6-硫代黄嘌呤互变异构体的密度泛函理论计算

在密度泛函B3LYP/6-311G**水平下,对14种气相和水相中可能存在的6-硫代黄嘌呤异构体进行了几何构型的全自由度优化,并计算出它们的总能量、焓、熵、吉布斯自由能。Onsager反应场溶剂模型用于水相的计算.计算结果表明,6-硫代黄嘌呤在气相中和水相中主要以硫酮的形式存在.在气相和水相中,硫酮-N7(H)均比硫酮-N9(H)更稳定.计算结果同已有实验结果一致.6-硫代黄嘌呤异构化的熵效应小,对互变异构平衡几乎没有显著的影响,而焓变对互变异构产生了主要的影响.较详细地讨论了水溶剂化作用对异构体的能量、几何结构、电荷分布和偶极矩的影响.     

2-硫代黄嘌呤互变异构体的密度泛函理论计算

在密度泛函B3LYP/6-311G**水平上,对14种气相和水相中可能存在的2-硫代黄嘌呤互变异构体进行了几何构型全自由度优化,并计算出它们的总能量、焓、熵、吉布斯自由能.Onsager反应场溶剂模型用于水相的计算.计算结果表明,2-硫代黄嘌呤在气相和水相中主要以酮式结构形式存在,与已有实验结果一致.在气相和水相中,酮式结构—N(7)(H)均比酮式结构—N(9)(H)更稳定.2-硫代黄嘌呤互变异构的熵效应小,对互变异构平衡没有显著的影响,而焓变对互变异构却产生了主要的影响.水溶剂化自由能与异构体的气相偶极矩存在相关性.另外,较详细地考察了2-硫代黄嘌呤与6-硫代黄嘌呤的相对稳定性.     

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...     

黄嘌呤异构体质子转移异构化反应机理的理论研究

采用量子化学密度泛函B3LYP/6-31G(d,p)和M06-2X/6-311++G(d,p)方法对黄嘌呤两种酮式异构体X(1,3,7)与X(1,3,9)间质子转移引起的互变异构反应机理进行了计算研究,获得了异构化反应过程的反应焓﹑活化吉布斯自由能和质子转移反应的速率常数等参数。水相计算采用极化连续(PCM)模型。结果表明,由于可能的氢迁移顺序差异,分子内由X(1,3,7)向X(1,3,9)异构化可能共有16条反应通道,涉及11个中间体和20个过渡态,其主反应通道速控步骤的活化吉布斯自由能为183.10k J/mol,速率常数为5.17×10-20s-1,其余各通道速控步骤活化吉布斯自由能均较高,而且整体水溶剂效应不利于质子转移的发生。     

吲唑、3-卤代吲唑互变异构反应机理的理论研究

The molecular structures of indazole and 3-halogeno-indazole tautomers were calculated by the B3LYP method at the 6-311G** level, both in the gaseous and aqueous phases, with full geometry optimization.The geometry and electronic structure of the tautomers of indazole, 3-halogeno-indazole and their transition states were obtained. The Onsager solvate theory model was employed for the aqueous solution calculations.The results of the calculation indicated that the N1-H form of the studied molecule is more stable than that of the N2-H form. The influences of the different 3-halogeno and solvent effects on the geometry, energy,charge and activation energy were discussed. The reaction mechanism of the tautomerization of indazole and 3-halogeno-indazole was also studied and a three-membered cyclic transition state of the tautomer reaction has been obtained.     

水助酮和烯醇类化合物互变异构反应机理的理论研究

采用MP2方法研究了水助MeC(＝O)(CH2R)与MeC(OH)(＝CHR)之间的异构化反应, 确定了相应的过渡态结构并计算了反应势垒. 研究结果表明, 常温下MeC(＝O)(CH2R)与MeC(OH)(＝CHR)之间的异构化反应容易通过2个水分子的参与而实现. 研究结果还表明, 发生质子转移的碳原子在过渡态中采取近sp3杂化. 凡是能够稳定该sp3轨道上的孤对电子的取代基, 都将使质子转移反应的势垒降低, 使异构化反应易于进行.     

2-硫代黄嘌呤与Hg2+配合物结构与性质的理论研究

在密度泛函B3LYP/6-31G**水平下,结合金属赝势基组Lanl2dz对14种2-硫代黄嘌呤异构体和Hg2+、Cl-可能形成的47种配合物结构进行了全自由度优化,并计算了键长、键角、零点能、吉布斯自由能、总能量和相对能量等数据。计算结果表明,2-硫代黄嘌呤异构体和Hg2+、Cl-形成的配合物结构中,2-硫代黄嘌呤酮式N(7)H形成的构型较稳定,能量相对较低的5种配合物中,能量由小到大的顺序依次为:2TX(1,3,7)-2<2TX(1,3,7)-3<2TX(1,3,7)-6<2TX(1,3,7)-4<2TX(1,3,7)-5。同时,将实验报道的红外光谱数据进行了归属,实验值和计算值比较吻合,从而说明了异构体的配位形式。     

卤代二硫烷互变异构体相对稳定性的理论研究

由于在有机合成化学、生命科学、大气化学和环境保护中的重要性,含有S=S键的过硫化物(硫烷)XSSX(X=H,CH3,F,Cl等)得到广泛研究。过硫化物XSSX可以有如图1所示的2种异构体,一种是与过氧化氢结构类似的非平面型的线型结构(属于C2点群),另一种是硫代亚砜型的分叉型构型(SSX2,属于C     

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。     

水助酮和烯醇类化合物互变异构反应机理的理论研揪靠

采用MP2方法研究了水助MeC(=O)(CH2R)与MeC(OH)(=CHR)之间的异构化反应,确定了相应的过渡态结构并计算了反应势垒.研究结果表明,常温下MeC(=O)(CH2R)与MeC(OH)(=CHR)之间的异构化反应容易通过2个水分子的参与而实现.研究结果还表明,发生质子转移的碳原子在过渡态中采取近sp3杂化.凡是能够稳定该sp3轨道上的孤对电子的取代基,都将使质子转移反应的势垒降低,使异构化反应易于进行.     

Cl+CH3OCl反应机理的理论研究

采用从头算方法,对多通道反应体系Cl+CH3OCl的反应机理进行了理论研究.在MP2/6-31+G(d,p)水平下优化了反应物、络合物、产物和过渡态的几何构型,并对得到的平衡几何构型进行了简谐振动频率分析.在相同水平下以过渡态为出发点,通过内禀反应坐标(IRC)理论计算了反应的最小能量路径.并且在MC-QCISD(T)/6-31G(d)高水平下进行了单点能量校正.研究结果表明,该反应存在4条可行的反应通道,其中生成HCl和CH2OCl的通道为主反应通道,其他反应通道为次反应通道.     

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...     

Theoretical study of the scavenging mechanism to 1,4-dicarbonyls by pyridoxamine: The water-assisted reaction

A theoretical study for the water-assisted scavenging mechanism of pyridoxamine with 1,4-dicarbonyls was investigated by density functional theory (DFT) method at B3LYP/6-31G(d) basis set. Two scavenging pathways were examined: imine formation vs. pyrrole ring formation. In addition, solvent effect was performed using the Onsager model. Our calculations indicated that the pyrrole ring formation was the preferred pathway for the reaction, which results were consistent with experimental data. The participation of one water molecule in the reaction would reduce the active energy considerably and the energy barriers of all the transition states in the water-assisted reaction were much lower than those of the non-assisted reaction. The presence of a solvent in the continuum model disfavors the reaction. Hydrogen-bonding interactions and steric hindrance effect play an important role in the scavenging reaction.     

Theoretical study of the mechanism of CH2CO + CN reaction

The potential energy surface information of the CH₂CO + CN reaction is obtained at the B3LYP/6‐311+G(d,p) level. To gain further mechanistic knowledge, higher‐level single‐point calculations for the stationary points are performed at the QCISD(T)/6‐311++G(d,p) level. The CH₂CO + CN reaction proceeds through four possible mechanisms: direct hydrogen abstraction, olefinic carbon addition–elimination, carbonyl carbon addition–elimination, and side oxygen addition–elimination. Our calculations demonstrate that R→IM1→TS3→P3: CH₂CN + CO is the energetically favorable channel; however, channel R→IM2→TS4→P4: CH₂NC + CO is considerably competitive, especially as the temperature increases (R, IM, TS, and P represent reactant, intermediate, transition state, and product, respectively). The present study may be helpful in probing the mechanism of the CH₂CO + CN reaction. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

CH2与HNCO反应机理的量子化学研究

异氰酸(HNCO)分解引发的一系列自由基反应是氮氧化物快速消除机理所研究的领域,由于该反应在燃烧化学中讨论氮氧化物NOx的消除过程十分重要,所以获得这些反应准确的位垒就成为实验化学和理论化学所要解决的问题,本文采用量子化学方法,研究了CH2与HNCO体系的反应机理,力求从理论角度给出合理的解释。     

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.     

Theoretical study on reaction mechanism of isocyanate radical NCO with ethene

The NCO + C₂H₄ reaction is simple and prototype for reaction of the NCO radical with unsaturated hydrocarbons, and it is considered to be important in fuel‐rich combustion. In this article, we for the first time perform detailed theoretical investigations for its reaction mechanism based on Gaussian‐3//B3LYP scheme covering various entrance and decomposition channels. The most favorable channel is firstly the NCO and C₂H₄ approach each other, forming a weakly‐bound complex L1 OCN···C₂H₄, followed by the formation of isomer L2 OCNCH₂CH₂ via a small barrier of 1.3 kcal/mol. Transition states of any decomposable or isomeric channels for L2 in energy are much higher than reactants, which indicate that adduct L2 has stabilization effect in this NCO + C₂H₄ reaction. The direct H‐abstraction channel leading to P1 HNCO + C₂H₃, might have an important contribution to the eventual products in high temperature. These results can well explain available kinetic experiment. Moreover, reaction mechanism for the title reaction is significantly different from the NCO + C₂H₂ reaction which proceeds on most favorably to generate the products HCN + HCCO and OCCHCN + H via a four‐membered ring intermediate. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Theoretical investigation on the mechanism and kinetics of OH radical with ethylbenzene

The OH hydrogen abstraction and addition with ethylbenzene have been studied in the range 298–1000 K using quantum chemistry methods. The geometries and frequencies of the reactants, transition states, and products were performed at BH and HLYP/6‐311++G(d,p) level, single point calculation for all the stationary points were carried out at CCSD(T) calculations of the optimized structures with the same basis set. Nine different reaction paths are considered corresponding to two side chain, three possible ring hydrogen abstraction, and four kinds different OH addition. The results of the theoretical study indicate that at the room temperature the reaction proceeds almost exclusively through OH addition, and is predicted to occur dominantly at the ortho position, the calculated overall rate constant is 6.72 × 10^?12 cm³ molecule^?1 s^?1, showing a very good agreement with available experimental data. Although negligible at low temperature, at 1000 K ring hydrogen abstraction accounts for about 32% of the total abstraction reaction, and the whole hydrogen abstraction makes up for 30% of the total reaction. This study may provide useful information on understanding the mechanistic features of OH‐initiated oxidation of ethylbenzene. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

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.