小型计算机的Hückel分子轨道(HMO)计算方法会话程序

量子化学近似计算方法中,Hückel分子轨道(HMO)计算方法简便迅捷,它对分析有机共轭分子的稳定性、化学反应能力和电子光谱,及其研究有机化合物结构与性能的关系都适用。目前在有机化学、药物化学和生物化学领域中得到广泛的应用。     

化学反应过渡态的计算方法

化学反应过渡态结构对化学反应机理的研究有重要作用, 但实验上无法直接得到。近年来发展了一些量子化学和分子力学方法, 可以从理论上计算出过渡态结构, 本文对这些计算方法进行了详细介绍。     

电化学表面增强拉曼光谱的量子化学研究

对于在分子水平上研究电化学表面吸附和反应过程,表面增强拉曼光谱(SERS)显示出了其独到的优势,提供了有力的技术方法,但对于其表面增强机理仍有待深入研究.本文总结了将量子化学计算应用于电化学表面增强拉曼光谱(EC-SERS)分析的研究,以电化学界面分子吸附、电化学反应以及光电化学反应的研究体系为模型,提取EC-SERS光谱所蕴藏的物理化学信息.通过对吡啶在电化学表面的吸附、水的吸附及其电化学反应、以及对巯基苯胺的电化学表面催化偶联反应等体系的研究,揭示了电化学表面吸附、反应和光电化学过程的本质.     

实时直接分析质谱分析中草药多糖

建立了一种机械力化学提取法和实时直接分析质谱(DART-MS)分析相结合的中草药多糖分析方法,同时测定了黄芪、银耳、百合、茯苓、鼓槌石斛、金钗石斛、铁皮石斛和党参8种中草药多糖。比较了机械力化学提取(MCE)法与传统方法的提取效果,并优化了MCE法的提取参数。通过体积排阻色谱分离中草药多糖,建立分子质量校准曲线,估算了多糖分子质量。利用DART-MS直接裂解多糖大分子,可瞬时获得m/z<350的碎片离子,不同的中草药多糖得到的特征性的质谱离子峰可用于多糖样品的有效区分。该方法具有简单、快速、高通量、无需预消化的特点,是直接从复杂多糖大分子中获取特征指纹图谱的有力工具。     

理论与计算化学在基础化学课程中的应用：1,2-环氧化物开环反应

1,2-环氧化物开环反应是化学类各专业有机化学课程中的重要教学内容，教材中的描述缺少定量、直观信息，学生通常采用机械记忆方式予以学习，不能完全理解相关的反应机制。本文设计了一个用理论化学方法研究化学反应的典型案例，通过量子化学计算探讨了酸、碱催化1,2-环氧化物开环反应的分子机制，揭示了反应的热力学和动力学性质，描绘了反应过程的直观物理图像，给出了理解反应区域选择性的定量信息。论文将计算量子化学与有机化学教学内容有机融合，不仅有助于学生加深对化学基本概念和基础理论的理解，而且有助于学生开阔视野、拓展思路，使学生充分认识到理论计算是开展化学研究的重要手段。     

乙醇分子内及分子间脱水反应机理的计算化学实验研究

介绍了一个面向大三下或大四上本科生的计算化学探索性实验。通过研究乙醇分子内及分子间脱水反应机理,从分子水平理解醇脱水反应过程中酸催化剂的作用机制和反应的能量路径。通过本实验,使学生初步掌握量子化学计算通用软件高斯(Gaussian)的使用方法,掌握结构优化、频率计算、过渡态搜索等基本计算操作,并学会采用量子化学计算手段来研究化学反应的基本过程,为将来科研工作打下重要基础。     

πMO的角边投影模型——直链烯烃πMO能级和系数的三角函数法

直链烯烃的分子轨道图象的建立与研究一直是结构化学和量子化学关注的重要课题。对初学者,如果能运用分子轨道理论的一般知识,建立一种简单通俗的模型,避开复杂的数学处理过程,应用初等数学方法进行计算又能得到同样结果,这是很有实际意义的。本文试图建立一个πMO的角边投影模型,应用初等三角函数法处理直链烯烃分子轨道能级和轨道系数。     

“化学反应速率”微观模型的创建探究

探讨了构建化学微观模型的价值与原理, 指出利用微观模型帮助学生理解抽象概念的重要意义, 并通过对"化学反应速率"教学现状的分析, 提出构建其微观模型的必要性和可能性。在此基础上, 构建了温度与分子运动、活化分子、活化能的微观模型以及化学反应微观过程与宏观、符号表征相关联的模型。     

拓扑量子方法及其在含C＝O和N＝O化合物性能估算中的应用

综合量子化学与拓扑化学知识, 提出了构建Y＝O (Y表示C, N)键拓扑量子键邻接矩阵的方法. 利用该矩阵特征根和量子化学原理, 计算了表征分子轨道能量、原子电荷和键级等的拓扑量子参数. 将这些参数对含C＝O, N＝O基团的有机分子的紫外吸收能量、红外伸缩振动频率和醛酮C＝O上的亲核加成反应速率等性质进行定量结构-性质相关, 得到的模型均具有良好的估算能力.     

计算化学在认识吸附现象中的应用*

吸附是重要的表面物理化学现象,其在工业实践及生活中有较多的应用。随着量子化学理论的发展及计算机技术的不断突破,采用计算化学的方法研究物质结构及化学反应已经变为现实,本文着重讲述如何采用计算化学方法研究吸附现象,在分子原子层次对吸附现象的本质进行模拟研究。     

Improving the Yield and Rate of Acid-Catalyzed Deconstruction of Lignin by Mechanochemical Activation

Lignin is a potential biomass feedstock from plant material, but it is particularly difficult to economically process. Inspired by recent ball-milling results, state-of-the-art quantum mechanochemistry calculations have been performed to isolate and probe the purely mechanochemical stretching effect alone upon acid-catalyzed deconstruction of lignin. Effects upon cleavage of several exemplary simple ethers are examined first, and with low stretching force they all are predicted to cleave substantially faster, allowing for use of milder acids and lower temperatures. Effects upon an experimentally known lignin fragment model (containing the ubiquitous β-O-4 linkage) are next examined; this first required a mechanism refinement (3-step indirect cleavage, 1-step side reaction) and identification of the rate-limiting step under zero-force (thermal) conditions. Mechanochemical activation using very low stretching forces improves at first only yield, by fully shutting off the ring-closure side reaction. At only somewhat larger forces, in stark contrast, a switch in mechanism is found to occur, from 3-step indirect cleavage to the direct cleavage mechanism of simple ethers, finally strongly enhancing the cleavage rate of lignin. It is concluded that mechanochemical activation of the common β-O-4 link in lignin would improve the rate of its acidolysis via a mechanism switch past a low force threshold. Relevance to ball-milling experiments is discussed.     

Prediction of reaction barriers and force-induced instabilities under mechanochemical conditions with an approximate model: a case study of the ring opening of 1,3-cyclohexadiene

Mechanochemistry, the use of mechanical stresses to activate chemical reactions, has emerged as a topic of significant interest. The present study examines the use of an approximate model for the prediction of reaction barriers under mechanochemical conditions using the ring opening of 1,3-cyclohexadiene along conrotatory and disrotatory directions as a specific test case. To do this, reaction barriers are evaluated using quantum chemical methods with an external force applied between various pairs of atoms. The results show that the consequent effects on the barrier exhibit a significant dependence on the locations of the atoms used to apply the external force, and in some cases, force-induced instabilities occur that alter the fundamental nature of the reaction pathway. The ability of an approximate model based on a second-order expansion of the force-modified potential energy with respect to nuclear coordinates to reproduce this behavior is then assessed. Good agreement between the results obtained through the quantum chemical calculations and approximate model is attained when force-induced instabilities do not occur. In addition, a strategy for predicting when such instabilities occur is presented and found to yield results that are in qualitative agreement with the quantum chemical calculations. Finally, the response of the system to the external force is interpreted in terms of the parameters entering the model, which correspond to interatomic distances and stiffnesses, and possibly sheds lights on ways to design molecules that exhibit a desired chemical response to mechanochemical conditions.     

Click mechanochemistry: quantitative synthesis of "ready to use" chiral organocatalysts by efficient two-fold thiourea coupling to vicinal diamines

Mechanochemical methods of neat grinding and liquid-assisted grinding have been applied to the synthesis of mono- and bis(thiourea)s by using the click coupling of aromatic and aliphatic diamines with aromatic isothiocyanates. The ability to modify the reaction conditions allowed the optimization of each reaction, leading to the quantitative formation of chiral bis(thiourea)s with known uses as organocatalysts or anion sensors. Quantitative reaction yields, combined with the fact that mechanochemical reaction conditions avoid the use of bulk solvents, enabled solution-based purification methods (such as chromatography or recrystallization) to be completely avoided. Importantly, by using selected model reactions, we also show that the described mechanochemical reaction procedures can be readily scaled up to at least the one-gram scale. In that way, mechanochemical synthesis provides a facile method to fully transform valuable enantiomerically pure reagents into useful products that can immediately be applied in their designed purpose. This was demonstrated by using some of the mechanochemically prepared reagents as organocatalysts in a model Morita-Baylis-Hillman reaction and as cyanide ion sensors in organic solvents. The use of electronically and sterically hindered ortho-phenylenediamine revealed that mechanochemical reaction conditions can be readily optimized to form either the 1:1 or the 1:2 click-coupling product, demonstrating that reaction stoichiometry can be more efficiently controlled under these conditions than in solution-based syntheses. In this way, it was shown that excellent stoichiometric control by mechanochemistry, previously established for mechanochemical syntheses of cocrystals and coordination polymers, can also be achieved in the context of covalent-bond formation.     

Click Mechanochemistry: Quantitative Synthesis of “Ready to Use” Chiral Organocatalysts by Efficient Two‐Fold Thiourea Coupling to Vicinal Diamines

Mechanochemical methods of neat grinding and liquid‐assisted grinding have been applied to the synthesis of mono‐ and bis(thiourea)s by using the click coupling of aromatic and aliphatic diamines with aromatic isothiocyanates. The ability to modify the reaction conditions allowed the optimization of each reaction, leading to the quantitative formation of chiral bis(thiourea)s with known uses as organocatalysts or anion sensors. Quantitative reaction yields, combined with the fact that mechanochemical reaction conditions avoid the use of bulk solvents, enabled solution‐based purification methods (such as chromatography or recrystallization) to be completely avoided. Importantly, by using selected model reactions, we also show that the described mechanochemical reaction procedures can be readily scaled up to at least the one‐gram scale. In that way, mechanochemical synthesis provides a facile method to fully transform valuable enantiomerically pure reagents into useful products that can immediately be applied in their designed purpose. This was demonstrated by using some of the mechanochemically prepared reagents as organocatalysts in a model Morita–Baylis–Hillman reaction and as cyanide ion sensors in organic solvents. The use of electronically and sterically hindered ortho‐phenylenediamine revealed that mechanochemical reaction conditions can be readily optimized to form either the 1:1 or the 1:2 click‐coupling product, demonstrating that reaction stoichiometry can be more efficiently controlled under these conditions than in solution‐based syntheses. In this way, it was shown that excellent stoichiometric control by mechanochemistry, previously established for mechanochemical syntheses of cocrystals and coordination polymers, can also be achieved in the context of covalent‐bond formation.     

Mechanochemistry of Gaseous Reactants

In recent years, the application of mechanical energy to chemical systems has repeatedly proven beneficial to facilitate chemical transformations in various areas in chemistry. Today, a systematic body of evidence indicates that mechanochemistry holds great promise to become a game‐changer in chemical synthesis. Not only does mechanochemistry permit access to products that are inaccessible by established means (e.g. purely thermal activation), mechanochemical reactions often outperform their solution‐based counterparts in terms of sustainability. Most mechanochemical reactions carried out by ball milling techniques involve transformations of solids and liquids, but the number of mechanochemical reactions with gaseous reactants is increasing. The aim of this Minireview is to provide an overview of recent chemical reactions involving gaseous samples by ball milling techniques and to highlight advances in ball milling technology for the safe handling of gaseous reagents. Examples of reactions proceeding at the interface of solid–/liquid–/gas–gas systems that led to significant improvements in reactivity or selectivity will also be highlighted.     

Mechanistic Insights on the Mechanosynthesis of Phenytoin,a WHO Essential Medicine**

In recent years, mechanochemistry has enriched the toolbox of synthetic chemists, enabling faster and more sustainable access to new materials and existing products, including active pharmaceutical ingredients (APIs). However, molecular-level understanding of most mechanochemical reactions remains limited, delaying the implementation of mechanochemistry in industrial applications. Herein, we have applied in situ monitoring by Raman spectroscopy to the mechanosynthesis of phenytoin, a World Health Organization (WHO) Essential Medicine, enabling the observation, isolation, and characterization of key molecular-migration intermediates involved in the single-step transformation of benzil, urea, and KOH into phenytoin. This work contributes to the elucidation of a reaction mechanism that has been subjected to a number of interpretations over time and paints a clear picture of how mechanosynthesis can be applied and optimized for the preparation of added-value molecules.     

Influence of external force on properties and reactivity of disulfide bonds

The mechanochemistry of the disulfide bridge--that is, the influence of an externally applied force on the reactivity of the sulfur-sulfur bond--is investigated by unrestricted Kohn-Sham theory. Specifically, we apply the COGEF (constrained geometry simulates external force) approach to characterize the mechanochemistry of the disulfide bond in three different chemical environments: dimethyl disulfide, cystine, and a 102-atom model of the I27 domain in the titin protein. Furthermore, the mechanism of the thiol-disulfide reduction reaction under the effect of an external force is investigated by considering the COGEF potential for the adduct and transition-state clusters. With the unrestricted Becke-three-parameter-Lee-Yang-Parr (UB3LYP) exchange-correlation functional in the 6-311++G(3df,3pd) orbital basis, the rupture force of dimethyl disulfide is 3.8 nN at a disulfide bond elongation of 35 pm. The interaction with neighboring groups and the effect of conformational rigidity of the protein environment have little influence on the mechanochemical characteristics. Upon stretching, we make the following observations: the diradical character of the disulfide bridge increases; the energy difference between the singlet ground state and low-lying triplet state decreases; and the disulfide reduction is promoted by an external force in the range 0.1-0.4 nN. Our model of the interplay between force and reaction mechanism is in qualitative agreement with experimental observations.     

Tinkering with Mechanochemical Tools for Scale Up

Mechanochemistry provides an environmentally benign platform to develop more sustainable chemical processes by limiting raw materials, energy use, and waste generation while using physically smaller equipment. A continuously growing research community has steadily showcased examples of beneficial mechanochemistry applications at both the laboratory and the preparative scale. In contrast to solution-based chemistry, mechanochemical processes have not yet been standardized, and thus scaling up is still a nascent discipline. The purpose of this Minireview is to highlight similarities, differences and challenges of the various approaches that have been successfully applied for a range of chemical applications at various scales. We hope to provide a discussion starting point for those interested in further developing mechanochemical processes for commercial use and/or industrialisation.     

Mechanochemical Synthesis of Catalytic Materials

The mechanochemical synthesis of nanomaterials for catalytic applications is a growing research field due to its simplicity, scalability, and eco-friendliness. Besides, it provides materials with distinct features, such as nanocrystallinity, high defect concentration, and close interaction of the components in a system, which are, in most cases, unattainable by conventional routes. Consequently, this research field has recently become highly popular, particularly for the preparation of catalytic materials for various applications, ranging from chemical production over energy conversion catalysis to environmental protection. In this Review, recent studies on mechanochemistry for the synthesis of catalytic materials are discussed. Emphasis is placed on the straightforwardness of the mechanochemical route—in contrast to more conventional synthesis—in fabricating the materials, which otherwise often require harsh conditions. Distinct material properties achieved by mechanochemistry are related to their improved catalytic performance.     

Mechanistic Insights into a Sustainable Mechanochemical Synthesis of Ettringite

Mechanochemistry offers an environmentally benign and facile synthesis method for a variety of cement paste constituents. In addition, these methods can be used to selectively tune the properties of cement components. The mineral ettringite is an important component of cementitious materials and has additional technological potential due to its ion exchange properties. Synthesis of ettringite via mechanochemistry is an environmentally friendly alternative to conventional wet-chemical synthesis established in industry. This contribution explores the mechanism of a two-step mechanochemical synthesis of ettringite, which was previously found to greatly improve the reaction conversion as compared with one-pot synthesis. The crystallinity of Al(OH)₃ was found to decrease during the first stage of this mechanochemical synthesis. This was correlated to a significant decrease in the particle size of Al(OH)₃ in this stage. No other significant changes were found for the other components, suggesting that mechanochemical activation of Al(OH)₃ is responsible for the enhanced formation of ettringite by the two-step approach. The environmentally friendly approach developed for ettringite synthesis offers a versatile synthetic strategy, which can be applied to synthesise further cementitious materials.