CH_3SGN与O_2气相反应机理的理论研究

在G3(MP2)水平上,通过对CH_3S与O_2rcyi2rvylce dm (PES)上关键驻点的能 量计算,共找到4种中间体,9个过渡态,6种产物通道,并对这些气相反应机理进 行了讨论,同时应用TST－RRKM理论对主要反应的速率进行计算。结果表明:CH_3S 与O_2反应在低温下以生成CH_3SOO为主,并与实验结果吻合;在中高温下以消去和 抽提反应为主,分别生成CH_3 + SO_2和CH_2S + HO_2,其它产物较少。     

Theoretical study of the methylbenzene side-chain hydrocarbon pool mechanism in methanol to olefin catalysis

Recent experimental work has shown that methanol to olefin (MTO) catalysis on microporous solid acids proceeds through a hydrocarbon pool mechanism with methylbenzenes frequently acting as the most important reaction centers. Other recent experimental evidence suggests that side-chain methylation is more important than an alternative paring (ring contraction-expansion) mechanism. The present investigation uses density functional theory B3LYP/cc-pVTZ//B3LYP/6-311G and G3(MP2) calculations to model many of the features of the side-chain mechanism. We first calculated at the G3(MP2) level the heats of formation of 43 neutral alkybenzenes to predict the thermodynamics for methylation reactions. The G3(MP2) results predict that sequential methylation of benzene rings with fewer than four methyl groups will preferentially occur on the ring, resulting in the series toluene, 1,3-dimethylbenzene, 1,2,4-trimethylbenzene, and 1,2,4,5-tetramethylbenzene. With the addition of another methyl group side-chain methylation becomes preferred, with 1-ethyl-2,4,5-tetramethylbenzene predicted to be more stable than pentamethylbenzene by 0.7 kcal/mol. We modeled the entire gas-phase side-chain reaction mechanism at the B3LYP/cc-pVTZ//B3LYP/6-311G level, using p-xylene, 1,2,3,5-tetramethylbenzene, and hexamethylbenzene as reaction centers and following the reaction to the point of producing both ethylene and propene. B3LYP/6-311G analytical frequencies were calculated in order to obtain the data needed for the prediction of enthalpies. For comparison, G3(MP2) enthalpies were also calculated for the mechanism based on p-xylene only. We also used a zeolite cluster model to more accurately describe the relative energetics of the reaction for the entire hexamethylbenzene mechanism and parts of the p-xylene mechanism. These calculations place the side-chain mechanism on a much stronger foundation and reproduce experimental structure-reactivity and structure-selectivity data for the methylbenzene hydrocarbon pool.     

A computational study on the amine-oxidation mechanism of monoamine oxidase: insight into the polar nucleophilic mechanism

The proposed polar nucleophilic mechanism of MAO was investigated using quantum chemical calculations employing the semi-empirical PM3 method. In order to mimic the reaction at the enzyme's active site, the reactions between the flavin and the p-substituted benzylamine substrate analogs were modeled. Activation energies and rate constants of all the reactions were calculated and compared with the published experimental data. The results showed that electron-withdrawing groups at the para position of benzylamine increase the reaction rate. A good correlation between the log of the calculated rate constants and the electronic parameter (sigma) of the substituent was obtained. These results agree with the previous kinetic experiments on the effect of p-substituents on the reduction of MAO-A by benzylamine analogs. In addition, the calculated rate constants showed a correlation with the rate of reduction of the flavin in MAO-A. In order to verify the results obtained from the PM3 method single-point B3LYP/6-31G*//PM3 calculations were performed. These results demonstrated a strong reduction in the activation energy for the reaction of benzylamine derivatives having electron-withdrawing substituents, which is in agreement with the PM3 calculations and the previous experimental QSAR study. PM3 and B3LYP/6-31G* energy surfaces were obtained for the overall reaction of benzylamine with flavin. Results suggest that PM3 is a reasonable method for studying this kind of reaction. These theoretical findings support the proposed polar nucleophilic mechanism for MAO-A.     

FC(O)O自由基与NO2反应的量子化学研究

用量子化学DFT, MP2, G3和G3MP2方法对FC(O)O自由基与NO₂的反应机理进行了理论研究. 优化了反应势能面上各驻点的几何结构, 通过内禀反应坐标(IRC)计算和振动分析, 确认了反应中的过渡态, 并用过渡态理论(TST)计算了相关反应的速率常数.     

Density functional theory (DFT) and DRIFTS investigations of the formation and adsorption of enolic species on the Ag/Al2O3 surface

Molecular structure and vibrational frequencies of the novel surface enolic species intermediate on Ag/Al2O3 have been investigated by means of density functional theory (DFT) calculations and in situ infrared spectroscopy. The geometrical structures and vibrational frequencies were obtained at the B3P86 levels of DFT and compared with the corresponding experimental values. Theoretical calculations show that the calculated IR spectra are in good agreement with the experimental spectroscopic results. In addition, the adsorption energy of enolic species on the Ag/Al2O3 catalyst surface was also evaluated. The reaction mechanism from C2H5OH to enolic species on Ag/Al2O3 catalyst was proposed.     

污泥热解中HCN与CaO的反应机理:密度泛函理论研究

采用密度泛函理论对污泥热解中CaO与HCN在低温段的反应进行了研究。在B3LYP/6-311++(3df,2p)水平上计算得到了反应路径上各驻点的几何构型与频率,并在此构型上使用CCSD(T)/cc-pVQZ进行单点能计算。结果表明,两个HCN分子吸附于CaO后,质子发生转移时出现反应路径中最大能垒(310.33 kJ/mol)。使用经典过渡态理论拟合了反应中各步骤的阿累尼乌斯公式,计算了三种典型温度下各步骤的反应速率,发现质子转移为该反应的决速步骤,且温度越高CaO对HCN的作用效果越好。     

9-氨基奎宁催化剂催化1-溴代硝基甲烷和亚苄基丙酮的共轭加成反应的理论研究

通过密度泛函理论(DFT) B3LYP, M062X和从头算方法MP2, 给出了9-氨基奎宁作为有机催化剂和苯甲酸作为辅助催化剂催化1-溴代硝基甲烷与亚苄基丙酮的不对称共轭加成反应的详细反应机理. 反应过程主要包括3个阶段: (1) 亚胺离子中间体的形成; (2) 亚胺离子与1-溴代硝基甲烷的亲核加成; (3) 水解并伴随催化剂的还原. 计算结果不仅解释了苯甲酸加合物在亚胺离子形成过程中所起的重要作用, 而且提供了一般反应模型以理解这个共轭加成反应的反应机理和对映选择性.     

A theoretical investigation of the mechanism for the reaction between a quebrachitol derivative and N3−

The reaction between a mesylated compound and sodium azide was previously studied experimentally at a temperature of 140 °C using dimethylformamide as a solvent. The product was assigned on the basis of the analysis of the NMR spectra. In this work semiempirical (AM1 and PM3), ab initio (Hartree–Fock and MP2) and density functional theory (BLYP functional) quantum mechanical calculations, using continuum models for describing the solvent effect, were carried out for this process to better understand the reaction mechanism. Three distinct mechanisms involving a carbocation and epoxide intermediates, and a transition-state structure for direct attack of the N₃ ⁻ species to the reactant were investigated. The theoretically calculated preferred reaction pathway passing through an epoxide intermediate agrees nicely with the experimental proposal, providing a good example of where theoretical calculations can be of great help to definitively elucidate the reaction mechanism. Received: 10 July 2001 / Accepted: 20 December 2001 / Published online: 8 April 2002     

Kinetic study of the reaction H2O2+H→H2O+OH by ab initio and density functional theory calculations

Theoretical investigations on the kinetics of the elementary reaction H₂O₂+H→H₂O+OH were performed using the transition state theory (TST). Ab initio (MP2//CASSCF) and density functional theory (B3LYP) methods were used with large basis set to predict the kinetic parameters; the classical barrier height and the pre-exponential factor. The ZPE and BSSE corrected value of the classical barrier height was predicted to be 4.1 kcal mol⁻¹ for MP2//CASSCF and 4.3 kcal mol⁻¹ for B3LYP calculations. The experimental value fitted from Arrhenius expressions ranges from 3.6 to 3.9 kcal mol⁻¹. Thermal rate constants of the title reaction, based on the ab initio and DFT calculations, was evaluated for temperature ranging from 200 to 2500 K assuming a direct reaction mechanism. The modeled ab initio-TST and DFT–TST rate constants calculated without tunneling were found to be in reasonable agreement with the observed ones indicating that the contribution of the tunneling effect to the reaction was predicted to be unimportant at ambient temperature.     

Cu(111)面上糠醇加氢生成2-甲基呋喃的反应机理

采用广义梯度近似的密度泛函理论并结合平板模型的方法,详细研究了糠醇在Cu(111)面上反应生成2-甲基呋喃的反应历程,优化了糠醇在Cu(111)面的吸附模型,并采用完全线性同步和二次同步变换的方法,对三种可能的反应机理中的各反应步骤进行了过渡态搜索.结果表明,糠醇主要通过支链上OH与Cu(111)面相互作用,易形成ψCH2和ψCH2O中间体(ψ代表呋喃环).糠醇进一步加氢机理很可能为:引入的氢物种明显降低了糠醇分解形成的中间体ψCH2的活化能,并促进了它的形成;中间体ψCH2更易从糠醇中获得H而生成2-甲基呋喃.该过程的控速步骤为ψCH2O*→ψCHO*+H*,活化能为199.0kJ/mol,总反应是2ψCH2OH=ψCH3+ψCHO+H2O.