Double Bond Migration Mechanism in Allyl Systems Involving the Hydroxide Ion. 3. Supermolecular Approach

The mechanism of a proton transfer in an allyl system involving the hydroxide ion was investigated by the RHF/6-31+G* and MP2/6-31+G* methods with one, two, and four water molecules (water regarded as a solvent) included in the calculation; in the fourth model, the reactant system and two H₂O molecules were placed into continuum. A possible mechanism of intramolecular proton transfer involving easy exchange of water molecules between the first and second coordination spheres of the propenide ion is considered. The results are compared with those obtained with gas-phase and polarizable continuum models.     

Double Bond Migration Mechanism in Allyl Systems Involving the Hydroxide Ion. 1. Gas-Phase and Born–Onsager Models

The reaction profile of the 1,3-prototropic rearrangement of propene involving the hydroxide ion was studied by the RHF/6-31+G*, MP2/6-31+G*, and B3LYP/6-31+G* ab initio methods within the framework of the gas-phase and Born–Onsager models (the latter including solvent effects). Propene isomerization in the presence of the hydroxide ion in the gas phase may occur with participation of a base proton with the intermediate formation of a water complex of the allyl ion. The transition state energy of this transformation is lower than the total energy of the starting hydroxide ion and propene and much lower than the sum of the energies of the isolated propenide ion and water molecule. An activation barrier arises when the solvent effect is included in calculation within the framework of the Born–Onsager model; the intermediate complex is much less stable than the complex considered in the gas-phase model. As in the latter, the mechanism of multiple bond migration is energetically preferable to the mechanism involving proton transfer to the reaction medium.     

分子筛催化cis-2-丁烯的双键异构反应机理的DFT研究

基于含有两个Si和一个Al的分子筛3T簇模型, 利用密度泛函方法(DFT)研究了分子筛催化1-丁烯双键异构为cis-2-丁烯的反应机理. 在B3LYP/6-31G(d,p)计算水平上对反应各驻点进行了全优化, 并计算了反应的活化能. 研究发现, 分子筛上的酸性OH基团首先通过物理吸附靠近1-丁烯的双键, 形成了π配位复合物后, 丁烯双键的端基C原子逐渐抽取这个质子, 同时相邻酸性位的一个O原子也抽取丁烯碳链上的一个H原子, 形成吸附态的cis-2-丁烯, 最后通过脱附形成产物, 使分子筛复原, 反应按照协同反应机理发生. 计算得到的表观活化能是55.9 kJ/mol, 与实验结果接近.     

<Emphasis Type="Italic">Ab initio</Emphasis> quantum-chemical study of the mechanism of methoxide ion formation in MOH/DMSO/CH<Subscript>3</Subscript>OH systems (M = Li,Na, K)

The profile of the reaction CH₃OH + MOH → CH₃OM + H₂O in the presence of an alkali (MOH, M = Li, Na, K) was investigated by the ab initio quantum-chemical method for the gas phase (with allowance for the solvent) within the continuum model. The proton transfer and the formation of the alkaline methoxide molecule in MOH/DMSO/CH₃OH systems (M = Li, Na, K) in the alkali-methanol pre-reaction complexes can take place without their preliminary dissociation and are barrier-free reactions.     

On the Origin of the Distinctly Different Reactivity of Ruthenium in [MO]+/CH4 Systems (M=Fe,Ru, Os)

The thermal gas‐phase reactions of [RuO]⁺ with methane have been explored by FT‐ICR mass spectrometry and high‐level quantum‐chemical calculations. In contrast to the previously studied [FeO]⁺/CH₄ and [OsO]⁺/CH₄ couples, which undergo oxygen/hydrogen atom transfers and dehydrogenation, respectively, the [RuO]⁺/CH₄ system produces selectively [Ru(CH)₂]⁺ and H₂O, albeit with much lower efficiency. Various mechanistic scenarios were uncovered, and the associated electronic origins were revealed by high‐level quantum‐chemical calculations. The reactivity differences observed for the [MO]⁺/CH₄ couples (M=Fe, Ru, Os) are due to the subtle interplay of the spin–orbit coupling efficiency, orbital overlap, and relativistic effects.     

Mechanism of Dissolution of a Lithium Salt in an Electrolytic Solvent in a Lithium Ion Secondary Battery: A Direct Ab Initio Molecular Dynamics (AIMD) Study

The mechanism of dissolution of the Li⁺ ion in an electrolytic solvent is investigated by the direct ab initio molecular dynamics (AIMD) method. Lithium fluoroborate (Li⁺BF₄^?) and ethylene carbonate (EC) are examined as the origin of the Li⁺ ion and the solvent molecule, respectively. This salt is widely utilized as the electrolyte in the lithium ion secondary battery. The binding of EC to the Li⁺ moiety of the Li⁺BF₄^? salt is exothermic, and the binding energies at the CAM–B3LYP/6‐311++G(d,p) level for n=1, 2, 3, and 4, where n is the number of EC molecules binding to the Li⁺ ion, (EC)_n(Li⁺BF₄^?), are calculated to be 91.5, 89.8, 87.2, and 84.0 kcal mol^?1 (per EC molecule), respectively. The intermolecular distances between Li⁺ and the F atom of BF₄^? are elongated: 1.773 Å (n=0), 1.820 Å (n=1), 1.974 Å (n=2), 1.942 Å (n=3), and 4.156 Å (n=4). The atomic bond populations between Li⁺ and the F atom for n=0, 1, 2, 3, and 4 are 0.202, 0.186, 0.150, 0.038, and 0.0, respectively. These results indicate that the interaction of Li⁺ with BF₄^? becomes weaker as the number of EC molecules is increased. The direct AIMD calculation for n=4 shows that EC reacts spontaneously with (EC)₃(Li⁺BF₄^?) and the Li⁺ ion is stripped from the salt. The following substitution reaction takes place: EC+(EC)₃(Li⁺BF₄^?)→(EC)₄Li⁺?(BF₄^?). The reaction mechanism is discussed on the basis of the theoretical results.     

Mechanism of reaction of n-butane with but-2-enes in the presence of LaCaX faujasites

The reactions of n-butane and an n-butane (80 mol.%)—isobutane (20 mol.%) mixture with but-2-enes in the presence of polycationic PdLaCaX faujasites were studied. Quantum-chemical calculation of the enthalpies of formation of alkanes C₄—C₈ and their cations showed that the reaction [Bnⁿ]⁺ [Buⁱ]⁺ is of crucial importance for the isomeric composition of the products of alkane alkylation. The general scheme of transformations of the hydrocarbons in the alkylation of n- and iso-paraffins was proposed based on the experimental data on the distribution of the C₈ isomers in the catalyzate at different temperatures and duration of the reaction.     

Synthesis and transformations of metallacycles. 27. Quantum-chemical study of the mechanism of styrene catalytic cyclometallation with triethylaluminum in the presence of Cp2ZrCl2

Cyclometallation of olefins with AlEt₃ in the presence of Cp₂ZrCl₂ affording five-membered monosubstituted cyclic organoaluminum compounds, was studied by the semiempirical quantum-chemical PM3 method. The proposed model of catalytic cycloalumination of styrene catalyzed by Cp₂ZrCl₂ is in a good accordance with experimental data obtained by dynamic NMR spectroscopy.     

Mechanism of the O2(1Δg) generation from the Cl2/H2O2 basic aqueous solution explored by the combined ab initio calculation and nonadiabatic dynamics simulation

In the present work, mechanism of the O₂(¹Δ_g) generation from the reaction of the dissolved Cl₂ with H₂O₂ in basic aqueous solution has been explored by the combined ab initio calculation and nonadiabatic dynamics simulation, together with different solvent models. Three possible pathways have been determined for the O₂(¹Δ_g) generation, but two of them are sequentially downhill processes until formation of the OOCl⁻ complex with water, which are of high exothermic character. Once the complex is formed, singlet molecular oxygen is easily generated by its decomposition along the singlet-state pathway. However, triplet molecular oxygen of O₂() can be produced with considerable probability through nonadiabatic intersystem crossing in the ¹Δ_g/ intersection region. It has been found that the coupled solvent, heavy-atom, and nonadiabatic effects have an important influence on the quantum yield of the O₂(¹Δ_g) generation. © 2018 Wiley Periodicals, Inc.     

Geometry of intramolecular silicon complexes: an ab initio estimation of the sensitivity to the effect of medium. (Aroyloxymethyl)trifluorosilanes

The effect of medium on the geometry of the (benzoyloxymethyl)trifluorosilane (1) molecule was studied by the HF and MP2 methods with the 6-31G(d), 6-311G(d), and 6-311+G(d,p) basis sets, as well as using the Onsager SCRF model, the PCM approach, and the data of X-ray diffraction study. Molecule 1 has a low complexation energy (5.4 kcal mol^–1 according to MP2(Full)/6-31G(d)+ZPE calculations and 6.8 kcal mol^–1 according to IR spectroscopy data), while its geometric parameters are the least sensitive to the effects of medium among all hypervalent silicon compounds studied to date. Nevertheless, the results obtained revealed a pronounced deformation of the SiO coordination bond in 1 on going from the gas phase to the polar solution and crystal. This serves as a theoretical substantiation of the hypothesis that substantial changes in the IR and NMR spectral characteristics of the (O—Si)-chelate (aroyloxymethyl)trifluorosilanes upon variation of external factors are due to geometric reasons. Analysis of the electron density distribution according to Bader indicates that the SiO bond in molecule 1 can be treated as a covalent bond of high polarity.     

镁铝双金属氢氧化物及氧化物去除硫离子(S2-)性能及其机理研究

利用层状双金属氢氧化物(LDHs)层间阴离子的可交换性及其结构记忆效应，研究了氯离子插层镁铝双金属氢氧化物(MgAl-Cl-LDHs)及镁铝双金属氧化物(MgAl-LDO)去除S^2-的性能及机理。采用X射线粉末衍射(XRD)、红外光谱(FTIR)、紫外-可见光谱(UV-Vis)和电感耦合等离子体原子发射光谱(ICP)等表征技术，详细探讨了焙烧复原法及离子交换法得到的不同产物的结构和去除S^2-性能，提出了MgAl-Cl-LDHs和MgAl-LDO对S^2-的不同去除机理。结果表明，MgAl-LDO具有较MgAl-Cl-LDHs更为优良的去除S^2-性能。文中所有去除实验的pH值均为9，此时S^2-发生水解且主要以HS-存在于溶液中。采用MgAl-Cl-LDHs为前体时，HS^-的离子交换能力比Cl^-弱，不能通过与Cl^-交换进入层间，S^2-的去除机理主要是表面吸附HS^-及将其氧化成S₂O₃^2-。以MgAl-LDO为前体时，HS^-通过LDO结构恢复能迅速进入层间，且受MgAl-LDO的催化氧化性能的影响，在结构恢复的同时HS-被氧化成S₂O₃^2-、S^2-和S^3-，可以高效去除溶液中的S^2-。n_Mg/n_Al比为2的LDHs在500 ℃下焙烧的产物MgAl-LDO对S^2-的去除率能够达到98%以上，以MgAl-LDO去除S^2-的方法可作为一种高效除S^2-的方法。     

Theoretical modeling of the symmetric (C2v) electrophilic attachment of chlorine to ethylene in aqueous solution

The electrophilic attachment of chlorine to ethylene in aqueous solution is studied using the complete active space self‐consistent field (CASSCF) method combined with the polarizable continuum model in a version which includes electrostatic, repulsion, and dispersion solute–solvent interactions. The C_2v symmetry is maintained for all the geometries considered, and the active space is generated distributing six electrons in five orbitals. After the CASSCF calculation a valence bond (VB) analysis has been performed along an approximate reaction coordinate by projecting the wave function onto a set of four classical structures; a reliable explanatory model of the rearrangement of the electronic structure for this process is then derived. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 74: 59–67, 1999     

2-(氟代己酰胺基)-苯甲醛与NH3反应生成3-(1-亚胺基氟烷基)- 1H-喹啉-2-酮的反应机理理论研究

采用密度泛函方法(DFT)研究了2-(氟代己酰胺基)-苯甲醛与NH3的微观反应机理. 在B3LYP/6-31G*水平上优化了反应物、过渡态、中间体及产物的几何构型, 通过振动分析确认了过渡态的结构, 并用内禀反应坐标方法(IRC)确认反应途径. 应用分子中的原子理论(AIM)分析了这些物质的成键特征. 报道了可能的反应通道, 其中Re→TS1→ IM1→TS2→IM2→TS3→IM3→TS4→IM7→TS11→IM9→TS12→IM10→TS13→IM11→TS14→P1具有相对较低的活化能, 是反应的主要通道, 理论预测的主要产物与实验吻合. 采用连续介质模型(PCM)方法研究了反应体系的溶剂化效应, 结果表明反应过程中各物质的能量比气相要低. 溶剂化效应使转化能垒有一定程度的升高.     

Towards a tunable tautomeric switch in azobenzene biomimetics: implications for the binding affinity of 2-(4'-hydroxyphenylazo)benzoic acid to streptavidin

The tautomeric equilibria of 2-(4'-hydroxyphenylazo)benzoic acid (HABA) and 2-(3',5'-dimethyl-4'-hydroxyphenylazo)benzoic acid (3',5'-dimethyl-HABA) have been studied by a combination of spectroscopic and computational methods. For neutral HABA in solvents of different polarity (toluene, chloroform, DMSO, DMF, butanol, and ethanol) the azo tautomer (AT) is largely predominant. For monoanionic HABA, the hydrazone tautomer (HT) is the only detected species in apolar solvents such as toluene and chloroform, while the AT is the only detected species in water and a mixture of both tautomers is detected in ethanol. Comparison of the results obtained for HABA and its 3',5'-dimethylated derivative shows that dimethylation of the hydroxybenzene ring shifts the tautomeric preferences towards the hydrazone species. These findings have been used to examine the differences in binding affinity to streptavidin, as the lower affinity of HABA can be explained in terms of the larger energetic cost associated with the tautomeric shift to the bioactive hydrazone species. Overall, these results suggest that a balanced choice of chemical substituents, embedding environment, and pH can be valuable for exploitation of the azo-hydrazone tautomerism of HABA biomimetics in biotechnological applications.