亚硝基苯与甲醛的反应机理和溶剂效应的理论研究

采用密度泛函理论方法RB3LYP/6-311＋＋G(d,p)研究了亚硝基苯与甲醛在单重态势能面上分别在气相和溶剂中的反应机理. 找到两条反应通道: 协同机理和分步机理, 均生成实验产物N-苯基氧肟酸C6H5NOHCHO. 计算结果表明: 亚硝基苯与甲醛在气相中分步机理为主要通道. 采用导电极化连续介质模型研究了反应体系在水、乙醇、乙腈、二氯甲烷、四氢呋喃、环己烷溶液中反应的溶剂化效应, 这些溶剂可降低反应的活化能, 但反应对溶剂的极性不敏感. 无论在气相还是溶剂中, 亚硝基苯与甲醛的分步机理为优势通道.     

Validation of the existence of tetrameric species of potassium trimethylsilanolate in the gas phase with a theoretical cluster model: role of the counterion as charge localizer in the structure

We have investigated, theoretically, the structural properties of potassium trimethylsilanolate in the gas phase at a B3LYP/6-31+G* level. For this purpose, a simplified ionic cluster model based on potassium trimethylsilanolate tetramers, proposed in the literature as the structural units of this compound in the solid state, was developed. Furthermore, we compared the validity of the model with two simpler ones: a monomer of potassium trimethylsilanolate and a trimethylsilanolate anion in the gas phase. The developed ionic cluster model was found to be best in reproducing the experimental structure of potassium trimethylsilanolate, supporting, at the same time, the existence of such tetrameric species (previously identified experimentally from mass spectrometry data by Weiss et al.) in the gas phase. Finally, NBO calculations highlighted the important role of the potassium counterion as a charge localizer in the structure of these chemical species.     

Theoretical study of reaction mechanisms for the ketonization of vinyl alcohol in gas phase and aqueous solution

Theoretical ab initio calculations are done on different mechanisms for the conversion of vinyl alcohol to acetaldehyde, both in gas phase and in solution. Several basis sets are used in order to assess the accuracy of the results in gas phase and a continuum model of the solvent is employed to mimic reactions in water solution. The results indicate a catalytic action of water in hydrated clusters in gas phase, whereas in solution, and within the error limits of our calculations, both neutral water-chain and ionic mechanisms appear to be equally probable. Finally, the action of acids or bases is tested through the analysis of the reaction of vinyl alcohol with H₃O⁺ and HO^–. The results of the calculations are shown to be in qualitative agreement with the experimental facts when 6-31++G basis set is used but not when either STO-3G or 4-31G basis sets are employed.     

亚硝基化合物与甲醛的反应机理和溶剂效应的理论研究

采用密度泛函方法B3LYP/6-311++G(d,p)研究了亚硝基苯C₆H₅-NO和2-甲基-2-亚硝基丙烷(CH₃)₃C-NO与甲醛分别在气相和溶剂中的反应机理. 在气相中均找到两条反应通道, 即协同机理和分步机理, 均生成实验产物氧肟酸, 而且分步机理均为优势通道; 除2-甲基-2-亚硝基丙烷的反应没有协同途径外, 在溶剂中反应机理与气相中的类似. 采用导电极化连续介质模型分别研究了在乙腈与水溶液中反应的溶剂化效应, 发现这些溶剂可降低反应的活化能, 但降低的程度比较小, 反应速率变化不大.     

Sulfur 1s near-edge x-ray absorption fine structure (NEXAFS) of thiol and thioether compounds

The speciation and quantification of sulfur species based on sulfur K-edge x-ray absorption spectroscopy is of wide interest, particularly for biological and petroleum science. These tasks require a firm understanding of the sulfur 1s near-edge x-ray absorption fine structure (NEXAFS) spectra of relevant species. To this end, we have examined the gas phase sulfur 1s NEXAFS spectra of a group of simple thiol and thioether compounds. These high-resolution gas phase spectra are free of solid-state broadening, charging, and saturation effects common in the NEXAFS spectra of solids. These experimental data have been further analyzed with the aid of improved virtual orbital Hartree-Fock ab initio calculations. The experimental sulfur 1s NEXAFS spectra show fine features predicted by calculation, and the combination of experiment and calculation has been used to improve assignment of spectroscopic features relevant for the speciation and quantification of the sulfur compounds.     

A theoretical study of the mechanism of phosphine-catalyzed hydroalkoxylation of methyl vinyl ketone

The mechanism of phosphine-catalyzed hydroalkoxylation of the methyl vinyl ketone has been investigated by the second-order M?ller-Plesset perturbation theory and the conductor-like polarized continuum model. The free energy reaction profiles of the reaction in both gas phase and solution phase are explored and compared. Our results suggest that the first stage of the studied reaction is the generation of the base (the methoxide anion) with the help of trialkylphosphine, and the second stage is the hydroalkoxylation of the methyl vinyl ketone catalyzed by this base. In the first stage, trialkylphosphine first adds to the methyl vinyl ketone to form a phosphonium enolate intermediate and then this species deprotonates a methanol molecule to generate a methoxide anion. Both steps involve free energy barriers of about 20 kcal/mol. In the second stage, both the addition of the methoxide anion to the methyl vinyl ketone and the proton transfer process from methanol to the methoxyl enolate anion intermediate have activation free energies of about 16 kcal/mol. The reaction in the second stage is exothermic by 10.2 kcal/mol at room temperature. A comparison of the free energy reaction profiles in the gas phase and the solution phase demonstrated that the generation of the methoxide anion could only occur in the presence of the polar solvents. The mechanism proposed in the present work is in reasonable agreement with the known experimental facts.     

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).     

Thiolysis and alcoholysis of phosphate tri- and monoesters with alkyl and aryl leaving groups. An ab initio study in the gas phase

Phosphate esters are important compounds in living systems. Their biological reactions with alcohol and thiol nucleophiles are catalyzed by a large superfamily of phosphatase enzymes. However, very little is known about the intrinsic reactivity of these nucleophiles with phosphorus centers. We have performed ab initio calculations on the thiolysis and alcoholysis at phosphorus of trimethyl phosphate, dimethyl phenyl phosphate, methyl phosphate, and phenyl phosphate. Results in the gas phase are a reference for the study of the intrinsic reactivity of these compounds. Thiolysis of triesters was much slower and less favorable than the corresponding alcoholysis. Triesters reacted through an associative mechanism. Monoesters can react by both associative and dissociative mechanisms. The basicity of the attacking and leaving groups and the possibility of proton transfers can modulate the reaction mechanisms. Intermediates formed along associative reactions did not follow empirically proposed rules for ligand positioning. Our calculations also allow re-interpretation of some experimental results, and new experiments are proposed to trace reactions that are normally not observed, both in the gas phase and in solution.     

A study of gas phase and surface formaldehyde polymerisation from first principles

First principles computational studies have been carried out on the gas phase and surface polymerisation of formaldehyde. Any gas phase polymerisation in dark conditions is shown to occur by way of neutral species, as opposed to earlier suggestions that a charged species may be involved. The resulting paraformaldehyde chains are shown to be stable only when fully bonded at both chain ends, as opposed to some of the earlier suggested mechanisms. Polymerisation is shown to be thermodynamically favourable in the gas phase, but kinetically limited. Derived rate constants are insufficient to account for experimental results, in agreement with prior studies suggesting that gas phase polymerisation actually occurs on surfaces rather than directly in the gas phase. The TiO(2) (110) surface is employed as a model surface to study formaldehyde adsorption and possible polymerisation mechanisms. Formaldehyde monomers are shown to weakly bind by way of coordinate bonding through the carbonyl group to surface Ti. A particularly strong dimerisation configuration is also found where the two ends of paraformaldehyde are terminated by a surface Ti and surface O, consistent with the earlier observation that paraformaldehyde chains are only stable when saturated at both ends.     

Ligand exchange chromatography: a vital dimension for the reliable characterization of heterocycles in crude oils and refined products

In the present study, we established a statistical distribution pattern of indigenous sulfur, nitrogen, and oxygen species in Arabian Heavy crude oil and its distilled fractions: naphtha, gas oil, and vacuum gas oil (VGO) using chemical derivatization with methyl iodide and subsequent characterization by positive electrospray Fourier transform mass spectrometry. It was observed that sulfur species for naphtha and gas oil were accumulated at lower double bond equivalent values and at lower carbon numbers compared to VGO, whereas crude oil encompassed a complete range of the sulfur species detected in all distilled fractions. Moreover, the use of alumina column chromatography and ligand exchange chromatography (LEC) on a palladium-bonded silica stationary phase revealed additional structural features of sulfur heterocycles in terms of condensed and non-condensed thiophenes. During LEC separation, in addition to sulfur heterocycles, interesting results were obtained for oxygen-containing compounds. Ortho-substituted alkyl phenols were separated from meta- and para-substituted alkyl phenols on a palladium-bonded silica stationary phase.     

Theoretical investigation of isotopic scrambling mechanisms in 2-chloroethyl methyl sulfide

Ab initio and semiempirical molecular orbital calculations have been applied to study the concerted and stepwise isotopic scrambling mechanisms of 2-chloroethyl methyl sulfide in the gas phase and in aqueous solution. The calculations reveal the structural details of the reactants, transition structures, and intermediates involved in this reaction and provide relative energy estimates. The concerted mechanism is found to be competitive with the stepwise mechanism in the gas phase, but the stepwise mechanism is favored in aqueous solution as no true transition structure for the concerted mechanism could be found using the solvation models. A combined approach of evaluating solvation energies with the generalized-Bom-plus-surface-tensions SM _x solvation models of Cramer and Truhlar at ab initio optimized geometries is found to deliver the best agreement with experimentally determined reaction barriers. Together with the recent experimental results of McManus and co-workers, the present study provides insights into the controlling factors involved in the elementary reaction steps of sulfur mustards and a solid foundation for investigations into more complex reactions of related compounds.     

Iodine oxidation by hydrogen peroxide and Bray-Liebhafsky oscillating reaction: effect of the temperature

This work presents a new experimental kinetic study at 39° and 50° of the iodine oxidation by hydrogen peroxide. The results allow us to obtain the temperature effect on the rate constants previously proposed at 25° for our model of the Bray-Liebhafsky oscillating reaction (G. Schmitz, Phys. Chem. Chem. Phys. 2010, 12, 6605.). The values calculated with the model are in good agreement with many experimental results obtained under very different experimental conditions. Numerical simulations of the oscillations observed formerly by different authors are presented, including the evolutions of the iodine, hydrogen peroxide, iodide ions and oxygen concentrations. Special attention is paid to the perturbing effects of oxygen and of the iodine loss to the gas phase.     

Unusual solvent effect on a SN2 reaction. A quantum-mechanical and kinetic study of the Menshutkin reaction between 2-amino-1-methylbenzimidazole and iodomethane in the gas phase and in acetonitrile

The quaternization reaction between 2-amino-1-methylbenzimidazole and iodomethane was investigated in the gas phase and in liquid acetonitrile. Both experimental and theoretical techniques were used in this study. In the experimental part of this work, accurate second-order rate constants were obtained for this reaction in acetonitrile from conductivity data in the 293-323 K temperature range and at ambient pressure. From two different empirical equations describing the effect of temperature on reaction rates, thermodynamic functions of activation were calculated. In the theoretical part of this work, the mechanism of this reaction was investigated in the gas phase and in acetonitrile. Two different quantum levels (B3LYP/[6-311++G(3df,3pd)/LanL2DZ]//B3LYP/[6-31G(d)/LanL2DZ] and B3LYP/[6-311++G(3df,3pd)/LanL2DZ]//B3LYP/[6-31+G(d)/LanL2DZ]) were used in the calculations, and the acetonitrile environment was modeled using the polarized continuum model (PCM). In addition, an atoms in molecules (AIM) analysis was made aiming to characterize possible hydrogen bonding. The results obtained by both techniques are in excellent agreement and lead to new insight into the mechanism of the reaction under examination. These include the identification and thermodynamic characterization of the relevant stationary species, the rationalization of the mechanistic role played by the solvent and the amine group adjacent to the nucleophile nitrogen atom, the proposal of alternative paths on the modeled potential energy surfaces, and the origin of the marked non-Arrhenius behavior of the kinetic data in solvent acetonitrile. In particular, the AIM analysis confirmed the operation of intermolecular hydrogen bonds between reactants and between products, both in the gas phase and in solution. It is also concluded that the unusual solvent effect on this Menshutkin reaction stems from the conjunction of a nucleophile possessing a relatively complex chemical structure with a dipolar aprotic solvent that is protophobic.     

Development of a simple desulfurization procedure for the determination of butyltins in complex sediment samples using gas chromatography-pulsed flame photometric detection

In this study a rapid solid phase extraction (SPE) procedure was developed to minimize the effect of different sulfur species for the determination of butyltin in sediments. The organosulfur species and organotins were firstly retained on C8 cartridges and then organotins were selectively eluted and analyzed by gas chromatography-pulsed flame photometric detection (GC-PFPD). Optimal conditions for the SPE procedure were obtained using an experimental design approach. The method's accuracy was established by analyzing a certified reference material (CRM), BCR-646 freshwater sediment. The experimental values were found to be in agreement with the assigned values for butyltins. Finally, complex sediment samples collected from a Chilean harbor were analyzed using this methodology to demonstrate its analytical potential for the determination of butyltin in environmental samples.     

A computational study on the decomposition of formic acid catalyzed by (H2O)x, x = 0-3: comparison of the gas-phase and aqueous-phase results

The mechanisms for the water-catalyzed decomposition of formic acid in the gas phase and aqueous phase have been studied by the high-level G2M method. Water plays an important role in the reduction of activation energies on both dehydration and decarboxylation. It was found that the dehydration is the main channel in the gas phase without any water, while the decarboxylation becomes the dominant one with water catalyzed in the gas phase and aqueous phase. The kinetics has been studied by the microcanonical RRKM in the temperature range of 200-2000 K. The predicted rate constant for the (H 2O) 3-catalyzed decarboxylation in the aqueous phase is in good agreement with the experimental data. The calculated CO 2/CO ratio is 200-74 between 600-700 K and 178-303 atm, which is consistent with the average ratio of 121 measured experimentally by Yu and Savage (ref 3).     

Experimental and mechanism studies on simultaneous desulfurization and denitrification from flue gas using a flue gas circulating fluidized bed

The oxidizing highly reactive absorbent was prepared from fly ash,industry lime,and an oxidizing additive M.Experiments of simultaneous desulfurization and denitrification were carried out in a flue gas circulating fluidized bed(CFB).The effects of influencing factors and calcium availability were also investigated on the removal efficiencies of desulfurization and denitrification.Removal efficiencies of 95.5%for SO2 and 64.8%for NO were obtained respectively under the optimal experimental conditions. The component of the spent absorbent was analyzed with chemical analysis methods.The results in- dicated that more nitrogen species appeared in the spent absorbent except sulfur species.A scanning electron microscope(SEM)and an accessory X-ray energy spectrometer were used to observe micro-properties of the samples,including fly ash,oxidizing highly reactive absorbent and spent absorbent.The simultaneous removal mechanism of SO2 and NO based on this absorbent was pro- posed according to the experimental results.     

Mechanism of thioredoxin-catalyzed disulfide reduction. Activation of the buried thiol and role of the variable active-site residues

Thioredoxins (Trx) are enzymes with a characteristic CXYC active-site motif that catalyze the reduction of disulfide bonds in other proteins. We have theoretically explored this reaction mechanism, both in the gas phase and in water, using density functional theory. The mechanism of disulfide reduction involves two consecutive thiol-disulfide exchange reactions, that is, nucleophilic substitutions at sulfur (S(N)2@S): first, by one Trx cysteine-thiolate group (Cys-32) at a sulfur atom of the disulfide substrate and, second, by the other Trx cysteine-thiolate group (the buried thiol of Cys-35) at the sulfur atom of the first Trx cysteine. We have investigated the intrinsic nature of such S(N)2@S substitution using the simple CH3S(-) + CH3SSCH3 model and how it is affected by solvation in aqueous solution. Next, we have examined how the behavior of the elementary S(N)2@S steps changes in the more realistic enzyme-substrate model CGPC + CH3SSCH3, which contains the active-site of Trx. In all model reactions, solvation turns the hypervalent trisulfide anion (i.e., the S(N)2@S transition species) from a stable complex into a transition state. Importantly, our analyses suggest that the deprotonation of the buried thiol (which is required before the latter can enter into the second S(N)2@S step) is done by the leaving group evolving from the first S(N)2@S step. Finally, molecular dynamics (MD) simulations, in the gas phase and in water, of CGPC, CGGC, and the corresponding wild-type Trx and P34G Trx show that the activity of the thioredoxin active-site motif (CXYC) is determined not only by the structural rigidity associated with the particular variable residues (XY) but also by the number of amide N-H groups. The latter are involved in the stabilization of the Cys-32 thiolate and thus affect the acidity and nucleophilicity of this residue.     

Radiation-induced copolymerization of ethylene and sulfur dioxide in the liquid and gas phases

The radiation-induced copolymerization of ethylene and sulfur dioxide has been studied in the liquid and gas phases. In the liquid phase, the copolymer composition remained equimolar over a temperature range of 20–160°C. and ethylene pressures of 50–680 atm. The rate of copolymerization in the liquid phase at 680 atm. increased with temperature to a maximum value at ～80°C. Above this temperature the rate steadily decreased to zero at 157°C. because of temperature-dependent depropagation reactions. In the gas phase, copolymers were formed that contained from 9 to 46 mole-% sulfur dioxide. Under constant conditions of temperature, pressure, and radiation intensity, the copolymerization rate in the gas phase increased with increasing sulfur dioxide in the initial gas mixture. The propagating species for the liquid-phase experiments is considered to consist of an equimolar complex molecule of ethylene and sulfur dioxide. For gas mixtures containing an excess molar concentration of ethylene, the propagating species are ethylene and the complex molecule. Infrared spectra show polysulfone structures. Calorimetric and x-ray diffraction analyses indicate crystalline structures for copolymers in the range 9–50 mole-% sulfur dioxide, although a melt transition temperature could not be observed for copolymer containing >31 mole-% sulfur dioxide. Clear uniform film was obtained with copolymers containing up to 31 mole-% SO₂.     

Chemical reactions in clouds

Summary In contrast to a fairly extensive knowledge of gas phase chemical reactions in the atmosphere, our current understanding of the chemistry in the aqueous phase of clouds is still inadequate. Particularly for continental clouds the difficulties arise primarily from uncertainties in reaction mechanisms for the oxidation of various sulfur(IV) species originating from the dissolution of sulfur dioxide in cloud water and the role of the ions of transition metals in this oxidation process. The importance of OH and other radicals in gas-phase reactions has led to models, in which radical reactions are held responsible for much of the chemical change also in the liquid phase. This viewpoint has gained support specifically from the identification of iron(III)-hydroxo complexes as a photochemical source of OH radicals in continental clouds. Their reactions with sulfur(IV) compounds in the aqueous phase initiate chain processes, which are currently being examined by laboratory studies.     

气相离子-分子反应B2H3-+CS2——B2H3S-+CS的理论研究

用密度泛函方法B3LYP/6-311++G(d,p)和高级电子相关的偶合簇法CCSD(T)/6-311++G(d,p)研究了气相离子-分子反应B₂H₃^-+CS₂B₂H₃S^-+CS的机理.结果表明,B₂H₃最可能进攻CS₂中碳原子形成三元环中间体,随后通过氢迁移和最终消除CS的反应步骤形成硫原子转移产物H₃BBS^-+CS,反应大量放热且不需要活化能.B₂H₃直接对CS₂中硫原子进攻夺取硫原子的反应方式存在一定能垒阻碍.计算结果有助于深入了解B₂H₃,B₃H-6和B₄H₇^-等缺电子硼氢负离子的反应行为.