Simulation model for urea-formaldehyde reactors

A comprehensive simulation model for urea/formaldehyde (UF)-resins has been developed. The model utilizes kinetic results reported in the literature for low molecular weight systems and extends these to higher molecular weight systems by using a functional group approach. The model consists of - 9 functional groups, 5 urea groups and 4 formaldehyde groups. - 9 reactions, 5 methylolation reactions and 4 condensation reactions. In all these reactions both the formation and the decomposition reactions are included. All the kinetic constants are pH and temperature dependent. Two batch runs with different U/F ratios were used to adjust a couple of model parameters. Samples from these batch runs were analyzed with HPLC for low molecular weight compounds, and GPC for the average molecular weight. A run with continuous IR measurement showed excellent fit for the formaldehyde concentration. The model also estimates the viscosity from the functional groups.     

On the use of surface-confined molecular catalysts in fuel cell development

Fuel cells have the potential to provide sustainable clean energy but are hampered by the reliance on Pt as an electrocatalyst for half-cell reactions. Bio-inspired molecular catalysts, composed of Earth-abundant elements, have shown promise as alternative electrocatalysts for fuel oxidation and oxygen reduction reactions. This article provides a concise overview of recent progress in this area with particular focus on (i) electrodes modified with molecular catalysts for fuel oxidation and O₂ reduction and (ii) fuel cells incorporating surface-confined molecular catalysts for both anodic and cathodic reactions. Finally, the prospect and challenges of using molecular catalysts in fuel cell assemblies are discussed.     

A comparison of various MO methods for predicting regioselectivity in Diels-Alder reactions

Various semiempirical MO methods for predicting regioselectivity in Diels-Alder reactions are compared in order to examine their reliability. They are the frontier molecular orbital method using CNDO/2, HMO and MNDO molecular orbitals and the HMO approach on pericyclic reactions presented by Tang.     

On-surface synthesis of cyclic organic molecules

Creating or connecting together large organic molecules, as polycyclic aromatic hydrocarbons (PAH), by chemical reactions readily on surfaces is the first step to a true advance in the field of molecular electronics. On-surface synthesis can be regarded as an efficient means to build new molecular species by using bottom-up strategies. Recently, a collection of different reactions leading to large tailor-made organic molecules on single-crystal metal surfaces has been reported. The fundamental mechanisms controlling these reactions can be investigated from a surface science perspective. This discipline skillfully combines the use of characterization techniques at the nanoscale, with single-crystal metallic surfaces able to catalyse these reactions. We present a tutorial review that highlights the relevance of the new bottom up strategies and classifies most of the different molecular on-surface reactions involving aromatic organic molecules that have been published up to date.     

Dynamics of reactions between two closed-shell molecules

The crossed molecular beam technique has been utilized to investigate a large number of elementary reactions. However, most of the studied reactions involve atoms or radicals; reactions between two stable molecular reactants are, in fact, seldom studied with the crossed molecular beam method. In this perspective, reactions between two stable molecules are reviewed and discussed. With crossed molecular beams and vacuum UV photoionization, the nascent products have been unambiguously identified. Consistent pictures of the reaction paths have been constructed based on the experimental data and ab initio calculations. Furthermore, there are intriguing features about the reaction barriers. The F(2) + organosulfur reactions are barrierless, demonstrating the first examples of such interactions between two closed-shell reactants. The barrier of F(2) + alkene reaction decreases with more methyl substitution groups at the C=C double bond, yet the absolute barrier heights from experiment and theory disagree with each other by ~2 kcal mol(-1), leaving an issue to be resolved in the future.     

燃料电池电极表界面催化氧还原反应的STM研究进展

催化氧还原反应的电催化剂是燃料电池的一个重要组成部分. 从分子尺度研究催化氧还原反应中所涉及的表界面反应机理,不仅有利于深入理解催化机理,更有利于指导人们合理地设计新型的电催化剂. 本文结合近年来国内外的研究工作,概述了通过扫描隧道显微镜研究燃料电池内部催化氧还原反应过程中所涉及的表面形貌变化、单分子结构变化、中间体的观测以及反应产物调控等方面最新进展,并展望了该研究领域的发展趋势.     

Electrolysis of water in the diffusion layer: first-principles molecular dynamics simulation

With Car-Parrinello molecular dynamics simulations the elementary reaction steps of the electrolysis of bulk water are investigated. To simulate the reactions occurring near the anode and near the cathode, electrons are removed or added, respectively. The study focuses on the reactions in pure water. Effects depending on a particular electrode surface or a particular electrolyte are ignored. Under anodic conditions, the reaction continues till molecular oxygen is formed, under cathodic conditions the formation of molecular hydrogen is observed. In addition the formation of hydrogen peroxide is observed as an intermediate of the anodic reaction. The simulations demonstrate that the electrochemistry of oxygen formation without direct electrode contact can be explained by radical reactions in a solvent. These reactions may involve the intermediate formation of ions. The hydrogen formation is governed by rapid proton transfers between water molecules.     

Molecular Networks in Dynamic Multilevel Systems

Dynamic multilevel systems can be assembled from molecular building blocks through two or more reversible reactions that form covalent bonds. Molecular networks of dynamic multilevel systems can exhibit different connectivities between nodes. The design and creation of molecular networks in multilevel systems require control of the crossed reactivity of the functional groups (how to connect nodes) and the conditions of the reactions (when to connect nodes). In recent years, the combination of orthogonal and communicating reactions, which can be simultaneous or individually activated, has produced a variety of systems that have given rise to macrocycles and cages, as well as molecular motors and multicomponent architectures on surfaces. A given set of reactions can lead to systems with unique responsiveness, compositions, and functions as a result of the relative reactivities. In this Concept article, different molecular networks from synthetic systems that can be produced by combinations of different reaction types are discussed. Moreover, applications of this chemistry are highlighted, and future perspectives are envisioned.     

Reaction Dynamics with Molecular Beams and Oriented Molecular Beams: A Tool for Looking Closer to Chemical Reactions and Photodissociations

Experimental studies on reaction dynamics by use of molecular beams and oriented molecular beams are reviewed in order for looking closer to chemical reactions as well as photodissociations at the molecular level. We discuss about versatility and usefulness of the electrostatic hexapole sate‐selector as a non‐destructive selector for molecular structure analysis. Some experimental evidences on novel reaction dynamics in photodissociation and stereodynamics are presented followed by concluding remarks and future perspectives for controlling chemical reactions from the point of view of green chemistry, by manipulating molecular orientation without any catalyst nor by applying any external forces like intense electromagnetic field.     

Electron-stimulated production of molecular hydrogen at the interfaces of amorphous solid water films on Pt(111)

The electron-stimulated production of molecular hydrogen (D(2), HD, and H(2)) from amorphous solid water (ASW) deposited on Pt(111) is investigated. Experiments with isotopically layered films of H(2)O and D(2)O are used to profile the spatial distribution of the electron-stimulated reactions leading to hydrogen within the water films. The molecular hydrogen yield has two components that have distinct reaction kinetics due to reactions that occur at the ASW/Pt interface and the ASW/vacuum interface, but not in the bulk. However, the molecular hydrogen yield as a function of the ASW film thickness in both pure and isotopically layered films indicates that the energy for the reactions is absorbed in the bulk of the films and electronic excitations migrate to the interfaces where they drive the reactions.     

Recent advancements in palladium-catalyzed reactions involving molecular oxygen

Oxidation reactions have significant value in organic chemistry, having been in focus continuously due to the high efficiency in building up molecular complexity. In the past few decades, transition metal-catalyzed oxidation reactions have been significantly explored and have played important roles in organic synthesis. Compared to the widely-used oxidants, such as inorganic salts, peroxides, hypervalent iodine reagents and quinones, molecular oxygen (O₂), which is natural, inexpensive, and environmentally friendly, is a highly appealing oxidant in academic and industry area for green and sustainable chemistry. Recently, significant advances have been made in palladium-catalyzed reactions using O₂ as the oxidant. This critical review highlights some of the recent developments in molecular oxygen-involved Pd-catalyzed oxidation reactions with a focus on mechanistic strategies and new reaction developments.     

应用分子形貌理论研究类SN2反应

应用分子形貌理论, 研究了类SN2反应过程中的沿着IRC路径上固定点的分子形貌的特征, 计算给出了形状和电子密度特征参数以及各键的Dpb值. 应用Matlab程序绘制了分子特征边界轮廓上的电子密度分布的三维图像, 即分子形貌像, 给出了这类反应的动态变化过程.     

硅磷酸铝分子筛催化羧酸的酯化反应研究

用新型硅磷酸铝分子筛和改性的硅磷酸铝分子筛催化脂肪族羧酸与脂肪族醇的酯化反应，以正戊酸分别与正丙醇、正丁醇、正戊醇和正辛醇以及正辛醇分别与乙酸、丙酸、正丁酸、正戊酸和正己酸的酯化反应作为探针实验进行了系统研究，实验结果显示，改性的硅磷酸铝分子筛比不改性的硅磷酸铝分子筛具有更高的酯化催化活性，实验发现，反应物的结构对酯化反应产生了显著的影响，文章揭示了新型硅磷酸铝分子筛和改性的硅磷酸铝分子筛对于羧酸的直接酯化反应的系列规律性。     

Optical manipulation in conjunction with photochemical/photothermal responses of materials

This article reviews optical manipulation coupled with photochemical/photothermal responses of nanometer sized materials including molecular systems, polymers, and inorganic nanoparticles. After the introduction, section 2 overviews the optical trapping of nanometer sized molecular systems including early-stage studies, such as trapping of polymer chains, micelles, and molecular aggregates in solution at room temperatures. Then, the conformation control of macromolecule assemblies and gels by optical force are introduced, followed by micro-fabrications achieved by combining optical trapping and photochemical reactions. Section 3 summarizes studies on the evaluation of optical force acting on nanometric molecular systems using fluorescence correlation techniques. Approaches to control optical force by using photochemical reactions are show in section 4, where the absorption band of target materials are modified through photochromic reactions, leading to micromechanical motion of small particles synchronizing with the photochemical reactions. Section 5 overviews photothermal effect in optical manipulation such as natural convection, Marangoni convection and thermophoresis, and applications of the thermal effects to develop new methods of micromanipulation achieved by combining optical force and photothermal responses.     

Pressure dependent reactions for atmospheric and combustion models

In order to characterize reactions as functions of temperature, pressure (molecular density) and the nature of the species that constitute that molecular density, master equation solutions are required. In this tutorial review, the application of the Multiwell suite of codes to some reactions of interest in atmospheric and combustion chemistry is discussed, with attention given to the details of the molecular and energy transfer values. Uncertainties in data and in structural and energetic molecular parameters combine to assure the need for optimization and collaborative processing of the entire data base when modeling practical systems.     

TTA的席夫反应和分子力学、量化计算

报导了噻吩甲酰三氟丙酮(TTA)与4 氨基安替比林(4 ATP)缩合反应生成的两个席夫碱化合物A和C,利用分子力学和分子动力学方法研究了该缩合反应可能生成的三个席夫碱化合物A、B和C的各种构象.从TTA的量化计算结果可以看出缩合反应生成的两个席夫碱化合物A和C是正确的.     

Accelerating chemical reactions: exploring reactive free-energy surfaces using accelerated ab initio molecular dynamics

A biased potential molecular dynamics simulation approach, accelerated molecular dynamics (AMD), has been implemented in the framework of ab initio molecular dynamics for the study of chemical reactions. Using two examples, the double proton transfer reaction in formic acid dimer and the hypothetical adiabatic ring opening and subsequent rearrangement reactions in methylenecyclopropane, it is demonstrated that ab initio AMD can be readily employed to efficiently explore the reactive potential energy surface, allowing the prediction of chemical reactions and the identification of metastable states. An adaptive variant of the AMD method is developed, which additionally affords an accurate representation of both the free-energy surface and the mechanism associated with the chemical reaction of interest and can also provide an estimate of the reaction rate.     

Modeling and Experimentation of RAFT Solution Copolymerization of Styrene and Butyl Acrylate,Effect of Chain Transfer Reactions on Polymer Molecular Weight Distribution

Chain transfer reactions widely exist in the free radical polymerization and controlled radical polymerization, which can significantly influence polymer molecular weight and molecular weight distribution. In this work, the chain transfer reactions in modeling the reversible addition–fragmentation transfer (RAFT) solution copolymerization are included and the effects of chain transfer rate constant, monomer concentration, and comonomer ratio on the polymerization kinetics and polymer molecular weight development are investigated. The model is verified with the experimental RAFT solution copolymerization of styrene and butyl acrylate, with good agreements achieved. This work has demonstrated that the chain transfer reactions to monomer and solvent can have significant impacts on the number‐average molecular weight (M_n) and dispersity (Ð).     

A study of the reaction of oxygen with graphite: model chemistry

A considerable amount of research has been directed towards the mechanism of oxidation of graphite as a model reaction system and because of its industrial importance. A number of recent studies have been concerned with ab initio molecular orbital calculations on graphite including model chemistry and the reactions with molecular oxygen. This study is concerned with oxidation steps involving the attachment of molecular oxygen to the graphene, the formation of carbon monoxide and, in particular, the subsequent oxidation reactions.