HNNO_2自身氢迁移及异构化机理的理论研究

采用CBS-QB3方法对二硝酰胺酸(HDN)裂解过程中的HNNO2自由基自身氢迁移及N—N键断裂异构化反应机理进行了研究.结果表明,HNNO2自由基自身氢迁移反应经历了N(4)—O(2)间的氢迁移、O(2)—O(3)间的氢迁移及内转化3个不同类型的基元反应,最终生成N2O分子与OH自由基.其中N(4)—O(2)间的氢迁移为HNNO2自由基自身氢迁移反应中的速率决定步.HNNO2自由基通过N(1)—N(4)键断裂以及O(2)—N(4)键形成异构化成产物NO+HNO,该过程的能垒为176.17kJ·mol-1,比氢迁移通道决速步能垒高出了47.59kJ·mol-1,表明氢迁移通道为HNNO2裂解过程中的优势通道.     

氟代乙炔氢迁移反应微正则系综速率常数的计算

以氟代乙炔的氢迁移反应为例, 计算了隧道效应、曲率因子和转动选态对微正则系综单分子反应速率常数的影响。结果表明, 在低能区域, 隧道效应和曲率因子对反应速率有较大影响, 均使反应速率增加, 而在高能区域作用不明显。与此同时还研究了不同对称态下的反应速率常数, 结果表明, 本例中的两对称态(A'态和A"态)的反应速率常数在能量较低时差异较大, 随能量升高, 两者趋于相等。     

甲酰胺水溶液的分子动力学模拟

为了解重要的生化模型甲酰胺在水溶液中的微观结构, 采用全原子力场在全浓度范围内对甲酰胺溶液进行了分子动力学模拟, 得到了溶液的径向分布函数, 分析计算了溶质和溶剂分子间的相互作用, 对甲酰胺和水分子的氢键缔合情况进行了分析. 研究发现羰基侧的H原子与水分子能形成C—H…O弱相互作用. 在作者早期的研究中发现, 此相互作用对于阻碍甲酰胺的异构化具有重要意义, 特别是当甲酰胺在溶液中含量增大时, 此相互作用更加不能忽视. 全浓度溶液的模拟表明, 甲酰胺在稀浓度区可以促进水局部结构的增强, 随FM浓度增加, 由水的自身缔合转变为水与FM的交叉缔合, 在FM高浓度区, 两者的交叉缔合将逐渐被甲酰胺自身的线状缔合代替.     

离子液体中卤代哒嗪氟代新方法的研究

在离子液体和氟化钾反应体系中，研究了卤代哒嗪化合物的氟代反应，讨论了微波辐射条件下由卤代哒嗪化合物合成氟代哒嗪化合物的方法。用离子液体作为反应介质替代了传统的分子溶剂，提高了产物收率，简化了操作。用微波反应不仅获得了较好的收率，也缩短了反应时间。利用离子液体和微波辐射条件进行氟代反应是一种绿色化学方法。     

钯催化烯烃的分子内氟芳基化反应（英文）

发展了一种钯催化的烯烃分子内氟芳基化反应,以Ar IF_2作为氟源和亲电试剂来活化烯烃,实现了4-芳基-1-丁烯的氟化关环反应,以中等到良好的收率得到氟芳基化产物.这类反应为从烯基芳烃出发合成氟代四氢萘和氟代色满提供了高效方法.     

氟代乙烷消除反应的量子拓扑研究

有关卤代乙烷消除卤化氢的反应已有一些实验和理论方面的研究^[1～8].本文通过跟踪氟代乙烷消除反应的IRC途径,研究其在IRC途径上体系的拓扑性质,进一步阐明了该反应为一协同的非同步过程.     

碳-氟键断裂用于构建氟代异噁唑化合物

含氟异噁唑骨架是许多药物及农药分子中的重要结构单元,其中单氟代异噁唑类化合物的合成方法还比较有限.从易得的α-三氟甲基-β-羰基酯出发,发展了基于碱促进的碳氟键断裂策略的氟代异噁唑化合物合成方法,该反应条件温和,操作简便,官能团兼容性良好,可以中等到较好的收率得到一系列3-氟代异噁唑化合物,且所得产物可进一步转化为生物活性分子的类似物.     

水助甲酰胺-H2O2氧化乙烯制环氧乙烷反应机理的理论研究

采用MP2方法研究了甲酰胺-H2O2氧化乙烯制取环氧乙烷的反应机理.优化得到了反应物、过渡态、中间体及产物的几何构型并计算了反应势垒.研究结果表明:没有水参与时,反应需要通过四元环过渡态完成,反应势垒很高,在常温下难以进行;有水参与时,在水分子的协助下,反应可以通过六元环过渡态完成,反应势垒较低,常温下反应容易进行.     

二氟亚烷基卡宾与甲醛环加成反应机理的理论研究

用二阶微扰理论研究单重态二氟亚烷基卡宾与甲醛发生的环加成反应机理,采用MP2/6-31G^*方法计算了势能面上各驻点的构型参数、振动频率和能量.结果表明,单重态二氟亚烷基卡宾与甲醛的环加成反应主要有两种反应通道,通道1中,两个反应物经a,b和c三条反应途径生成三元环构型的产物P1,其中途径c是主反应途径,该途径有两步组成:(Ⅰ)二氟亚烷基卡宾与甲醛生成了1个富能中间体(INT1c),是无势垒放热反应,放出能量为219.18kJ/mol;(Ⅱ)中间体(INT1c)异构化为产物二氟亚烷基环氧乙烷,其势垒为134.71kJ/mol.通道2的反应途径由三步组成:(Ⅰ)反应物首先生成了1个富能中间体(INT1b),为无势垒的放热反应,放出的能量142.77kJ/mol;(Ⅱ)中间体(INT1b)异构化成另一中间体(INT2),其势垒为22.31kJ/mol;(Ⅲ)中间体(INT2)异构化成四元环构型产物P2,其势垒为11.98kJ/mol.     

CN自由基与乙烯酮反应机理的理论研究

采用B3LYP和QCISD(T)方法计算得到了CN自由基与乙烯酮(CH2CO)双分子单碰撞反应势能面.结果表明,CN自由基与CH2CO的单碰撞反应存在三个最可能的反应通道.一是CN中C原子进攻CH2CO中亚甲基碳原子生成中间体NCCH2CO,然后中间体NCCH2CO中和—CO基团相接的C—C键断裂得到产物CH2CN CO;二是CN与CH2CO分子直接加成生成中间体CH2C(O)CN,然后这个中间体通过—CN基团的转移异构化到中间体NCCH2CO,进而通过第一条通道得到产物CH2CN CO;三是CN自由基直接从CH2CO中夺氢的氢迁移反应,由于存在一个15.44 kJ/mol的反应势垒及产物的能量较高,这个通道在整体反应动力学里是可以忽略的.目前的理论计算结果与实验结果符合,并有效地解释了此反应的具体机理过程.     

A computational study of the mechanism for the transmetalation of 2-trimethylstannylbuta-1,3-diene with SnCl4

Electronic structure calculations were performed at the B3LYP/6-31G level to identify the stationary structures on the potential energy surfaces for the transmetalation of 2-trimethylstannylbuta-1,3-diene with SnCl(4). The reaction pathways were characterized by locating the transition states on the intrinsic reaction coordinate. The calculations showed that the reaction between the reactant and SnCl(4), which generates 1-trichlorostannylbuta-2,3-diene via transmetalation, has a low energy barrier of 78.1 kJ.mol(-)(1). The following isomerization process is the rate-controlling step. It turned out that the isomerization process from 1-trichlorostannylbuta-2,3-diene to 2-trichloro-stannylbuta-1,3-diene via transmetalation with SnCl(4) is more energetically favorable than other possible isomerization processes.     

用常温正丁烷异构化反应表征固体超强酸性

研究了室温下固体超强酸催化剂上正丁烷反应，发现转化率低于50％时，异构化选择性高于95％，正丁烷异构化反应动力学符合一级可逆反应规律，固体超强酸的酸强度与正丁烷异构化反应转化率和速率常数呈顺变关系，与反应表现活化能呈逆变关系．报出了一种新的表征固体超强酸性的实验方法．     

Theoretical study on the formation mechanism of iso‐CH2I‐Cl

The dissociation and isomerization reaction mechanism on the ground‐state potential energy surface for CH₂ClI are investigated by ab initio calculations. It is found that the isomer iso‐CH₂I‐Cl can be produced from either the recombination of the photodissociation fragments or the isomerization reaction of CH₂ClI, rather than from isomerization reaction of iso‐CH₂Cl‐I. Further explanations of experimental results are also presented. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Ab initio interpolated potential energy surface and classical reaction dynamics for HCO+ + H, HOC+ + H, and deuterated analogues

Classical simulations of the reactions between HCO+/COH+ and hydrogen atoms, as well as their deuterated variants, have been carried out on an ab initio interpolated potential energy surface. The surface is constructed at the quadratic configuration interaction with single and double excitation level of ab initio calculation. At low energies we observe reaction channels associated with the isomerization of the cation, hydrogen/deuterium exchange, and the combination of isomerization with exchange. The HCO+/DCO+ ions only undergo exchange, and deuteration is more facile than the release of deuterium. The COH+/COD+ ions undergo isomerization or isomerization combined with exchange, the latter being the dominant reaction channel. Deuteration is again more facile than the release of deuterium, in combination with isomerization. These results are consistent with experimental measurements and with hypotheses on the deuteration of molecules in the interstellar medium.     

A DFT study on the double bond migration of butene catalyzed by ionic pair of 1-ethyl-3-methyl-imidazolium fluoride

The double bond migration of butene catalyzed by 1-ethyl-3-methyl-imidazolium fluoride (EmimF) has been studied using quantum chemical method. The geometries of reactant, transition state and product for the isomerization have been optimized by density functional theory (DFT) at the B3PW91/6-31G(d,p), 6-311++G(d,p) and aug-cc-PVDZ levels. The computed results show that the 4-H atom on imidazole ring of EmimF has a good catalytic activity to the double bond migration of butene and the catalytic reaction of 1-butene to 2-butene is a synergetic and elementary process. The apparent activation energy of isomerization is about 197 kJ/mol.     

Competitive 1,2- and 1,5-hydrogen shifts following 2-vinylbiphenyl photocyclization

[reaction: see text] The photocyclization of 2-vinylbiphenyl and its derivatives has been proposed to occur via a two-step mechanism: photocyclization to form an unstable 8a,9-dihydro-phenanthrene intermediate, followed by exothermic unimolecular isomerization to a 9,10-dihydrophenanthrene. The mechanism of the hydrogen shift process has been investigated using deuterated derivatives of 2-isopropenylbiphenyl and 2,6-diphenylstyrene. 1H NMR analysis of the photoproducts indicates that the thermally allowed 1,5-hydrogen or deuterium shift is a minor product-forming pathway and that an unusual double 1,2-hydrogen or deuterium shift is the major product-forming pathway. The potential energy surface for photocyclization and hydrogen shift processes has been explored computationally. The calculated barrier for the 1,5-shift is predicted to be significantly lower than that for the 1,2-shift. Alternative mechanisms for the occurrence of 1,2-hydrogen or deuterium migration are presented.     

Keto-to-enol isomerization in the molecular ion of dimethylmethylphosphonate

A mechanism for keto-to-enol isomerization in dimethylmethylphosphonate (DMMP) has been proposed based on deuterium-labeling studies, a model compound and thermodynamic data. An electron ionization study of H/D exchange occurring in CD₃-labeled DMMP suggests that rapid keto-to-enol isomerization occurs in the ion source and a reaction mechanism based on sequential 1,4-H migrations rather than by the direct 1,3-H transfer or by sequential 1,2-H migrations is proposed. The examination of the mass-analyzed ion kinetic energy/collision-induced dissociation spectrum of the methylphosphonic acid molecular ion suggests that keto-to-enol isomerization does not occur for this species and that 1,2- and 1,3-H migrations are not favored. Available thermodynamic data were employed to construct a potential energy surface for keto-to-enol isomerization of the DMMP molecular ion. The thermodynamic data show that the energy barrier to isomerization is below the internal energy required for decomposition of the DMMP molecular ion. Additionally, the ΔH_f° for the intermediate and enolic isbmers are shown to be significantly less than the ΔH_f° for the keto form of the DMMP molecular ion.     

Sulfur atom exchange in the reaction of SH radicals with S atoms

The structural and energetic properties of the HSS-->SSH transition state are examined using the single and double coupled-cluster method. The energy change for the isomerization reaction is estimated to be 31.7+/-1 kcal mol(-1). The results suggest that the reaction between SH radicals and S atoms should isotopically exchange because the isomerization barrier is significantly less than the S-S bond dissociation energy in the HSS radical.     

Ab Initio MO Studies on the Reaction Mechanism for Carbonyl Insertion Catalyzed by Carbonyl Cobalt Complex

IntroductionSome of ah l'nito MO studies on elementary reactions catalyzed by organometallic compounds have been published[' 41, which are of momentous significance for better understanding the catalytic action and developing the catalytic agent. At the same time, there still aremany unknown aspects that remain to be studied. Carbonyl insertion is one of the key reac.tlons, and it can go in three possible pathsL'--'j, i. e., carbonyl migration, ethyl migrationand concerted reaction paths. Th…     

Exploring reaction pathways with transition path and umbrella sampling: application to methyl maltoside

The transition path sampling (TPS) method is a powerful approach to study chemical reactions or transitional properties on complex potential energy landscapes. One of the main advantages of the method over potential of mean force methods is that reaction rates can be directly accessed without knowledge of the exact reaction coordinate. We have investigated the complementary nature of these two differing approaches, comparing transition path sampling with the weighted histogram analysis method to study a conformational change in a small model system. In this case study, the transition paths for a transition between two rotational conformers of a model disaccharide molecule, methyl beta-D-maltoside, were compared with a free energy surface constrained by the two commonly used glycosidic (phi,psi) torsional angles. The TPS method revealed a reaction channel that was not apparent from the potential of mean force method, and the suitability of phi and psi as reaction coordinates to describe the isomerization in vacuo was confirmed by examination of the transition path ensemble. Using both transition state theory and transition path sampling methods, the transition rate was estimated. We have estimated a characteristic time between transitions of approximately 160 ns for this rare isomerization event between the two conformations of the carbohydrate. We conclude that transition path sampling can extract subtle information about the dynamics not apparent from the potential of mean force method. However, in calculating the reaction rate, the transition path sampling method required 27.5 times the computational effort than was needed by the potential of mean force method.