氢抽提反应CHBr2+HBr→CH2Br2+Br的直接动力学研究

采用直接动力学方法,对CHBr2+HBr→CH2Br2+Br反应通道进行了理论研究,在B3LYP/6-311+G(d,p)水平下获得了优化几何构型、频率以及最小能量路径,更精确的单点能在B3LYP/6-311++G(3df,2pd)水平下完成.利用正则变分过渡态理论,结合小曲率隧道效应校正方法计算了反应通道在220 K～2 000 K温度范围内的速率常数.在整个反应区间,隧道效应对反应的影响比较大;变分效应在低温时有一定的影响,在高温区间的影响很小可以忽略.计算得到的速率常数和已有实验值很好地吻合.     

用变分过渡态理论研究CH3SiH3+H→CH3SiH2+H2反应动力学

用变分过渡态理论对CH3SiH3与H的抽提反应进行了理论研究;利用从头算计算了反应体系的构型、振动频率和能量等信息;计算了温度在298 ～1700K内反应的速率常数和穿透系数。结果表明,在室温下,变分对于此反应影响较大,隧道效应特别明显,计算得到的速率常数和实验值符合得很好。     

用变分过渡态理论对CH3SiH3+O反应体系的动力学研究

用变分过渡态理论对CH3SiH3与氧原子O的抽提反应进行了理论研究。利用从头算计算了反应体系的构型、振动频率和能量等信息,分析了此反应的反应机理;在298～1000 K计算了主要反应通道的速率常数。结果表明,在低温下,变分对于此反应影响较大,隧道效应较明显;计算得到的室温速率常数和实验符合很好。     

CH_3CN+CH_3→CH_2CN+CH_4氢消除反应的直接动力学研究

采用直接动力学方法,对乙腈与甲基的反应进行了理论研究.在BHandHLYP/6-311G(d,p)和MP2/6-311G(d,p)水平下获得,稳定点的几何结构、振动频率及最小能量路径(MEP),在G3(MP2)和MC-QCISD水平下对能量信息进一步确认.利用正则变分过渡态理论,结合小曲率隧道效应校正(CVT/SCT)方法计算了该反应在220K～2000K的速率常数,与实验值符合得很好.     

CF_2ClC(O)OCH_2CH_3+OH的反应机理及动力学性质的理论研究

利用双水平直接动力学方法,在MCG3-MPWB//M06-2X/aug-cc-pVDZ水平上研究了CF_2ClC(0)OCH_2CH_3+OH的微观反应机理.得到了反应物CF_2ClC(O)OCH_2CH_3的5种稳定构象(RCl~RC5),并对每一构象考察了发生在-CH_3-和-CH_2-基团上的所有可能氢提取反应通道.利用改进的变分过渡态理论(ICVT)结合小曲率隧道效应校正(SCT)计算了各反应通道的速率常数,分析了各构象反应位点选择性.结果表明,对于构象RCl和RC2,低温时氢提取反应主要发生在-CH_2-基团上;而对于构象RC3RC4和RC5,发生在-CH_3基团上的氢提取反应通道在整个温度区间内占绝对优势.根据Boltzmann配分函数计算总包反应速率常数,在298 K温度下计算的体系总包反应速率常数与实验值相符,进而给出200~1000 K温度范围内拟合了速率常数的三参数Arrhenius表达式:k_(overall)=5.45×10~(25)T~(4.54)exp(-685/T).     

用变分过渡态理论研究NH_3+NH→2NH_2反应动力学

利用变分过渡态理论的“直接动力学”方法对ＮＨ３分子和基态ＮＨ（Ｘ３∑）自由基的反应进行了理论研究；利用从头算计算得到反应体系的电子结构能和能量梯度等信息，计算了２００～２５００Ｋ温度范围该反应的速率常数和穿透系数，分析了影响隧道效应和反应速率常数的一些因素．结果表明，在低温情况下，变分对于此反应影响较大，隧道效应特别明显．计算得到的速率常数和实验值符合得很好．     

微正则变分过渡态理论直接动力学计算反应速率常数的一种新方法

在微正则过渡态理论(mTST)和微正则变分过渡态理论(mVT)的基础上, 构造了一种量子变分RRKM速率常数的新算法, 用于计算化学反应的k(E, J), k(E)和k(T). 该算法包括了Beyer-Swinehart的隧道校准方法的微正则速率常数的计算. 在该方法中发展了一种截断式内插法, 可有效的计算热速率常数中的Boltzmann积分. 通过对H(D)2CO→H(D)₂ + CO, CH₂CO→CH₂ + CO和CH₄ + H→CH₃ + H₂反应的检验可看出, 其计算结果与采用更精确计算方法的结果相当符合, 但本方法可节省大量机时.     

CF3CH2CH3+OH的反应机理及动力学性质的理论研究

采用密度泛函理论BB1K/6-31+G(d,p)计算了反应CF₃CH₂CH₃+OH各反应通道上驻点的稳定结构和振动频率, 并分别在BMC-CCSD, MC-QCISD和G3(MP2)水平上进行了单点能校正. 运用变分过渡态理论, 在BMC-CCSD//BB1K, MC-QCISD//BB1K, G3(MP2)//BB1K以及BB1K水平上计算了各反应通道的速率常数, 讨论了-CH₂和-CH₃基团上H提取通道对总反应的贡献, 并与已有实验和理论结果进行了对比. 计算结果表明, BMC-CCSD水平上的速率常数与实验测量值符合得很好, 进而给出了该水平上反应在200~1000 K温度范围内速率常数k(cm³?molecule^-1?s^-1)的三参数表达式: k=1.90×10^-21T^3.21exp(-292.62/T).     

反应Si+HCl→SiCl+H的直接动力学研究

利用从头算直接动力学方法，研究反应Si HCl→SiCl H的动力学性能，在QCISD／6－311＋G（d,p)和CCSD（T）／aug-cc-pvtz(单点）水平上，得到体系的势能面信息，进而利用变分过渡态理论计算了反应的速率常数及其与温度的关系。计算结果与实验符合得很好。     

Cl原子与CH3COCH2Cl反应动力学的理论研究

在B3LYP/6-31G(d,p)水平上优化了Cl原子与CH3COCH2Cl反应的各驻点的几何构型,并在相同水平上通过频率计算和内禀反应坐标(IRC)分析对过渡态的结构和反应物、产物的连接性进行了验证。采用高精确模型G3MP2方法进行单点能计算,构建了反应的势能剖面。计算结果表明,标题反应有抽氢反应、加成-消除反应、取代反应3种反应机理6条反应通道。利用经典过渡态理论(TST)和正则变分过渡态理论(CVT)计算了各反应通道在200~2000 K温度范围内的速率常数,并用小区率隧道效应模型(SCT)对抽氢反应进行校正。计算结果显示,反应有一定的变分效应,计算的总反应速率常数与文献报道的实验值符合得较好,速率常数的三参数表达式为k=2.33×10-19T2.54exp(567.07/T)cm3·mol-1·s-1。     

Direct dynamic study on the hydrogen abstraction reaction CH3CN + OH-->CH2CN + H2O

The reaction of acetonitrile with hydroxyl has been studied using the direct ab initio dynamics methods. The geometries, vibrational frequencies of the stationary points, as well as the minimum energy paths were computed at the BHandHLYP and MP2 levels of theory with the 6-311G(d, p) basis set. The energies were further refined at the PMP4/6-311+G(2df, 2pd) and QCISD(T)/6-311+G(2df, 2pd) levels of theory based on the structures optimized at BHandHLYP/6-311G(d, p) and MP2/6-311G(d, p) levels of theory. The Polyrate 8.2 program was employed to predict the thermal rate constants using the canonical variational transition state theory incorporating a small-curvature tunneling correction. The computed rate constants are in good agreement with the available experimental data.     

Theoretical study on the Br + CH3SCH3 reaction

The multiple-channel reactions Br + CH(3)SCH(3) --> 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 G3(MP2) (single-point) theory. The rate constants for every reaction channels, Br + CH(3)SCH(3) --> CH(3)SCH(2) + HBr (R1), Br + CH(3)SCH(3) --> CH(3)SBr + CH(3) (R2), and Br + CH(3)SCH(3) -->CH(3)S + CH(3)Br (R3), are calculated by canonical variational transition state theory with small-curvature tunneling correction over the temperature range 200-3000 K. The total rate constants are in good agreement with the available experimental data, and the two-parameter expression k(T) = 2.68 x 10(-12) exp(-1235.24/T) cm(3)/(molecule s) over the temperature range 200-3000 K is given. Our calculations indicate that hydrogen abstraction channel is the major channel due to the smallest barrier height among three channels considered, and the other two channels to yield CH(3)SBr + CH(3) and CH(3)S + CH(3)Br are minor channels over the whole temperature range.     

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.     

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 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.     

Theoretical study on the reaction CX3 + SiH(CH3)3 (X = H, F)

Theoretical investigations are carried out on the multiple-channel reactions, CH(3) + SiH(CH(3))(3) → products and CF(3) + SiH(CH(3))(3) → products. The minimum energy paths (MEP) are calculated at the MP2/6-311 + G(d,p) level, and energetic information is further refined by the MC-QCISD (single point) method. The rate constants for major reaction channels are calculated by the canonical variational transition state theory (CVT) with small-curvature tunneling (SCT) correction over the temperature range 200-1500 K. The theoretical rate constants are in good agreement with the available experimental data and are found to be k(1a)(T) = 1.93 × 10(-24) T(3.15) exp(-1214.59/T) and k(2a)(T) = 1.33 × 10(-25) T(4.13) exp(-397.94/T) (in unit of cm(3) molecule(-1) s(-1)). Our calculations indicate that hydrogen abstraction channel from SiH group is the major channel due to the smaller barrier height among five channels considered.     

Theoretical Investigation on the Reactions of H with (CH3)4‐nSiHn (n = 1–4)

The abstractions of H with (CH₃)_4‐nSiH_n (n = 1–4) have been investigated at high levels of ab initio molecule orbital theory. Geometries have been optimized at the MP‐2 level with 6–31G(d) basis set, and G2MP2 level has been used for the final energy calculations. Theoretical analysis provided conclusive evidence that the main process occurring in each case is the abstraction of H from the Si? H bond leading to the formation of the H₂ and silyl radicals; the abstraction of H from C? H bond has higher barrier and is difficult to react in each case. The kinetics of the title reactions have been calculated with variational transition state theory over the temperature range 200–1000 K, and the theoretical rate constants match well with the experimental values.     

Theoretical study on the reaction of SiH(CH(3))(3) with SiH(3) radical

The multiple-channel reactions SiH(3) + SiH(CH(3))(3) --> products are investigated by direct dynamics method. The minimum energy path (MEP) 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 channels are calculated by the improved canonical variational transition state theory with small-curvature tunneling correction over the temperature range of 200-2400 K. The theoretical three-parameter expression k(T) = 2.44 x 10(-23)T(3.94) exp(-4309.55/T) cm(3)/(molecule s) is given. Our calculations indicate that hydrogen abstraction channel R1 from SiH group is the major channel because of the smaller barrier height among five channels considered.     

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.