2,6-硫代黄嘌呤质子互变异构反应机理的理论研究

在密度泛函B3LYP/6-311G~(**)理论水平上,对气相和水相中2,6-硫代黄嘌呤各烯醇式与酮式水助质子互变异构体及其过渡态进行几何构型全自由度优化,获得它们在气相和水相中的几何结构和电子结构,PCM反应场溶剂模型用于水相计算.结果显示在气相和水相中,水参与反应降低了互变异构质子迁移的反应活化能,对互变异构质子迁移的反应起到催化作用,但是没有改变各异构体的稳定性顺序,其顺序为W1＞W3＞W2.进一步研究了2,6-硫代黄嘌呤各烯醇式与酮式水助质子互变异构的反应机理,提出了2,6-硫代黄嘌呤各烯醇式与酮式互变异构质子迁移的反应为平面六元环的过渡态结构.探讨了溶剂化效应对互变异构体的几何结构、能量、电荷分布以及互变异构反应活化能的影响等.     

胞嘧啶一水复合物质子转移异构化反应机理的理论研究

采用密度泛函B3LYP方法,在6-311＋G＊＊基组水平上对胞嘧啶一水复合物质子转移引起的氨-酮式、氨-烯醇式与亚胺-酮式互变异构反应机理进行了计算研究,获得了互变异构过程的反应焓、活化能、活化吉布斯自由能和质子转移反应速率常数等参数。将Onsager反应场溶剂模型用于水相的计算,结果表明,胞嘧啶一水复合物无论在气相中...     

5-氟胞嘧啶气相及水助质子转移异构化的理论研究

采用密度泛函B3LYP/6-311G**方法,对6种5-氟胞嘧啶异构体孤立分子的稳定性及质子转移引起的酮式-烯醇式、氨基式-亚胺式互变异构反应机理进行了计算研究,获得了零点能、吉布斯自由能及质子转移过程的反应焓、活化能、活化吉布斯自由能和速率常数等参数.计算结果表明,气相中烯醇-氨基式FC4是最稳定的异构体.分子内质子转移设计了FC1→FC2和FC1→FC6两条通道,分别标记为P(1)和P(2),各通道速控步骤的活化能和速率常数分别为155.9 kJ·mol-1,4.70×10-15 s-1和173.1 kJ·mol-1,1.41×10-18 s-1.水助催化时,相应通道P(3) 和P(4) 速控步骤的活化能和速率常数分别为51.0 kJ·mol-1,1.41×103 s-1和88.2 kJ·mol-1,4.53×10-3 s-1.可见,水分子的加入极大地降低了质子转移的活化能垒.另外发现,水分子参与形成协同的双质子转移机理比水助单质子转移机理更利于降低活化能垒.     

哒嗪裂解反应机理的密度泛函理论研究

利用密度泛函理论(DFT)方法, 以哒嗪为煤的模型化合物, 在(U)B3LYP/6-31G(d,p)水平下计算了标题化合物的键布居数和键裂解能, 并对其热解机理进行了探讨. 在(U)B3LYP/6-311++G(d,p)水平下计算了热解过程中各物种的单点能, 并对总能量进行了校正. 结果表明, 标题物N-N键的布居数和裂解能均最小, 且各键裂解能随温度变化不大, 热解最终产物为HCN和乙炔, NH₃可能是HCN经过二次转换得到的. 采用过渡态理论计算了300~1900 K温度范围内热解的速率常数, 求得lnk与1/T的线性关系.     

SiHCl3-H2气相外延生长Si单晶反应机理的理论研究

采用密度泛函理论计算方法, 在B3LYP/6-311G*水平下, 计算并得到了SiHCl3与H2反应各反应通道上各驻点的构型、振动频率和能量. 结果表明, 在气相中SiHCl3分解的通道d和SiHCl3与H2反应的通道c为竞争反应, 但其均未还原出Si原子, 只有衬底Si参与SiHCl3-H2的反应, Si原子才淀积在Si衬底上.     

黄嘌呤及其互变异构体的密度泛函理论研究

嘌呤碱及其衍生物在生物系统中起重要作用。对人具有兴奋和利尿作用的茶碱和咖啡碱就是黄嘌呤的甲基衍生物。黄嘌呤存在多种互变异构体,从理论计算的角度研究这些互变异构体的几何结构、电子结构及相对稳定性,进一步研究溶剂对其结构和性质的影响是有意义旧。密度泛函理论方法既考虑了电子相关,又较其它CI（组态相互作用）或MPn（n级微扰）方法节省机时。其计算结果较好,被广泛应用于研究各种化合物。本文采用密度泛函B3LYP／6—311G方法对14种黄嘌呤可能的互变异构体（见图1）,分别在气相和水相中进行几何构型全自由度优化和能量计算,讨论了异构体的相对稳定性,水的溶剂化作用对异构体的能量、几何构型、电荷分布和偶极矩的影响,探讨了溶剂极性对异构体的能量和偶极矩的影响。     

过亚硝酸异构化反应机理的密度泛函理论研究

用密度泛函理论方法研究了过亚硝酸在水溶液中的异构化反应机理。在 B3LYP/6-31G水平基础上用梯度解析技术全自由度优化了反应物、产物和反应途径 中的中间产物及过渡态的几何构型，并通过振动频率分析加以确认，进行了内禀反 应坐标计算，确定了该反应的可能通道。结果表明：该反应为多通道强放热反应， 其中以羟基直接转移途经能垒最小，绝对反应速率常数值最大，因此，推测该通道 为主要的反应通道。     

非金属氧化—还原对MO2／MO2^—（M=N,S,CI）的电子转移反应动力学

以Marcus-Hush电子转移理论为基础，提出了用量子化学密度泛函方法研究自交换和异交换电子转移反应的理论方案。在DFT B3LYP／6－311＋G（2D）水平上研究了溶液中MO2／MO2^-，SO2／SO2^-和CIO2／CIO2^-等3个氧化－还原对的自交换以及它们之间的6个交叉电子转移反应的动力学性质，获得了与实验较为一致的结果。     

尿嘧啶水助质子转移反应机理的研究

用密度泛函理论,在B3LYP/6-311++G**计算水平下分别对尿嘧啶所有的气相、液相、过渡态和质子转移异构体的结构进行全优化,获得它们在气相和水相中的几何结构和电子结构,PGM反应场溶剂模型用于水相计算.结果显示:在气相和水相中,水参与反应降低了互变异构质子迁移的反应活化能,对互变异构质子迁移的反应起到催化作用,但...     

H-ZSM-5催化环己酮肟制ε-己内酰胺反应机理的ONIOM研究

采用ONIOM和DFT方法研究了H-ZSM-5分子筛催化环己酮肟制ε-己内酰胺的贝克曼重排反应机理.所有的构型优化使用ONIOM(B3LYP/6-31+G(d):PM3)方法进行,并在此基础上对得到的最优构型应用多种密度泛度方法,如B3LYP/6-31+G(d),PBE/6-31+G(d),M062X/6-31+G(d)和ωB97XD/6-31+G(d)进行完整46T簇模型的单点能计算.B3LYP,PBE和M062X泛函使用dftd3程序计算了额外色散校正,以考虑分子筛内部的弱相互作用.计算结果显示,经色散校正的DFT方法的计算精度被大大提高,达到一个与MP2方法相媲美的精度.反应的决速步是由第一步1,2-H转移和重排步共同决定,内部反应能垒为44.5 kcal/mol.反应速率常数表明,在293~393 K时,在H-ZSM-5上贝克曼重排反应进行得相当缓慢;当温度达到423 K时,正逆反应速率相当,反应开始发生;当达到623 K时,反应速率常数为130 1/s.     

Theoretical study of the reaction mechanism of proton transfer in glycinamide

To investigate the tautomerism of glycinamide that is induced by proton transfer, we present detailed theoretical studies on the reaction mechanism of both the isolated gas phase and H₂O‐assisted proton transfer process of glycinamide, using density functional theory calculations by means of the B3LYP hybrid functional. Twenty‐six geometries, including 10 significant transition states, were optimized, and these geometrical parameters are discussed in detail. The relative order of the activation energy for hydrogen atom transfer of all the conformers has been systematically explored in this essay. For the amido hydrogen atom transfer process, the relative order of the activation energy is: IV < II < III < I, while in the carbonic hydrogen atom transfer process, the relative order is IV > II > III > I. Meanwhile, the most favorable structure for both the amido hydrogen atom transfer and the carbonic hydrogen atom transfer has been found. The involvement of the water molecule not only can stabilize the transition states and the ground states, but can also reduce the activation energy greatly. The superior catalytic effect of H₂O has been discussed in detail. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Negative influence of pKa on activation energy barrier: A case study for double proton transfer reaction in inorganic acid dimers

Strength of acid can be determined by means of pK_a value. Attempts have been made to find a relationship between pK_a and activation energy barrier for a double proton transfer (DPT) reaction in inorganic acid dimers. Negative influence of pK_a is observed on activation energy (E_a) which is contrary to the general convention of pK_a. Four different levels of theories with two different basis sets have been used to calculate the activation energy barrier of the DPT reaction in inorganic acid dimers. A model based on first and second order polynomial has been created to find the relationship between activation energy for DPT reaction. © 2018 Wiley Periodicals, Inc.     

Substituent effect on the reaction mechanism of proton transfer in formamide

MP2 and B3LYP methods at 6‐311++G** basis set have been used to explore proton transfer in keto‐enol forms of formamide and to investigate the effect of substituent, i.e., H, F, Cl, OH, SH, and NH₂ on their transition states. Additionally, the vibrational frequencies of aforementioned compounds are calculated at the same levels of theory. It is proposed that the barrier heights values in kJ/mol for F, Cl, OH, and SH substituents are significantly greater than that of the bare tautomerization reaction, implying the importance of the substituents effect on the intramolecular proton transfer. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Theoretical analysis on the hydrogen bonding and reactivity that associated with the proton transfer reaction of carboxylic acid dimers and their monosulfur derivatives

Carboxylic acid dimers and their monosulfur derivatives are investigated by density functional theory calculations. Basis set superposition error (BSSE) counterpoise correction is included to compare the influence of BSSE on the interaction energies as well as on the geometries. The nature of hydrogen bond is determined on the basis of atoms in molecules (AIM) and natural bond orbital (NBO) analyses. Good correlations have been established between H‐bond length versus AIM topological parameter, orbital interaction, and barrier height for proton transfer. The reactivity behavior along the reaction path of the double proton transfer reaction has also been studied. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

3-卤代吡唑质子转移过程的理论研究

采用密度泛函B3LYP/6-311G**方法,对3-卤(-F、-Cl、-Br)代吡唑几何构型进行了全自由度优化,获得了它们的几何结构和电子结构。计算结果显示,N1-H型的稳定性大于N2-H型。计算并考察了3-卤代吡唑进行结构互变的质子转移过程的四种可能途径:(a)分子内质子转移;(b)水助质子转移;(c)同种二聚体双质子转移;(d)异种二聚体双质子转移。计算结果表明(以3-氟代吡唑为例),途径d所需要的活化能最小(54.89 kJ/mol),而途径a所需要的活化能最大(198.83kJ/mol),途径b和c的活化能居中间分别为(104.05 kJ/mol和69.05 kJ/mol)。研究还表明氢键在降低活化能方面起着重要的作用,卤素(-F、-Cl、-Br)对活化能的影响不大。     

Theoretical investigation of the proton affinities of benzazoles in the gas phase and in solution

The ground‐state equilibrium geometries of benzothiazole, benzoxazole, and benzimidazole were optimized at the density functional theory (DFT)/6‐31G** level of theory. Proton affinities on each of the possible sites in the studied series of compounds have been calculated at the DFT/6‐31G**/6‐311++G** level. The results indicate clearly that N‐site protonation is strongly favored over X‐site protonation (X = NH, O, S) for the series studied. Correlation of the computed proton affinities to the energy (E_HOMO) of the highest occupied MO in the gas phase and in solution has been explored and discussed. A comprehensive investigation of the effect of solvent on the process of protonation of the studied compounds has been performed. Different dielectric continuum models (i.e., Onsager, PCM, and IPCM) have been tested; their performance and range of applicability are reported and discussed. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

NO2^-/NO2体系电子转移的外电场效应和温度效应

用半经典模型研究NO2^-/NO2体系自交换电子转移的反应机理和速率常数, 探讨了外电场对反应过程势能面, 反应能垒及电子转移速率常数的影响, 确定了能垒崩溃的阈值; 讨论了低能垒条件下电子转移反应的速率常数随温度变化的特征。研究表明, 一定方向的外电场能显著降低电子转移反应的活化能垒并提高反应速率常数k, 而对非绝热电子转移反应, 当温度T和活能能垒Ec满足T=2Ec/R时, k取得极大值。     

Photo-excited intramolecular charge transfer and excited state intramolecular proton transfer behaviors for HPIBT system: A theoretical investigation

Given facile synthetic route and excellent photo stability, excited state intramolecular proton transfer (ESIPT)-active luminous materials have gained more and more attention. Here, we focus on photo-induced excitation process and the ESIPT reaction process for the novel 5-benzothiazol-2-yl-6-hydroxy-2-methyl-isoindole-1,3-dione (HPIBT) molecule. On the level of chemical geometries and infrared spectra, we verify that O─H⋯N of HPIBT should be enhanced. We find that a proton is likely to be attracted by enhanced electronic densities around N, that is, charge transfer impetus ESIPT trend. Combing potential energy curves and searching for transition state, we clarify the ultrafast ESIPT mechanism of HPIBT due to a low barrier, which legitimately explains previous experimental characteristics.