A theoretical study on the mechanism of the addition reaction between carbene and epoxyethane

The mechanism of addition reaction between carbene and epoxyethane has been investigated employing the MP2 and B3LYP/6-311+G* levels of theory. Geometry optimization, vibrational analysis, and energy property for the involved stationary points on the potential energy surface have been calculated. Based on the calculated results at the MP2/6-311+G* level of theory, it can be predicted that there are two reaction mechanisms (1) and (2). In the first reaction carbene attacks the atom O of epoxyethane to form an intermediate 1a (IM1a), which is a barrier-free exothermic reaction. Then, IM1a can isomerize to IM1b via a transition state 1a (TS1a), where the potential barrier is 48.6 kJ/mol. Subsequently, IM1b isomerizes to a product epoxypropane (Pro1) via TS1b with a potential barrier of 14.2 kJ/mol. In the second carbene attacks the atom C of epoxyethane firstly to form IM2 via TS2a. Then IM2 isomerizes to a product allyl alcohol (Pro2) via TS2b with a potential barrier of 101.6 kJ/mol. Correspondingly, the reaction energies for the reactions (1) and (2) are −448.4 and −501.6 kJ/mol, respectively. Additionally, the orbital interactions are also discussed for the leading intermediate. The results based on the B3LYP/6-311+G* level of theory are paralleled to those on the MP2/6-311+G* level of theory. Furthermore, the halogen and methyl substituent effects of H₂C: on the two reaction mechanisms have been investigated. The calculated results indicate that the introductions of halogen or methyl make the addition reaction difficult to proceed.     

A theoretical determination of the electronic spectrum of formaldehyde

Summary The electronically excited states of formaldehyde are examined by means of multiconfigurational second-order perturbation (CASPT2) theory with extended ANO-type basis sets. The calculations comprised five valence excited states plus all singlet 3s, 3p, and 3d members of the Rydberg series converging on the first ionization. The computed vertical excitation energies were found to be within 0.2 eV of the available experimental energies. Full geometry optimization has been performed for five valence excited states. Assuming a planar geometry, the 0-0 transition for the valence¹A₁( *) state is calculated to appear near 7.9 eV, close to the (n _y 3p) region. This state is, however, not planar and the true adiabatic energy is 7.4 eV, which is 2.3 eV below the corresponding vertical transition.     

Gas-phase thermolysis reaction of formaldehyde diperoxide. Kinetic study and theoretical mechanisms

Gas-phase thermolysis reaction of formaldehyde diperoxide (1,2,4,5-tetroxane) was performed in an injection chamber of a gas chromatograph at a range of 463-503 K. The average Arrhenius activation energy and pre-exponential factor were 29.3 ± 0.8 kcal/mol and 5.2 × 10¹³ s⁻¹, respectively. Critical points and reaction paths of the ground singlet and first triplet potential energy surfaces (PES) were calculated, using DFT method at BHANDHLYP/6-311+G^∗∗ level of the theory. Also, G3 calculations were performed on the reactant and products. Reaction by the ground-singlet and first-triplet states turned out to be endothermic and exothermic, respectively. The mechanism in three steps seemed to be the most probable one. An electronically non-adiabatic process appeared, in which a crossing, at an open diradical structure, from the singlet to the triplet state PES occurred, due to a spin-orbit coupling, yielding an exothermic reaction. Theoretical kinetic constant coming from the non- adiabatic transition from the singlet to the triplet state agrees with the experimental values.     

Mechanism for the gas-phase reaction between formaldehyde and hydroperoxyl radical. A theoretical study

We present a high-level theoretical study on the gas-phase reaction between formaldehyde and hydroperoxyl radical carried out using the DFT-B3LYP, QCISD, and CCSD(T) theoretical approaches in connection with the 6-311+G(d,p), 6-311+G(2df,2p), and aug-cc-pVTZ basis sets. The most favorable reaction path begins with the formation of a pre-reactive complex and produces the peroxy radical CH(2)(OO)OH in a process that is computed to be exothermic by 16.8 kcal/mol. This reaction involves a process in which the oxygen terminal of the HO(2) moiety adds to the carbon of formaldehyde, and, simultaneously, the hydrogen of the hydroperoxyl group is transferred to the oxygen of the carbonyl in a proton-coupled electron-transfer mechanism. Our calculations show that this transition state lies below the sum of the energy of the reactants, and we computed a rate constant at 300 K of 9.29 x 10(-14) cm(3) molecule(-1) s(-1), which is in good agreement with the experimental results. Also of interest in combustion chemistry, we studied the hydrogen abstraction process by HO(2), the result of which is the formation of HCO + H(2)O(2). We found two reaction paths with activation enthalpies close to 12 kcal/mol. For this process, we computed a rate constant of 1.48 x 10(-16) cm(3) molecule(-1) s(-1) at 700 K, which also agrees quite well with experimental results.     

Using 2,4 DNPH-PVC membrane sensors for indirect determination of formaldehyde in urea glue and wastewater

Three simple, rapid, and sensitive ion-selective electrodes for indirect determination of free formaldehyde in urea glue and wastewater have been developed. The methods are based on the formation of the membrane sensors 2,4-dinitrophenylhydrazine-phosphtungestic acid (DNPH-PTA), 2,4-dinitrophenylhydrazine-phosphomolybdic acid (DNPH-PMA), and 2,4-dinitrophenylhydrazine-tetraphenylborate (DNPH-TPB) as neutral carriers. The sensors are stable and show fast potential responses of 30?s, and near-Nernstian cationic slopes of 56.2?±?0.5, 54.3?±?0.5, and 53.8?±?0.4?mV per decade of activity between pH 0.5 and 3.5 over a wide range of 2,4-dinitrophenylhydrazine concentrations (1?×?10^?5 to 1?×?10^?2?M). These sensors were used for indirect determination of formaldehyde over concentration range (1?×?10^?4 to 1?×?10^?1?M). The selectivity coefficients of the developed sensors indicate excellent selectivity for 2,4 DNPH over a large number of organic and inorganic species. The mediator o-nitrophenyloctyl ether has a significant affect on the lifetime of the fabricated sensors. The analytical applications of the proposed sensors showed good results for indirect determination of formaldehyde in formaldehyde solutions, wastewater solutions, and free formaldehyde in urea-formaldehyde liquid and powder glues. The results were compared favourably with that obtained by ASTM, colorimetric, and British Standard methods.     

小麦粉中甲醛(吊白块)的测定方法研究

对小麦粉中甲醛提取方法、甲醛与2,4-二硝基苯肼的衍生化反应条件以及甲醛衍生物的提取净化方法进行了研究, 建立了一种小麦粉中甲醛(吊白块)的高压液相色谱测定方法.该方法在甲醛浓度为0.026～0.832 μg/mL范围内与其衍生物的色谱峰面积呈显著线性相关, 相关系数r=0.9986. 小麦粉中甲醛不同添加量的平均回收率＞99%;吊白块不同添加量的平均回收率＞94%(以甲醛量计). 重复测定相对标准偏差平均为4.5%, 甲醛的检出限为9.6 μg/L, 相当于小麦粉中甲醛的最小检出量为0.24 mg/kg.     

A density functional study towards the preferential binding of anions to urea and thiourea

A DFT study predicted the trend towards binding of anions with simple urea/thiourea receptor units, which has been found to be in good agreement with experimentally observed results. Our results provide a trend for the interaction of anions of different shapes with urea/thiourea receptor molecules, which do not always follow the basicity scale of anions.     

An ab initio molecular orbital study on the addition reaction of triplet nitrene to ethylene

A minimal basis set, contracted from an extensive set of primitive Gaussian type functions (GTF), was used to expand the molecular orbitals (MO) within the framework of self consistent field (SCF) theory. The results revealed that aziridine is formed in its first excited triplet state (T ₁) when ethylene is reacted with triplet nitrene. The equilibrium geometry of aziridine in its (T ₁) state had a tetrahedral CCN bond angle.     

High-accuracy theoretical study on the thermochemistry of several formaldehyde derivatives

In the case of several formaldehyde derivatives, with importance in atmospheric and combustion chemistry, the currently available thermochemical values suffer from considerably large uncertainties. In this study a high-accuracy theoretical model chemistry has been used to provide accurate thermochemical data including heats of formation at 0 and 298 K and standard molar entropies at 298 K for CF(2)O, FCO, HFCO, HClCO, FClCO, HOCO, and NH(2)CO. For most of the thermochemical quantities studied here, this investigation delivers the best available estimate.     

N-4-戊烯基硝酮的分子内环加成反应的区域选择性的理论研究

用MINDO/3方法求出了N-4-戊烯基硝酮分子内环加成反应的过渡态和反应途径.两个环加成区域异构体是由N-4-戊烯基硝酮的两个不同的构象经过各自的过渡态得到的. 理论分析满意地解释了实验结果.     

Kaolinite–urea complexes obtained by mechanochemical and aqueous suspension techniques—A comparative study

Intercalation compounds of low- and high-defect kaolinites have been prepared by direct reaction with urea aqueous solution as well as by co-grinding with urea in the absence of water (mechanochemical intercalation). The complexes formed were studied by X-ray diffraction, thermal analysis, DRIFT spectroscopy, and scanning electron microscopy. In aqueous solution the degree of intercalation for the low- and high-defect kaolinites was found to be 77 and 65%, respectively. With mechanochemical intercalation, both kaolinites were almost fully expanded after 1 h of grinding. Based on the results of DRIFT spectroscopy, a structural model for the bonding of urea to the siloxane surface is proposed. The kaolinite–urea intercalation compounds produced by mechanochemical intercalation have crystallite sizes lower than those obtained by the aqueous solution method.     

A novel urea conductometric biosensor based on zeolite immobilized urease

A new approach was developed for urea determination where a thin film of silicalite and zeolite Beta deposited onto gold electrodes of a conductometric biosensor was used to immobilize the enzyme. Biosensor responses, operational and storage stabilities were compared with results obtained from the standard membrane methods for the same measurements. For this purpose, different surface modification techniques, which are simply named as Zeolite Membrane Transducers (ZMTs) and Zeolite Coated Transducers (ZCTs) were compared with Standard Membrane Transducers (SMTs). Silicalite and zeolite Beta with Si/Al ratios 40, 50 and 60 were used to modify the conductometric electrodes and to study the biosensor responses as a function of changing zeolitic parameters. During the measurements using ZCT electrodes, there was no need for any cross-linker to immobilize urease, which allowed the direct evaluation of the effect of changing Si/Al ratio for the same type of zeolite on the biosensor responses for the first time. It was seen that silicalite and zeolite Beta added electrodes in all cases lead to increased responses with respect to SMTs. The responses obtained from ZCTs were always higher than ZMTs as well. The responses obtained from zeolite Beta modified ZMTs and ZCTs increased as a function of increasing Si/Al ratio, which might be due to the increased hydrophobicity and/or the acid strength of the medium.     

A theoretical study of malononitrile addition to carbonyl compounds

The nucleophilic addition of the malononitrile anion (MN⁻) to formaldehyde was studied theoretically by the AM1 semiempirical MO method. The addition is found to be endothermic with a late productlike transition state on the reaction coordinate. Additions of MN⁻ to a series of carbonyl compounds were studied in order to investigate the substituent effect on the energetics of the title addition and the nucleophilic attack reactivity. The solvent effect was stimulated by hydrogen bonding a single molecule of water to the formaldehyde oxygen and/or to the MN⁻ anion. Its influence on the energetics and the transition-state geometry was estimated. The Hammond postulate was satisfied for the studied additions. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 62 : 419–426, 1997     

On the performance of time-dependent density functional theory and polarizable continuum model in the study of aqueous formaldehyde

In this paper Time-Dependent Density Functional Theory (TD-DFT) with Becke's three-parameter hybrid method using the Lee–Yang–Parr correlation functional (B3LYP) in conjunction with different basis sets is applied to calculate the solvatochromic shift on the lowest singlet transition of formaldehyde in vacuum and in solution. Our results, in excellent agreement with solvent excitation energy shift of acetone in water at 298–300 K (no experimental data on formaldehyde in water are available so far) and with previous high level quantum chemical calculations, help in appreciating the potentialities of the combined DFT/PCM approach on aquo-complexes for the study of structural and spectroscopic properties of molecule in solution.     

Molecular dynamics simulation study of the water-mediated interaction between zwitterionic and charged surfaces

We calculated the potential of mean force (PMF) for the interaction between a model zwitterionic bilayer and a model charged bilayer. To understand the role of water, we separated the PMF into two components: one due to direct interaction and the other due to water-mediated interaction. In our calculations, we observed that water-mediated interaction is attractive at larger distances and repulsive at shorter. The calculation of the entropic and enthalpic contributions to the solvent-mediated components of the PMF showed that attraction is entropically dominant, while repulsion is dominated by the enthalpy.     

A pyridine-based macrocyclic host for urea and acetone

A pyridine-based macrocycle with a polyether chain has been designed and synthesized. The macrocycle shows strong binding for acetone involving both conventional and unconventional hydrogen bonds. The acetone in the cavity is exchangeable in CHCl₃ by urea. This has been studied thoroughly by ¹H NMR, ¹³C NMR, mass and X-ray analyses.     

过氧化氢尿素加合物的热分解动力学

把H_2O_2转化成固态化H_2O_2化合物,能使H_2O_2的使用更加方便。在这类固态H_2O_化合物中,过碳酸钠、过硼酸钠等氧系漂白、杀菌剂在国外已得到广泛应用,在我国则处在开发试产阶段,我们已开发成功过氧化氢尿素加合物这一新型氧系漂白、杀菌剂,其活性氧含量比过碳酸钠过硼酸纳高,其性能更优越。为了考察过氧化氢尿素加合物的热稳定性,我们从热失重分析资料研究了过氧化氢尿素加合物的热分解动力学。     

A theoretical study of the reaction of lithium aluminum hydride with formaldehyde and cyclohexanone.

Geometries and energies of the reactants, complexes, and transition states for the reactions of lithium aluminum hydride with formaldehyde and cyclohexanone were obtained using ab initio and density functional (Becke3LYP/6-31G**) molecular orbital calculations. Two pathways for reaction with formaldehyde and four transition states corresponding to axial and equatorial attack at cyclohexanone were located. The transition state structures had reactant-like geometries. Predicted stereoselectivity of the reduction of cyclohexanone strongly favors axial approach of hydrogen, in agreement with experimental data. Analysis of the transition state structures suggests that electronic effects are more important than torsional effects in controlling stereoselectivity.