rate constants show a slightly negative temperature dependence, which is consistent with the determination in the literature. The kinetic isotope effects for the three reactions are "inverse". The values of kH/kD are 0.88, 0.91, and 0.69 at room temperature, respectively, and they increase as the temperature increases.     

Theoretical study for the reaction of CH3OCl with Cl atom

A direct dynamics method is employed to study the kinetics of the multiple channel reaction CH(3)OCl + Cl. The potential energy surface (PES) information is explored from ab initio calculations. Two reaction channels, Cl- and H-abstractions, have been identified. The optimized geometries and frequencies of the stationary points and the minimum-energy paths (MEPs) are calculated at the MP2 level of theory using the 6-311G(d, p) and cc-pVTZ basis sets, respectively. The single-point energies along the MEPs are further refined at the G3(MP2)//MP2/6-311G(d, p), G3//MP2/6-311G(d, p), as well as by the multicoefficient correlation method based on QCISD (MC-QCISD) using the MP2/cc-pVTZ geometries. The enthalpies of formation for the species CH(3)OCl and CH(2)OCl are calculated via isodesmic reactions. The rate constants of the two reaction channels are evaluated by using the variational transition-state theory over a wide range of temperature, 220-2200 K. The calculated rate constants exhibit the slightly negative temperature dependence and show good agreement with the available experimental data at room temperature at the G3(MP2)//MP2/6-311G(d, p) level. The present calculations indicate that the two channels are competitive at low temperatures while H-abstraction plays a more important role with the increase of temperature. The calculated k(1a)/k(1) ratio of 0.5 at 298 K is in general agreement with the experimental one, 0.8 +/- 0.2. The high rate constant for CH(3)OCl + Cl shows that removal by reaction with Cl atom is a potentially important loss process for CH(3)OCl in the polar stratosphere.     

Implementation of a microcanonical variational transition state theory for direct dynamics calculations of rate constants

Calculation of microcanonical rate constants has been an important field in chemical dy-namic studies for many years because it can be used not only to give good prediction of rate con-stants in microcanonical assembly, but also to calculate rate constants with certain conserved quantum numbers such as the total angular momentum, and in turn, can be easily converted into thermal rate constants[1—3]. The widely used method for calculating microcanonical rate constants of unimolecular reac-tions…     

Direct dynamics study on the mechanism and the kinetics of the reaction of CH3NH2 with OH

A theoretical study of the mechanism and the kinetics for the hydrogen abstraction reaction of methylamine by OH radical has been presented at the CCSD(T)/6‐311 ++G(2d,2p)//CCSD/6‐31G(d) level of theory. Our theoretical calculations suggest a stepwise mechanism involving the formation of a prereactant complex in the entrance channel and a preproduct complex in the exit channel, for the two hydrogen abstraction channels involving the methyl and amine groups. For clarity, the diagram of potential for the reaction is given. The calculated standard reaction enthalpies are ?98.48 and ?76.50 kJ mol^?1 and barrier heights are 0.36 and 25.25 kJ mol^?1, respectively. The rate constants are evaluated by means of the improved canonical variational transition state theory with small‐curvature tunneling correction (ICVT/SCT) in the temperature range of 299–3000 K. The calculated results show that the rate constants at experimentally measured temperatures are in good agreement with the experimental values. It is shown that the calculated rate constants exhibit a non‐Arrhenius behavior. Moreover, the variational effect is obvious in the calculated temperature range. The dominant product channel is to form CH₂NH₂ and H₂O via hydrogen abstraction from the CH₃ group of CH₃NH₂ by OH in the calculated temperature range. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Theoretical study of the reactions CF3CH2OCHF2 + OH/Cl and its product radicals and parent ether(CH3CH2OCH3) with OH

A dual-level direct dynamic method is employed to study the reaction mechanisms of CF3CH2OCHF2 (HFE-245fa2; HFE-245mf) with the OH radicals and Cl atoms. Two hydrogen abstraction channels and two displacement processes are found for each reaction. For further study, the reaction mechanisms of its products (CF3CH2OCF2 and CF3CHOCHF2) and parent ether CH3CH2OCH3 with OH radical are investigated theoretically. The geometries and frequencies of all the stationary points and the minimum energy paths (MEPs) are calculated at the B3LYP/6-311G(d,p) level. The energetic information along the MEPs is further refined at the G3(MP2) level of theory. For reactions CF3CH2OCHF2 + OH/Cl, the calculation indicates that the hydrogen abstraction from --CH2-- group is the dominant reaction channel, and the displacement processes may be negligible because of the high barriers. The standard enthalpies of formation for the reactant CF3CH2OCHF2, and two products CF3CH2OCHF2 and CF3CHOCHF2 are evaluated via group-balanced isodesmic reactions. The rate constants of reactions CF3CH2OCHF2 + OH/Cl and CH3CH2OCH3 + OH are estimated by using the variational transition state theory over a wide range of temperature (200-2000 K). The agreement between the theoretical and experimental rate constants is good in the measured temperature range. From the comparison between the rate constants of the reactions CF3CH2OCHF2 and CH3CH2OCH3 with OH, it is shown that the fluorine substitution decreases the reactivity of the C--H bond.     

Theoretical studies and rate constant calculations of the reactions C2F5CHO with OH radicals and Cl atoms

The hydrogen abstraction reactions of C₂F₅CHO with OH radicals and Cl atoms have been investigated theoretically by a dual-level direct dynamics method. In this study, the optimized geometries and frequencies of the stationary points are calculated at the MP2/cc-pVDZ level of theory. The energies of the stationery points and the selected points along the minimum energy paths are further refined at the MC-QCISD level using the MP2 geometries. Complexes with energies less than those of the reactants or products are located at the entrance or the exit channels of the two reactions. This result indicates that both of reactions proceed via indirect reaction mechanisms. The enthalpies of formation for the reactant C₂F₅CHO and the product radical C₂F₅CO are estimated by isodesmic reactions at the MC-QCISD//MP2/cc-pVDZ level. At the same level, the rate constants are calculated by canonical variational transition state theory (CVT) incorporating with the small-curvature tunneling correction (SCT) in the temperature range 200–1000 K. Good agreement between the calculated and experimental rate constants is obtained at the room temperature. Due to the lack of the kinetic data of these reactions, the fitted three-parameter expressions based on the CVT/SCT rate constants within 200–1000 K are k₁ = 1.64 × 10⁻²⁴ T^4.33 exp (−566.1/T) and k₂ = 6.33 × 10⁻¹⁵ T^1.35 exp (550.3/T) cm³ molecule⁻¹ s⁻¹, respectively.     

Global dynamics and transition state theories: Comparative study of reaction rate constants for gas-phase chemical reactions

In this review article, we present a systematic comparison of the theoretical rate constants for a range of bimolecular reactions that are calculated by using three different classes of theoretical methods: quantum dynamics (QD), quasi-classical trajectory (QCT), and transition state theory (TST) approaches. The study shows that the difference of rate constants between TST results and those of the global dynamics methods (QD and QCT) are seen to be related to a number of factors including the number of degrees-of-freedom (DOF), the density of states at transition state (TS), etc. For reactions with more DOF and higher density of states at the TS, it is found that the rate constants from TST calculations are systematically higher than those obtained from global dynamics calculations, indicating large recrossing effect for these systems. The physical insight of this phenomenon is elucidated in the present review.     

Direct dynamics study on mechanism and kinetics of the biradical self‐reaction of HOO

A theoretical study of the mechanism and the kinetics for the hydrogen abstraction reaction of the biradical hydroperoxy radical has been presented at the CCSD(T)/6‐311++G(3d,2p)//CCSD/6‐31+G(d,p) level of theory. Our theoretical calculations suppose a stepwise mechanism involving the formation of a postreactant complex in the triplet and singlet entrance channels. Four transition states of the six‐membered chain complexes (³TS1 and ¹TS1) and six‐membered ring complexes (³TS2 and ¹TS2) are located at the high dual level CCSD(T)/6‐311++G(3d,2p)//CCSD/6‐31+G(d,p) method. The rate constants of Path 1 ～ Path 4 at the CCSD(T)/6‐311++G(3d,2p)//CCSD/6‐31+G (d,p) level are calculated by means of the conventional transition state theory (TST) and canonical variational TST without and with small‐curvature tunneling (SCT) correction within the temperature range of 200–2,500 K. The calculated results show that the triplet channel is the dominating reaction channel and Path 2 is found to be the most favorable pathway. The rate constants of Path 2 are in good agreement with the experimental values at the experimentally measured temperatures. Moreover, the variational effect is not obvious in the low temperature range but is not neglectable in the high temperature range. The SCT plays an important role particularly in the low temperature range. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

TheRate: Program for ab initio direct dynamics calculations of thermal and vibrational-state-selected rate constants

We introduce TheRate (THEoretical RATEs), a complete application program with a graphical user interface (GUI) for calculating rate constants from first principles. It is based on canonical variational transition-state theory (CVT) augmented by multidimensional semiclassical zero and small curvature tunneling approximations. Conventional transition-state theory (TST) with one-dimensional Wigner or Eckart tunneling corrections is also available. Potential energy information needed for the rate calculations are obtained from ab initio molecular orbital and/or density functional electronic structure theory. Vibrational-state-selected rate constants may be calculated using a diabetic model. TheRate also introduces several technical advancements, namely the focusing technique and energy interpolation procedure. The focusing technique minimizes the number of Hessian calculations required by distributing more Hessian grid points in regions that are critical to the CVT and tunneling calculations and fewer Hessian grid points elsewhere. The energy interpolation procedure allows the use of a computationally less demanding electronic structure theory such as DFT to calculate the Hessians and geometries, while the energetics can be improved by performing a small number of single-point energy calculations along the MEP at a more accurate level of theory. The CH₄+H↔CH₃+H₂ reaction is used as a model to demonstrate usage of the program, and the convergence of the rate constants with respect to the number of electronic structure calculations. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1039–1052, 1998     

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 CF 3 CHO + Cl using duallevel direct dynamics method. The potential energy surface information was obtained at the MCQCISD/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.     

Calculating rate constants with updated Hessians using variational transition state theory with multidimensional tunneling

Variational transition state theory with multidimensional tunneling (VTST/MT) has been used for calculating the rate constants of reactions. The updated Hessians have been used to reduce the computational costs for both geometry optimization and trajectory following procedures. In this paper, updated Hessians are used to reduce the computational costs while calculating the rate constants applying VTST/MT. Although we found that directly applying the updated Hessians will not generate good vibrational frequencies along the minimum energy path (MEP), however, we can either re-compute the full Hessian matrices at fixed intervals or calculate the Block Hessians, which is constructed by numerical one-side difference for the Hessian elements in the "critical" region and Bofill updating scheme for the rest of the Hessian elements. Due to the numerical instability of the Bofill update method near the saddle point region, we have suggested a simple strategy in which we follow the MEP until certain percentage of the classical barrier height from the barrier top with full Hessians computed and then performing rate constant calculation with the extended MEP using Block Hessians. This strategy results a mean unsigned percentage deviation (MUPD) around 10% with full Hessians computed till the point with 80% classical barrier height for four studied reactions. This proposed strategy is attractive not only it can be implemented as an automatic procedure but also speeds up the VTST/MT calculation via embarrassingly parallelization to a personal computer cluster.     

Theoretical studies on the reactions CH3SCH3 with OH,CF3, and CH3 radicals

The multiple‐channel reactions OH + CH₃SCH₃ → products, CF₃ + CH₃SCH₃ → products, and CH₃ + CH₃SCH₃ → products are investigated by direct dynamics method. The optimized geometries, frequencies, and minimum energy path are all obtained 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 eight reaction channels are calculated by the improved canonical variational transition state theory with small‐curvature tunneling contribution over the temperature range 200–3000 K. The total rate constants are in good agreement with the available experimental data and the three‐parameter expressions k₁ = 4.73 × 10^?16T^1.89 exp(?662.45/T), k₂ = 1.02 × 10^?32T^6.04 exp(933.36/T), k₃ = 3.98 × 10^?35T^6.60 exp(660.58/T) (in unit of cm³ molecule^?1 s^?1) over the temperature range of 200–3000 K are given. Our calculations indicate that hydrogen abstraction channels are the major channels and the others are minor channels over the whole temperature range. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Theoretical study on the kinetics of OH radical reactions with CH3OOH and CH3CH2OOH

The mechanisms and kinetics studies of the OH radical with alkyl hydroperoxides CH(3)OOH and CH(3)CH(2)OOH reactions have been carried out theoretically. The geometries and frequencies of all the stationary points are calculated at the UBHandHLYP/6-311G(d,p) level, and the energy profiles are further refined by interpolated single-point energies method at the MC-QCISD level of theory. For two reactions, five H-abstraction channels are found and five products (CH(3)OO, CH(2)OOH, CH(3)CH(2)OO, CH(2)CH(2)OOH, and CH(3)CHOOH) are produced during the above processes. The rate constants for the CH(3)OOH/CH(3)CH(2)OOH + OH reactions are corrected by canonical variational transition state theory within 250-1500 K, and the small-curvature tunneling is included. The total rate constants are evaluated from the sum of the individual rate constants and the branching ratios are in good agreement with the experimental data. The Arrhenius expressions for the reactions are obtained.     

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 k ₁(T) = 9.97 × 10^?13 T ^0.54exp(613.22/T) and k ₂(T) = 1.16 × 10^?17 T ^2.30exp(?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.