OH+C_2H_2N←C_2H_3+NO→OH+C_2H_2N反应机理的密度泛函理论研究

应用密度泛函理论研究了反应通道(a)C2H3+NO→CH3+NCO和(b)C2H3+NO→OH+C2H2N的反应机理.在B3LYP/6-31G(d)水平上优化了反应物、中间体、过滤态、产物的几何构型,通过频率分析确定了11个中间体和10个过渡态.所有的反应物、中间体、过渡态、产物都在CCSD/6-311++G(d,p)水平上进行了单点能较正.并讨论了反应的异构化过程.计算结果表明10是能量最低的中间体,比反应物的能量低308 479kJ/mol;过渡态1/3,2/5,3/4,4/8比反应物的能量高,其中3/4是能量最高的过渡态,比反应物的能量高91 894kJ/mol.通道(a)和(b)的理论放热值分别为111 059和96 619kJ/mol.     

NH_2(~2B_1)与CH_4反应机理的理论研究

在密度泛函理论 B3LYP/6 -31 1 G*水平下 ,研究了 NH2 与 CH4的反应机理 .通过振动频率和内禀反应坐标 ( IRC)分析 ,对反应过渡态进行了确认 .在 QCISD( T) /6 -31 1 G*水平下进行了单点能计算 ,并进行了零点能校正 ,结果表明 ,反应 NH2 + CH4NH3 + CH3 是主要的反应通道 .     

1-氟-1,1- 二氯乙烷与氟原子反应的理论研究

利用密度泛函理论研究了CH3CCl2F与F原子的反应机理.在MPW1K水平下计算了反应物、过渡态和产物的几何构型和频率,并进一步利用内禀反应坐标理论获得了反应的最小能量路径;在G3(MP2)水平下对所有驻点进行了单点能量校正.结果表明,CH3CCl2F与F原子的反应存在两个H迁移反应通道:CH2H′CCl 2F+F→C...     

三氯乙烷与氟原子的反应机理研究和速率常数计算

采用双水平直接动力学方法研究了反应CH3CCl3+F→CH2CCl3+HF的反应机理和动力学性质;在MPW1K/6-311+G(d,p)水平上优化了所有稳定点的几何构型,并通过频率分析进行了验证;随后在MCG3-MPWPW91//MPW1K水平上进行了能量校正,并利用变分过渡态理论,在MCG3-MPWPW91//MPW1K/6-311+G(d,p)水平上计算了反应在200～2 000K温度区间内的速率常数.结果表明,反应物CH3CCl3属于Cs点群,-CH3基团上的3个H原子等同,反应存在单个氢迁移反应通道.     

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.     

CH3SS与OH自由基反应机理的理论研究

应用密度泛函理论(DFT)对CH3SS与OH自由基单重态反应机理进行了研究.在B3PW91/6-311+G(d,p)水平上优化了反应通道上各驻点(反应物、中间体、过渡态和产物)的几何构型,用内禀反应坐标(IRC)计算和频率分析方法对过渡态进行了验证.在QCISD(T)/6-311++G(d,p)水平上计算了各物种的单点能,并对总能量进行了零点能校正.研究结果表明,CH3SS与OH反应为多通道反应,有5条可能的反应通道.反应物首先通过不同的S—O键相互作用形成具有竞争反应机理的中间体IM1和IM2.再经过氢迁移、脱氢和裂解等机理得到主要产物P1(CH2SS+H2O),次要产物P2(CH2S+HSOH),P3(CH3SH+1SO)和P4(CH2SSO+H2),其中最低反应通道的势垒为174.6kJ.mol-1.     

三氟化氯和环氧丙烷反应的理论研究

应用密度泛函理论对三氟化氯和环氧丙烷反应产生C3H5O和C1F2自由基的机理进行了研究。在B3PW91/6-31+G(d,p)水平优化了12个不同反应通道上各驻点(反应物、中间体、过渡态和产物) 的几何构型,并计算了它们的振动频率和零点振动能。采用CCSD(T)/6-31+ G(d,p) // B3PW91/6-31+G(d,p)单点能计算方法求得各物种的能量,并作了零点能校正。计算结果表明,三氟化氯和环氧丙烷反应可经过不同的反应路径引发C3H5O自由基和C1F2自由基,其中,三氟化氯呈对称的F原子与环氧丙烷的C(1)上与CH3在同一侧的上的H原子结合的活化能最低,仅为16.81 kJ/mol。     

OH+ C2H2N←C2H3 + NO→CH3 + NCO反应机理的密度泛函理论研究

应用密度泛函理论研究了反应通道(a)C2H3 NO→CH3 NCO和(b)C2H3 NO→OH C2H2N的反应机理．在B3LYP／6-31G(d)水平上优化了反应物、中间体、过滤态、产物的几何构型，通过频率分析确定了11个中间体和10个过渡态．所有的反应物、中间体、过渡态、产物都在CCSD／6-311 G(d，P)水平上进行了单点能较正．并讨论了反应的异构化过程．计算结果表明10是能量最低的中间体，比反应物的能量低308．479kJ／mol；过渡态1／3，2／5，3／4，4／8比反应物的能量高，其中3／4是能量最高的过渡态，比反应物的能量高91．894kJ／mol．通道(a)和(b)的理论放热值分别为111．059和96．619kJ／mol．     

HOSO+NO反应的机理及动力学

在B3LYP/6-311++G(d,p)水平上研究了HOSO+NO的反应机理. 优化得到了反应势能面上各驻点的几何构型, 通过内禀反应坐标(IRC)确认了反应物、中间体、过渡态和产物的相关性. 在CCSD(T)/6-311++G(d,p)水平上对计算得到的构型进行了能量校正. 应用经典过渡态理论(TST)与变分过渡态理论(CVT), 并结合小曲率隧道(SCT)效应模型校正的方法计算了标题反应在200-3000 K温度范围内的速率常数k^TST、k^CVT和k^CVT/SCT. 计算结果表明: HOSO+NO反应在单重态和三重态条件下均可发生, 其中单重态反应为主反应通道, HNO+SO₂为主产物. 并利用电子密度拓扑分析方法研究主反应通道反应过程中的化学键变化.     

CH3SH与NO2的反应机理及动力学

在G3B3,CCSD(T)/6-311++G(d,p)//B3LYP/6-311++G(d,p)水平上详细研究了CH3SH与基态NO2的微观反应机理.在B3LYP/6-311++G(d,p)水平得到了反应势能面上所有反应物、过渡态和产物的优化构型,通过振动频率分析和内禀反应坐标(IRC)跟踪验证了过渡态与反应物和产物的连接关系.在CCSD(T)/6-311++G(d,p)和G3B3水平计算了各物种的能量,得到了反应势能面.利用经典过渡态理论(TST)与变分过渡态理论(CVT)并结合小曲率隧道效应模型(SCT),分别计算了在200～3000K温度范围内的速率常数kTST,kCVT和kCVT/SCT.研究结果表明,该反应体系共存在5个反应通道,其中N进攻巯基上H原子生成CH3S+HNO2的通道活化势垒较低,为主要反应通道.动力学数据也表明,该通道在200～3000K计算温度范围内占绝对优势,拟合得到的速率常数表达式为k1CVT/SCT=1.93×10-16T0.21exp(-558.2/T)cm3·molecule-1·s-1.     

Computational study of the deamination reaction of cytosine with H2O and OH-

The mechanism for the deamination reaction of cytosine with H(2)O and OH(-) to produce uracil was investigated using ab initio calculations. Optimized geometries of reactants, transition states, intermediates, and products were determined at RHF/6-31G(d), MP2/6-31G(d), and B3LYP/6-31G(d) levels and for anions at the B3LYP/6-31+G(d) level. Single-point energies were also determined at B3LYP/6-31+G(d), MP2/GTMP2Large, and G3MP2 levels of theory. Thermodynamic properties (DeltaE, DeltaH, and DeltaG), activation energies, enthalpies, and free energies of activation were calculated for each reaction pathway that was investigated. Intrinsic reaction coordinate analysis was performed to characterize the transition states on the potential energy surface. Two pathways for deamination with H(2)O were found, a five-step mechanism (pathway A) and a two-step mechanism (pathway B). The activation energy for the rate-determining steps, the formation of the tetrahedral intermediate for pathway A and the formation of the uracil tautomer for pathway B, are 221.3 and 260.3 kJ/mol, respectively, at the G3MP2 level of theory. The deamination reaction by either pathway is therefore unlikely because of the high barriers that are involved. Two pathways for deamination with OH(-) were also found, and both of them are five-step mechanisms. Pathways C and D produce an initial tetrahedral intermediate by adding H(2)O to deprotonated cytosine which then undergoes three conformational changes. The final intermediate dissociates to product via a 1-3 proton shift. Deamination with OH(-), through pathway C, resulted in the lowest activation energy, 148.0 kJ/mol, at the G3MP2 level of theory.     

Theoretical study and rate constant computation on the reaction HFCO + OH --> CFO + H2O

The potential energy surface, including the geometries and frequencies of the stationary points, of the reaction HFCO + OH is calculated using the MP2 method with 6-31+G(d,p) basis set, which shows that the direct hydrogen abstraction route is the most dominating channel with respect to addition and substitution channels. For the hydrogen abstraction reaction, the single-point energies are refined at the QCISD(T) method with 6-311++G(2df,2pd) basis set. The calculated standard reaction enthalpy and barrier height are -17.1 and 4.9 kcal mol(-1), respectively, at the QCISD(T)/6-311++G(2df,2pd)//MP2/6-31+G(d,p) level of theory. The reaction rate constants within 250-2500 K are calculated by the improved canonical variational transition state theory (ICVT) with small-curvature tunneling (SCT) correction at the QCISD(T)/6-311++G(2df,2pd)//MP2/6-31+G(d,p) level of theory. The fitted three-parameter formula is k = 2.875 x 10(-13) (T/1000)1.85 exp(-325.0/T) cm(3) molecule(-1) s(-1). The results indicate that the calculated ICVT/SCT rate constant is in agreement with the experimental data, and the tunneling effect in the lower temperature range plays an important role in computing the reaction rate constants.     

A computational investigation of HCN2+ isomeric structures: implications for the chemistry of Titan's atmosphere.

The structure and stability of various HCN2+ isomeric structures have been investigated at the complete active space SCF (CASSCF) and multireference-configuration interaction [MR-Cl-SD(Q)] levels of theory with the 6-31G(d) and 6-311G(d,p) basis sets. The investigated species include the singlet (S) and triplet (T) open-chain H-N-C-N+ ions 1S, 1S', and 1T, the open-chain H-C-N-N+ ions 2S, 2S', and 2T, the HC-N2+ cyclic structures 3S and 3T, and the HN-CN+ cyclic structures 4S and 4T. All these species have been identified as true energy minima on the CASSCF(8,7)/6-31G(d) potential energy surface, and their optimised geometries, refined at the CASSCF(8,8)/6-31G(d) level of theory, have been used to perform single point calculations at the [MR-Cl-SD(Q]/6-311G(d,p) computational level. The most stable structure was the H-N-C-N+ ion 1T, whose absolute enthalpy of formation at 298.15 K has been estimated as 333.9 +/- 2 kcalmol(-1) using the Gaussian-3 (G3) procedure. The two species closest in energy to 1T are the triplet H-C-N-N+ ion 2T and the singlet diazirinyl cation 3S, whose G3 enthalpies of formation at 298.15 K are 343.5 +/- 2 and 340.6 +/- 2 kcalmol(-1), respectively. Finally, we have discussed the implications of our calculations for the detailed structure of the HCN2+ ions formed in the reaction between N3+ and HCN, experimentally observed by flowing after-glow-selected ion flow/drift tube mass spectrometry and possibly occurring in Titan's atmosphere.     

COS与O2反应的密度泛函理论研究

用量子化学密度泛函理论（DFT）方法，对COS与O2的反应进行了理论研究．在UB3LYP／6—31G^＊，UB3LYP／6—311＋＋G^＊＊水平上，优化了反应势能面上各驻点（反应物、产物、中间体和过渡态）的几何构型，在UB3LYP／6—31G^＊水平上通过内禀反应坐标（IRC）计算和振动分析，对过渡态进行了确认．在CCSD（T）／6—311＋＋G（2d，2p）水平上进行了单点能量计算，并确定了反应机理．研究结果表明，反应主要产物为CO2和SO.     

ClONO2异构化反应和分解反应的理论研究

The potential energy surface for the decomposition and isomerization of Chlorine Nitrate（ClONO2）is calculated using the G3 theory. Geometries of related species are optimized at the MP2（ full）/6-31+G（d）and B3LYP/6-31+G（d）level. Vibrational frequencies and IR intensities of ClONO2 have been calculated at the same level. Obtained geometries and vibrational frequencies as well as IR intensities for ClONO2 are in good agreement with experimental values. A new stable stereoisomer is verified by CCSD（T）and QCISD（T）methods at 6-311G （d）basis set. Calculated geometries using above methods for this stereoisomer are in accord with each other. The calculated reaction heat of ClONO2 are also in good agreement with the available data in the literature. Among these reactions of ClONO2,the stereoisomer reaction is the hardest one. The barrier height for this reaction is 481.52 kJ/mol relative to ClONO2 and the reaction is endothermic by 299.85 kJ/mol. On the other hand,among these unimolecular reactions of ClONO2,the decomposition reaction of NO2+ClO is the easiest one. These results indicate that ClONO2 is very stable.     

HCO＋HCN→HCHO＋CN反应机理研究

用ａｂｉｎｉｔｉｏ方法在ＵＨＦ／６－３１Ｇ基组上研究了ＨＣＯ　＋ＨＣＮ→ＨＣＨＯ＋ＣＮ　反应的反应机理。经寻找反应的过渡态和计算内禀反应坐标（ＩＲＣ）表明，该反应经一过渡态而形成产物，具有较宽的位垒，并属于后位垒反应。用ＵＭＰ２∥６－３１Ｇ方法计算的活化位垒为１４９．６０２ｋＪ／ｍｏｌ，与实验值（１４３．００１ｋＪ／ｍｏｌ）相当一致。     

Mechanistic study of the deamination reaction of guanine: a computational study

The mechanism for the deamination of guanine with H(2)O, OH(-), H(2)O/OH(-) and for GuaH(+) with H(2)O has been investigated using ab initio calculations. Optimized geometries of the reactants, transition states, intermediates, and products were determined at RHF/6-31G(d), MP2/6-31G(d), B3LYP/6-31G(d), and B3LYP/6-31+G(d) levels of theory. Energies were also determined at G3MP2, G3MP2B3, G4MP2, and CBS-QB3 levels of theory. Intrinsic reaction coordinate (IRC) calculations were performed to characterize the transition states on the potential energy surface. Thermodynamic properties (ΔE, ΔH, and ΔG), activation energies, enthalpies, and Gibbs free energies of activation were also calculated for each reaction investigated. All pathways yield an initial tetrahedral intermediate and an intermediate in the last step that dissociates to products via a 1,3-proton shift. At the G3MP2 level of theory, deamination with OH(-) was found to have an activation energy barrier of 155 kJ mol(-1) compared to 187 kJ mol(-1) for the reaction with H(2)O and 243 kJ mol(-1) for GuaH(+) with H(2)O. The lowest overall activation energy, 144 kJ mol(-1) at the G3MP2 level, was obtained for the deamination of guanine with H(2)O/OH(-). Due to a lack of experimental results for guanine deamination, a comparison is made with those of cytosine, whose deamination reaction parallels that of guanine.     

Computational study of the aminolysis of anhydrides: effect of the catalysis to the reaction of succinic anhydride with methylamine in gas phase and nonpolar solution

Different possible pathways of the aminolysis reaction of succinic anhydride were investigated by applying high level electronic structure theory, examining the general base catalysis by amine and the general acid catalysis by acetic acid, and studying the effect of solvent. The density functional theory at the B3LYP/6-31G(d) and B3LYP/6-311++G(d,p) levels was employed to investigate the reaction pathways for the aminolysis reaction between succinic anhydride and methylamine. The single point ab initio calculations were based on the second-order M?ller-Plesset perturbation theory (MP2) with 6-31G(d) and 6-311++G(d,p) basis sets and CCSD(T)/6-31G(d) level calculations for geometries optimized at the B3LYP/6-311++G(d,p) level of theory. A detailed analysis of the atomic movements during the process of concerted aminolysis was further obtained by intrinsic reaction coordinate calculations. Solvent effects were assessed by the polarized continuum model method. The results show that the concerted mechanism of noncatalyzed aminolysis has distinctly lower activation energy compared with the addition/elimination stepwise mechanism. In the case of the process catalyzed by a second methylamine molecule, asynchronous proton transfer takes place, while the transition vectors of the acid-catalyzed transition states correspond to the simultaneous motion of protons. The most favorable pathway of the reaction was found through the bifunctional acid catalyzed stepwise mechanism that involves formation of eight-membered rings in the transition state structures. The difference between the activation barriers for the two mechanisms averages 2 kcal/mol at various levels of theory.     

An assessment of theoretical procedures for predicting the thermochemistry and kinetics of hydrogen abstraction by methyl radical from benzene

The reaction enthalpy (298 K), barrier (0 K), and activation energy and preexponential factor (600-800 K) have been examined computationally for the abstraction of hydrogen from benzene by the methyl radical, to assess their sensitivity to the applied level of theory. The computational methods considered include high-level composite procedures, including W1, G3-RAD, G3(MP2)-RAD, and CBS-QB3, as well as conventional ab initio and density functional theory (DFT) methods, with the latter two classes employing the 6-31G(d), 6-31+G(d,p) and/or 6-311+G(3df,2p) basis sets, and including ZPVE/thermal corrections obtained from 6-31G(d) or 6-31+G(d,p) calculations. Virtually all the theoretical procedures except UMP2 are found to give geometries that are suitable for subsequent calculation of the reaction enthalpy and barrier. For the reaction enthalpy, W1, G3-RAD, and URCCSD(T) give best agreement with experiment, while the large-basis-set DFT procedures slightly underestimate the endothermicity. The reaction barrier is slightly more sensitive to the choice of basis set and/or correlation level, with URCCSD(T) and the low-cost BMK method providing values in close agreement with the benchmark G3-RAD value. Inspection of the theoretically calculated rate parameters reveals a minor dependence on the level of theory for the preexponential factor. There is more sensitivity for the activation energy, with a reasonable agreement with experiment being obtained for the G3 methods and the hybrid functionals BMK, BB1K, and MPW1K, especially in combination with the 6-311+G(3df,2p) basis set. Overall, the high-level G3-RAD composite procedure, URCCSD(T), and the cost-effective DFT methods BMK, BB1K, and MPW1K give the best results among the methods assessed for calculating the thermochemistry and kinetics of hydrogen abstraction by the methyl radical from benzene.