氟氯甲醇及其异构体的气相离解反应研究

在G3(MP2)水平上,通过对CH2ClFO的势能面(PES)上关键驻点的能量计算,共找到3种中间体,14个过渡态,20种产物通道,并对氟氯甲醇(CHClFOH)及其异构体(CH2FOCl和CH2ClOF)的气相解离机理进行了讨论。结果表明：四中心的1,2-HX(X=Cl,F)消去反应是氟氯甲醇的主要通道,但对于其同分异构体,OCl和OF键断裂又是强竞争过程。     

基于DFT密度泛函理论的聚酯热降解反应机理研究

本文应用密度泛函理论（DFT）研究钛系催化剂催化聚酯热降解的反应机理。分别以Ti(OEt)4化合物以及Ti(OEt)3 阳离子化合物为催化剂,探讨了二苯甲酸乙二酯（EDB）解聚反应的Lewis酸催化热降解反应机理（M1机理）和烷氧基配位催化热降解反应机理（M2机理）。结果表明,Lewis酸催化热降解反应机理在两种催化剂下的解聚反应能垒与无催化剂反应非常相似,未表现出明显促进作用;Ti(OEt)3 催化剂在M2机理中明显降低了解聚反应能垒,是聚酯热降解反应的催化活性中心。经轨道相互作用分析发现,阳离子催化剂和反应物EDB之间存在较为明显的轨道相互作用。     

庚酸甲酯高温燃烧化学动力学机理的系统简化和分析

采用详细化学反应动力学机理的系统简化方法, 对庚酸甲酯高温燃烧化学动力学机理进行了系统简化. 首先采用两步直接关系图法(Directed relation graph method, DRG)和主成分分析(Principle component analysis, PCA)方法对由1087个物种、4592步可逆反应组成的庚酸甲酯燃烧的详细机理进行框架简化, 得到了包含108个物种, 547步基元反应的框架机理. 在此框架机理基础上, 进一步采用计算奇异值摄动法(Computational singular perturbation, CSP)对框架机理进行时间尺度分析, 再选取30个准稳态物种, 采用准稳态近似(Quasi steady state approximation, QSSA)方法构建了包含78个物种、74步总包反应的全局简化机理. 模拟结果表明, 在较宽的参数范围内, 框架机理和全局简化机理均能重现庚酸甲酯高温燃烧时的点火延迟、物种浓度分布和熄火等燃烧特性. 此外, 基于框架机理阐明了庚酸甲酯高温燃烧的反应路径和对点火有重要影响的基元反应. 与详细机理相比, 框架机理保留了良好的精确性和全局性, 可以很好地反映庚酸甲酯的燃烧特性, 有助于对生物柴油的燃烧过程的理解.     

正十二烷高温燃烧详细化学动力学机理的系统简化

采用详细化学反应动力学机理的系统简化方法, 以典型航空燃料的替代组分正十二烷为研究对象, 开展了正十二烷高温燃烧化学动力学机理的系统简化. 首先采用多步直接关系图法(DRG)和基于计算奇异值摄动法(CSP)重要性指标的反应移除方法对由1279个组分, 5056个基元反应组成的正十二烷燃烧详细机理进行框架简化, 得到了包含59 个组分, 222 个基元反应的框架机理; 进一步采用CSP对框架机理进行时间尺度分析, 选出了10个准稳态物种, 采用准稳态近似方法(QSSA)构建了包含49个组分的全局简化机理. 计算结果表明, 在较宽的参数范围内, 框架机理和全局简化机理均能够重现正十二烷详细机理在高温燃烧的点火延迟时间、熄火以及物种浓度分布等方面的模拟结果.     

Development and testing of a detailed kinetic mechanism of natural gas combustion in internal combustion engine

A detailed chemical mechanism to describe the combustion of natural gas in internal combustion (IC) engine has been developed,which is consisting of 233 reversible reactions and 79 species.This mechanism accounts for the oxidation of methane,ethane,propane and nitrogen.It has been tested using IC engine model of CHEMKIN 4.1.1 and experimental measurements.The performance of the proposed mechanism was evaluated at various equivalence ratios (φ=0.6 to φ=1.3),initial reactor conditions (Tini=500 to 3500 ℃;Pini...     

Chemically induced dynamic electron polarization investigation of the triplet-radical system in the solution of the triplet quencher

The chemically induced dynamic electron polarization (CIDEP) of the triplet molecule/triplet quencher/2,2,6,6-te-tramethyl-1-piperidinyloxyl (TEMPO) systems were measured using the high time-resolved ESR spectrometer.The competition between the radical-triplet pair mechanism (RTPM) and triplet mechanism (TM) or radical pair mechanism (RPM) polarization in the solution of the triplet quencher was investigated,and the relationship between reaction rate of the radical-triplet pair and quenching rate of triplet was deduced.     

Mechanistic study of the gas-phase decomposition of methyl formate

The major products of the thermal decomposition of methyl formate in the gas phase are CH(3)OH, CH(2)O, and CO. Experimental studies have proposed that the mechanism to describe these observations involves two key steps: (1) unimolecular decomposition of methyl formate to yield CH(3)OH + CO, followed by (2) thermal decomposition of methanol to yield CH(2)O + H(2). The present study shows that there exists an alternative mechanism that is energetically more favorable. The new mechanism involves two competing parallel unimolecular decomposition pathways to yield the observed major products.     

Te,CdTe,HgCdTe的电沉积及其成核机理

应用自装微机联用恒电位系统研究了在玻璃碳或铁电极上Te、CdTe、HgCdTe的电沉积及其电结晶成核机理。结果表明,在酸性溶液中,HTeO_2~+的阴极还原符合4电子还原机理,其电结晶生长表现为由HTeO_2~+扩散控制的三维瞬时成核机理;CdTe沉积层的形成是亚碲酸还原的延续,其电结晶成核机理因电位阶跃值、沉积温度及溶液pH值的改变而由HTeO_2~+扩散控制的三维瞬时成核转变为二维瞬时成核机理;对HgCdTe,其电沉积过程的动力学步骤可设想为: Hg~(2+)+2e—→Hg,HTeO_2~++3H~++4e—→Te+2H_2O xHg+Te—→Hg_2Te,Hg_2Te+(1—x)Cd~(2+)+2(1—x)e—→Hg_2Cd_(1-2)Te 相关的结晶生长除受各种实验因素影响外,还与CdTe的成核过程有关。在本文实验条件下,大体遵循二维瞬时成核机理。     

Fe(III)与2,3-二羟基苯甲酸配位反应的动力学和机理

研究了水溶液中三价铁与2,3-二羟基苯甲酸络合反应的机理与动力学. 测定并计算了络合作用的反应速度常数. 活性 和活性焓随氢离子浓度增加而降低, 指明了FeCH2(3+)的缔合机理和FeOH(2+)与Fe2(OH)2(4+)的解离活化机理, 反应速率方程表明氢离子浓度的依赖关系对Fe(III)络合作用是典型的. 但是在[Fe(III)]的二级反应动力学则有一个反常贡献.     

三种Au(111)催化水煤气变换反应机理的比较

采用密度泛函理论对三种水煤气变换反应(WGSR)机理(氧化还原机理、羧基机理、甲酸基的生成机理)在Au(111)面上的反应历程进行详细讨论.通过对表面吸附物种(H2O、CO、OH、O、H、CO2、COOH、HCOO)的吸附行为进行研究,得到最佳活性吸附中心.对三种机理中的14个基元反应的活化能进行分析,得出WGSR在Au(111)上按照羧基机理和氧化还原机理进行的可能性较大,按照甲酸基的生成机理进行的可能性较小.相比较羧基机理和氧化还原机理,反应更有可能按照羧基机理进行,最佳反应途径为H2O-H→OH+CO→COOH+OH→CO2.     

Prediction and measurement of pollutant emissions in CNG fired internal combustion engine

In the present study, the detailed reaction mechanisms were developed and Chemkin 4.1.1 was implemented to predict the formation of pollutant species in compressed natural gas (CNG) fired internal combustion (IC) engine. The proposed mechanisms were developed by coupling the EXGAS (an automatic mechanism generation tool for alkane oxidation) mechanisms with the Leed's NOx mechanism (version 2.0). The simulation results of each proposed mechanism were validated by the experimental measurements for profiles of temperature, pressure and pollutant species (CO, NOx). The rate of production analysis of each mechanism identified the important reactions in each mechanism which contributed to the formation of pollutant species. In spite of some discrepancies, the experimental measurements indicate that Mechanism-IV (consisting of 208 reactions and 78 species) showed closer agreement for each of the predicted profiles of temperature, pressure and pollutant species (CO, NOx).     

Polymerization of α=Aminoisobutyric Acid NCA: Interpretation According to the Hypothesis of a Multiple Mechanism

The basic salt-initiated polymerization of α-aminoisobutyric acid NCA in acetonitrile was studied using various alkaline alcoholates and in the presence or absence of various protic (very weak acid) additives. The cation effect observed was the one expected from either the N-carboxy-α-amino acid anhydride (NCA) anion mechanism (activated monomer mechanism) or the alcoholate anion mechanism (Blout's mechanism). The anion effect appeared to be abnormal for the former mechanism, but did not agree nor disagree substantially with the latter. Furthermore, such additives as methanol (conjugate acid of the initiator), 3-methylhydantoin, 2-oxazolidone, and N-acetylglycine NCA (prototype of the chain growing through the NCA anion mechanism) considerably enhanced the rate of initiation. A still higher rate of initiation could be obtained by the combined use of two additives. IR and DTA analyses of the polymerization products showed the formation of 5,5-dimethylhydan-toin-3-isobutyric acid in the sample using the alcohol-free initiator, hence the NCA anion mechanism is operative. This acid was absent in the low DP polymer obtained in the presence of added methanol, and this agrees with the alcoholate anion mechanism without, however, proving it. Thus, while only part of the results could be explained by one or the other of the previous interpretations, all the experimental facts were accounted for, without noticeable contradiction, by the hypothesis of a multiple mechanism which contains both interpretations among its elements.     

Microwave-assisted organocatalytic anomerization of alpha-C-glycosylmethyl aldehydes and ketones

The use of l-proline (30 mol %) and MW irradiation (13 W) with cooling promotes in a few hours the almost quantitative anomerization of alpha-C-glycosylmethyl aldehydes into beta-isomers. An open-chain enamine-based mechanism is postulated for this transformation. The anomerization of alpha-ketones was instead achieved by the pyrrolidine/TFA couple and MW irradiation at 120 degrees C (enamine mechanism) and by DBU as Br?nsted base (enolate mechanism).     

Mechanism of the rhodium-catalyzed asymmetric isomerization of allylamines to enamines

A theoretical study of the mechanism of the rhodium-catalyzed asymmetric isomerization of allylamines to enamines by using density functional theory with the B3LYP functional leads us to discard the so far accepted nitrogen-triggered mechanism, in which the isomerization occurs on N-bonded intermediates and transition states, in favor of a variation of the classical allylic mechanism for olefin isomerization. The modified allylic mechanism consists of four main steps: 1) N-coordination of the allylamine to Rh(I); 2) intramolecular isomerization from kappa(1)-(N)-coordination to eta(2)-(C,C)-coordination of the allylamine; 3) oxidative addition of C(1)--H to form a distorted octahedral eta(3)-allyl complex of Rh(III); and 4) hydrogen transfer to C(3) (reductive C(3)--H elimination). The two hydrogen transfer steps (oxidative addition and reductive elimination) have the highest barriers of the overall process. The oxidative addition barrier, which includes solvent effects, is 28.4 kcal mol(-1). For the reductive elimination, the value in solvent is 28.6 kcal mol(-1), very similar to the oxidative addition barrier.     

Mechanistic investigation of oxidation of glycine and alanine by bis(dihydrogen-tellurto)argentite(III) ion in alkaline medium. A kinetic study

The oxidation of glycine and alanine by bis(dihydrogen-tellurto)argentite(III) ion was studied by stopped-flow spectrophotometery. The reaction is of first order in Ag(III) complex and has less than unit order in glycine and alanine. A plausible mechanism was proposed form the kinetic studies. The rate equations derived from mechanism found to explain all experimental phenomena. The activation parameters with respect to the rate determining step of the mechanism were calculated.     

Relativistic DFT study on the reaction mechanism of second-row transition metal Ru with CO2

To estimate the importance of relativistic effects on the reaction mechanisms between Ru and CO2, the potential energy surfaces have been performed in the triplet and quintet electronic states using quasi-relativistic (Pauli), zero-order regularly approximated (ZORA), and nonrelativistic (NR) density functional theory (DFT) at the PW91/TZP level. The results demonstrate that there are two rival reaction mechanisms: one is an addition mechanism and the other is an insertion mechanism in the triplet state. The only mechanism in the quintet state is the insertion mechanism. The most favored reaction mechanism in Ru + CO2 is that the Ru atom in its ground state first attacks the CO bond of CO2, forming q-Ru(CO)O (5A') with the insertion mechanism, and then undergoes an intersystem crossing to t-Ru(CO)O (3A'). Then it crosses t-TS3 to produce t-ORuCO molecule. The relativistic effects are important for reactivity of the second-row transition metal to CO2. In the key step of t-Ru(CO)O via t-TS3 to t-ORuCO, relativistic effects reduce the barrier energy by 10.3 kcal/mol, which is nearly half the nonrelativistic barrier energy.     

Theory favors a stepwise mechanism of porphyrin degradation by a ferric hydroperoxide model of the active species of heme oxygenase

The report uses density functional theory to address the mechanism of heme degradation by the enzyme heme oxygenase (HO) using a model ferric hydroperoxide complex. HO is known to trap heme molecules and degrade them to maintain iron homeostasis in the biosystem. The degradation is initiated by complexation of the heme, then formation of the iron-hydroperoxo species, which subsequently oxidizes the meso position of the porphyrin by hydroxylation, thereby enabling eventually the cleavage of the porphyrin ring. Kinetic isotope effect studies indicate that the mechanism is assisted by general acid catalysis, via a chain of water molecules, and that all the events occur in concert. However, previous theoretical treatments indicated that the concerted mechanism has a high barrier, much higher than an alternative mechanism that is initiated by O-O bond homolysis of iron-hydroperoxide. The present contribution studies the stepwise and concerted acid-catalyzed mechanisms using H(3)O(+)(H(2)O)(n)(), n = 0-2. The effect of the acid strength is tested using the H(4)N(+)(H(2)O)(2) cluster and a fully protonated ferric hydroperoxide. All the calculations show that a stepwise mechanism that involves proton relay and O-O homolysis, in the rate-determining step, has a much lower barrier (>10 kcal/mol) than the corresponding fully concerted mechanism. The best fit of the calculated solvent kinetic isotope effect, to the experimental data, is obtained for the H(3)O(+)(H(2)O)(2) cluster. The calculated alpha-deuterium secondary kinetic isotope effect is inverse (0.95-0.98), but much less so than the experimental value (0.7). Possible reasons for this quantitative difference are discussed. Some probes are suggested that may enable experiment to distinguish the stepwise from the concerted mechanism.     

Microsomal P450-catalyzed N-dealkylation of N,N-dialkylanilines: evidence for a C(alpha)-H abstraction mechanism

The early proposal that P450-catalyzed N-dealkylation of N,N-dialkylamines proceeds through a single-electron-transfer (SET) mechanism was later challenged in favor of the C(alpha)-H abstraction mechanism. In the present study, a series of N-alkyl-N-cyclopropyl-p-chloroaniline probes have been used to examine whether the P450-catalyzed N-dealkylations proceed through a C(alpha)-H abstraction and/or a SET mechanism, using phenobarbital-induced rat liver microsomal P450 enzymes as a model system. While the findings are highly consistent with a C(alpha)-H abstraction mechanism, further experimental evidence may be necessary to completely rule out the SET mechanism.     

荧光光谱法研究卟啉与金属离子配位反应机理初探

本文用荧光光谱法初步研究了卟啉与金属离子配位反应机理和部分催化剂的催化机理.实验发现,在一定条件下,卟啉以一种与其主要存在形式不同的变形体H₂P^*存在,根据H₂P^*的存在和产生的条件,对卟啉与金属离子配位反应的一般条件作出了较为满意的阐述.