密度泛函方法的ZSM-5分子筛上苯与乙烯烷基化反应机理研究

采用密度泛函(DFT)方法对于ZSM-5分子筛上苯与乙烯的烷基化反应机理进行研究。选取了包含酸性质子的8T原子簇模型代表ZSM-5分子筛的部分结构。采用B3LYP/6-31G(d)水平从生成能与反应活化能角度对于烷基化反应发生时可能存在的2种联合机理和一种分步机理进行了计算与比较。结果表明,联合反应机理中乙烯的质子化和C-C键的形成同时发生,而分步机理开始于乙醇盐中间体的生成,随后与苯反应生成反应产物。分步机理中乙醇盐中间体生成步骤的活化能(124.55 kJ/mol)低于联合机理的反应活化能(168.98 kJ/mol和156.06 kJ/mol),而烷基化步骤的活化能(209.35 kJ/mol)高于联合机理的反应活化能,由此可以推断ZSM-5分子筛上苯与乙烯的烷基化反应时2种机理同时发生,存在竞争关系。     

HSS和OH反应机理的理论研究

应用量子化学计算方法研究HSS和OH的反应机理。在B3LYP/6-311+G(2df,p)水平上对HSS和OH反应过程中所有物种进行了构型优化和频率计算。采用QCISD(T)/6-311+G(3df,2p)方法的能量数据,构建了标题反应的势能剖面。结果表明,HSS和OH反应体系中存在4条通道,主通道为OH中O原子夺取HSS中H原子形成产物P1(H_2O+S_2),该通道包含3条路径其表观活化能分别为-72.15,-33.07和-31.41 kJ·mol~(-1),优势路径经过环状过渡态TS1。基于统计热力学原理,预测了稳定物种的热力学性质。     

米力农互变异构的理论计算

采用B3LYP/6-311G(d,P)法,分析米力农分子的酮式与烯醇式构型的构象.再采用相同方法,在气相和水相中计算并考察烯醇式与酬式结构互变时,质子迁移的两种可能途径:(a)分子内质子迁移,(b)水助质子迁移.结果表明,后者所需的活化能较小,氢键在降低反应活化能方面起重要作用,PCM溶剂化对分子几何结构参数和反应活化能的影响较小.     

论1-吡啶-3-[4-(苯基偶氮)苯基]-三氮烯的三氮异构化

采用3种不同泛函(B3LYP,BP86,PBE1PBE)在6-31G*和6-311+G*基组下,计算有机材料1-吡啶-3-[4(苯基偶氮)苯基]-三氮烯(PYPAPT)不同结构上三氮异构化的途径有2种可能:(a)分子内质子迁移,(b)水助质子迁移,因此获得它们的互变异构过程活化能、活化吉布斯自由能和质子转移反应的速率常数等性质.采用PCM法研究反应体系的溶剂化效应.证明孤立分子和一水合物最稳定的异构体相同都为[J],计算结果也与实验值符合得很好,溶剂化效应对异构化能垒的影响较大.最稳定的异构体分了内质子转移的速控步骤的活化能为170.48 kJ/mol,速率常数为3.86×10-15s-1:当水分子参与反应以双质子转移机理异构化时,活化能显著降低,有利于三氮异构化,其最稳定异构体的速控步骤的活化能为47.41kJ/mol,速率常数为9.73×104s-1.计算结果还表明,氢键作用在增大PYPAPT水合物相对稳定性、降低质子转移异构化反应活化能等方面起着重要作用.     

煤自燃生成一氧化碳和水的反应机理研究

应用傅立叶变换红外光谱仪研究煤在氧化自燃中不同温度所生成的气体产物.在30%～100%左右有水和二氧化碳气体析出,温度升至105℃～150%左右时,有一氧化碳生成.采用密度泛函B3LYP法,在6-311G基组水平上研究煤与氧发生自燃反应生成水和一氧化碳的反应体系,对反应势能面上各驻点的几何构型全优化,用频率分析法和内禀反应坐标(IRC)法验证过渡态.计算结果表明,煤自燃生成一氧化碳的反应是氧分子攻击苯环侧链上丙基末端的碳原子,使丙基生成带醛的基团(-CH2-CH2-CO-H)和水,而带醛的基团继续分解生成一氧化碳.由反应活化能可知,生成水和一氧化碳的反应是一个自发式反应.     

用第一性原理方法研究磷铝沸石合成初期生成的低聚物

磷铝分子筛的合成初期,小分子间发生缩合反应形成二聚体、三聚体、四聚体等聚合物,这一阶段的物质形成时间很短,在实验中难以测得.本文采用量子化学方法对沸石合成初期低聚物的稳定结构进行探索和研究.使用密度泛函(DFT)理论,B3LYP泛函,6-31G(d,p)基组,从56种初始结构中我们得到了4种二聚体,7种三聚体,6种四聚体的稳定结构.根据分子相对自由能,在生成的稳定结构中,链状的水合物与环状化合物更为稳定.这些稳定结构在连续介质水溶剂化模型(PCIW)下计算得到的相对自由能表明:在水环境下(Al-P-Al)三元环以及四元环状化合物更为稳定.其中环状四聚体与实验中观察到沸石骨架中广泛存在的四元环结构相吻合.计算得到的结构数据、振动频率和NMR化学位移与AIPO沸石相似.说明这种环状结构可能足形成沸石的关键结构单元,在合成初期的溶液中大量存在.最后我们探讨了在中性条件下缩合反应的机理.     

氰酸和异氰酸异构化反应的量子化学计算研究

主要采用密度泛函理论(DFT)方法,在B3LYP/6-31 G(d,p)水平上,优化了氰酸、异氰酸的结构,分别得到氰酸的能量为-168.69356 a.u,异氰酸的能量为-168.73227 a.u,并对过渡态进行了反应的内禀反应曲线IRC计算.计算结果表明氰酸、异氰酸的异构化反应易于进行,反应以生成异氰酸为主,而且这个反应是一个典型的氢迁移反应.     

叠氮酸与氨氰环加成反应的理论研究(英文)

采用B3LYP、QCISD、MP2方法在6-311+G(d)基组水平上对叠氮酸与氨氰环加成反应进行了详细研究。首先对所有的反应物、过渡态和产物的构型进行了全参数优化,并用频率分析和内禀反应坐标(IRC)方法对过渡态进行确认。以步长为0.1(amu)1/2 bohr的内禀反应坐标方法在B3LYP/6-311+G(d)方法和基组水平上计算了内禀反应坐标s为(-3.00～3.00)(amu)1/2 bohr范围内反应的键长、振动频率、NBO电荷变化情况。根据统计热力学方法和用Wigner校正的Eyring过渡态理论,计算了(200～450)K温度范围内反应热力学函数及速率常数,探讨了温度对反应的影响。结果表明,反应经过过渡态TS生成5-AT的活化能为117.14 kJ/mol(B3LYP)、130.18 kJ/mol(QCISD)和106.24 kJ/mol(MP2),产物的相对能量为-74.24 kJ/mol(B3LYP)、-87.01 kJ/mol(QCISD)和-79.09 kJ/mol(MP2);反应随温度的升高更具有动力学优势,但在热力学上低温下更易于进行,因此,结合热力学和动力学因素,我们认为(300～350)K是该反应的适宜温度。     

亚硝酰氢与羟基自由基反应的密度泛函理论研究

用密度泛函理论研究了HNO OH反应机理。在(U)B3LYP/aug-cc-pVTZ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,获得了零点能校正后的反应势能曲线。研究表明:根据进攻方式的不同,有3个反应通道,反应通道不同则产物不同。反应的主要产物是NO H_2O,次要产物是NH_2 O_2;主要产物和次要产物与反应物的总能量之差(经零点能校正后)分别为-133.42 kJ/mol和150.44 kJ/mol。生成主要产物时主要反应通道的活化能为102.11 kJ/mol。     

Au催化水煤气变换反应的密度泛函理论研究

应用密度泛函理论(DFT)对Au催化水煤气变换反应的3种可能机理(氧化还原机理、羧基机理、甲酸根机理)进行比较。通过对各个基元反应的反应物、反应中间体、过渡态和产物的研究表明:水煤气变换反应在Au(111)面有3种可能机理,并且每种机理各有2种反应途径.对氧化还原机理来说,反应易按照2个OH的歧化反应进行:对羧基机理来说,反应易按照COOH和OH发生歧化反应进行;对甲酸根机理来说,反应易按照HCOO和OH反应进行。纵观整个反应历程,从活化能比较可知,按甲酸根机理进行的可能性最小,按照羧基机理反应的可能性最大。     

Pyrolysis mechanism of carbon matrix precursor cyclohexane--the formation of condensed-ring aromatics and the growing process of molecules

Based on the experiments, the UAM1 method was adopted to investigate benzene condensation of an important intermediate and the molecule growing mechanism during the cyclohexane pyrolysis process. The conclusions were drawn as follows: (1) from the viewpoint of thermodynamics, the condensation of benzene and C4H5* is a spontaneous reaction and the rising temperature will increase the spontaneous tendency of the reaction. (2) From the viewpoint of kinetic, the condensation of benzene and C4H5* is a two-step reaction. The rate-determining step is step 2 of hydrogen removal from intermediate C10H10 (I1) with the activation energy of 350.61 kJ/mol below 1473 K while the rate-determining step is step 1 that free radical C4H5* attacks benzene to form intermediate C10H10 (I1) with the activation energy DeltaE(0)(not equal to theta)=31.74 kJ/mol above 1473 K. (3) The space structure, electronic structure and thermodynamics parameters of molecular reaction of dense-ring aromatizing compounds can be used to replace the resonance energy and free valence to judge the activation of thermodynamic reaction of compounds. And (4) the analysis of the space structure, electronic structure and thermodynamic parameters show that the growing process of molecules with benzene used as initial reactants becomes more easier as the multi-ring aromatizing molecular system increases.     

CH3S与CH2SH自由基反应的密度泛函理论研究

用量子化学从头算和密度泛函理论(DFT)对CH3S与CH2SH自由基反应进行了研究。在B3LYP/6-311+G(d,p)水平上优化了反应通道各驻点物种(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)计算和频率分析的方法对过渡态结构及连接性进行了验证。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,CH3S与CH2SH反应为多通道反应,有3条可能的反应通道。反应物首先通过C-S键相互作用形成链状碳-硫偶合中间体IM1,再经过裂解和脱氢气机理生成主要产物P1(CH3SH+CH2S)和次要产物P2和P3。根据势能面分析,所有反应均为吸热反应。相对于CH3S与CH2SH,各产物能量分别为(-175.2,335.5和331.5)kJ.mol-1。     

2-(4-甲氧基苯基)亚肼基乙酸与乙醛酸反应密度泛函理论研究

本文应用量子化学密度泛函理论(DFT)研究了2-(4-甲氧基苯基)亚肼基乙酸与乙醛酸的反应机理及动力学行为,并研究了不同取代基对反应的影响.在RB3LYP/6-31G*水平上,优化了各反应通道的反应物、中间体、过渡态及产物的几何构型,计算了各驻点的振动频率、零点能和电荷分布.计算结果表明,该反应有2条反应通道,分别生成(2Z)和(2E)-3(4-甲氧基苯基二氮烯基)丙烯酸.产物发生了键长的平均化和电荷的重新分布.两反应通道具有相同的反应入口,Z式产物为主要产物,2-(4-甲氧基苯基)亚肼基乙酸中苯环对位被给电子基团取代,有利于反应进行.     

双氧/硫杂紫菜嗪内氢迁移反应的理论研究

本文在确定各反应体系反应物和产物稳定构象的基础上,采用B3LYP/6-31G~(**)方法利用Gaussian 03程序,对质子化的双氧/硫杂紫菜嗪的内氢迁移反应进行了研究,寻找并优化相应的过渡态,比较了其迁移反应速率的差异。计算结果表明,在中心空穴较小的体系中,空间位阻和静电作用对构象稳定性的影响显著增强,并有可能超越共轭效应而成为影响构象的首要因素。另外,在体系的总电子不变的前提下,缩小中心空穴,提高芳香性,有助于电荷的平均分布,可以明显减弱取代基对IHAT反应速率的影响。     

分子筛催化甲苯歧化SE1反应机理的分子模拟研究

应用分子模拟半经验量子力学Mopac 6．0-AM1近似计算方法分析了分子筛催化甲苯歧化反应的S_E~1反应历程,确定了反应历程中的反应态、过渡态和产物态,得到了反应活化能和反应热等相关信息,对内禀反应坐标的计算进一步验证了反应过程中的能量变化。计算结果表明,分子筛催化的甲苯歧化反应沿S_E~1反应历程可通过两步基元反应完成;质子由分子筛向甲苯分子转移的过程为反应的快速步骤,其活化能达到428．54 kJ·mol~(-1),需要在高温下进行;甲苯歧化总反应的热效应很小,与实验数据相吻合。     

Efficient combinatorial filtering for desired molecular properties of reaction products

Two combinatorial filtering methods for efficiently selecting reaction products with desired properties are presented. The first, "direct reactants" method is applicable only to those molecular properties that are strictly additive or approximately additive, with relatively small interference between neighboring fragments. This method uses only the molecular properties of reactants. The second, "basis products" method can be used to filter not only the strictly additive properties but also the approximately additive molecular properties where a certain degree of mutual influence occurs between neighboring fragments. This method requires the molecular properties of the "basis products," which are the products formed by combining all the reactants for a given reaction component with the simplest set of complementary reactant partners. There is a one-to-one correspondence between the reactants and the "basis products." The latter is a product representation of the former. High efficiency of both methods is enhanced further by a tree-sorting and hierarchical selection algorithm, which is performed on the reaction components in a limited space determined systematically from the filtering criteria. The methods are illustrated with product logPs, van der Waals volumes, solvent accessible surface areas, and other product properties. Good results are obtained when filtering for a number of important molecular properties in a virtual library of 1.5 billion.     

Influence of density and environmental factors on decomposition kinetics of amorphous polylactide – Reactive molecular dynamics studies

In this work, we investigate the influence of the surrounding environment and the initial density on the decomposition kinetics of polylactide (PLA). The decomposition of the amorphous PLA was investigated by means of reactive molecular dynamics simulations. A computational model simulates the decomposition of PLA polymer inside the bulk, due to the assumed lack of removal of reaction products from the polymer matrix. We tracked the temperature dependency of the water and carbon monoxide production to extract the activation energy of thermal decomposition of PLA. We found that an increased density results in decreased activation energy of decomposition by about 50%. Moreover, initiation of decomposition of the amorphous PLA is followed by a rapid decline in activation energy caused by reaction products which accelerates the hydrolysis of esters. The addition of water molecules decreases initial energy of activation as well as accelerates the decomposition process. Additionally, we have investigated the dependency of density on external loading. Comparison of pressures needed to obtain assumed densities shows that this relationship is bilinear and the slope changes around a density equal to 1.3 g/cm³. The conducted analyses provide an insight into the thermal decomposition process of the amorphous phase of PLA, which is particularly susceptible to decomposition in amorphous and semi-crystalline PLA polymers.     

Reaction centre accessibility. II. Role of reaction centre congestion in the calculation of reaction centre accessibility

Accessibility to reaction centres is as important as electronic reactivity in determining the success of a reaction. The possibility of its calculation becomes a necessary requisite in the prediction of reaction products. Using a recently proposed approach to the calculation of reaction centre congestion based on a two dimensional representation of molecules, a new system has been realised that can quickly evaluate the desired accessibility. The system is based on the simulation of the steric interaction between reactants in different orientations. The calculation of an interaction energy for each orientation and their combination permits the approximate estimation of the reaction probability for the steric factors concerned. Even though all the operations were performed using a two-dimensional representation the results are encouraging. It is obvious that at this level it is impossible to predict face accessibility preference.