Mechanisms and Stereochemistry of Heterolytic Fragmentation

Heterolytic fragmentation can proceed by several mechanisms. A study of the solvolysis of γ-aminoalcohol derivatives N? C? C? C? X, where X equals halogen or ? OSO₂R, shows that fragmentation may occur by a one-step mechanism or by several multi-step mechanisms depending on structural, electronic, and steric factors. The one-step synchronous mechanism has rigorous stereoelectronic requirements. It is associated with an increased reaction rate as evidenced by the “frangomeric” effect. Ketoxime derivatives RR′C?N? X may undergo Beckmann fragmentation by the synchronous mechanism or by prior rearrangement depending on their α-substituents. Fragmentation is frequently accompanied by conventional substitution, elimination, and rearrangement reactions. Under such circumstances knowledge of all possible mechanisms of a given substrate is essential to bring about a desired transformation.     

Heterolytic Fragmentation. A Class of Organic Reactions

Heterolytic fragmentation is a widespread but neglected class of organic reactions. It involves the regulated cleavage of molecules containing certain combinations of atoms such as carbon, oxygen, nitrogen, sulfur, phosphorus, silicon, boron and halogens. Fragmentation reactions are useful in degradation and structure elucidation, some are also of preparative value. A knowledge of the structural and electronic requirements makes it possible to predict and influence the course of reactions. From a theoretical viewpoint the recognition of heterolytic fragmentation has lead to the classification and correlation of a large number of apparently diverse reactions. Selected cases are reviewed in this article.     

Heterolytic cleavage of homoallylic alcohols—I: Fragmentation of 6-hydroxycamphene derivatives

Heterolytic fragmentation of homoallylic alcohols incorporated in a bicyclo[2,2,1]heptane system with electrophilic reagents or that of the corresponding epoxides with acids, is described. A short route, via this fragmentation, to synthon 25, potentially useful in the syntheses of cis-jasamone and prostanoids, is reported.     

Nanoscale Effects on Thermodynamics and Phase Equilibria in Oxide Systems

Because different solid materials (phases) have different surface energies, equilibria among them will be significantly affected by particle size. This Minireview summarizes experimental (calorimetric) data for the surface energies of oxides and discusses shifts in the stability of polymorphs, the thermodynamics of hydration, and oxidation–reduction reactions in nanoscale oxide systems.     

分子轨道法在若干C_7脂肪酮异构体的质谱电离断裂过程中的应用

本文利用自编的LCBO—MO(成键轨道线性组合分子轨道)法计算程序,对若干C_7脂肪酮异构体的主要骨架键断裂几率进行理论计算。并与Abbatt等所提供的质谱实验数据进行比较,获得基本一致的定性结果。     

土壤有机质对菲的吸附-解吸平衡的影响

以自然土壤和过氧化氢分级土壤为实验模拟样品,测定了菲在这些样品上的吸附一解吸等温线,用线性和Freundlich模型拟合了这些等温线．^13C NMR谱表明,随着土壤有机质腐殖化程度的加深,有机质将含有较多的长链烷烃化合物,含氧、氮化合物有所减少,芳香环的数量变化不大．吸附实验结果表明,土壤有机质含量与菲的吸附容量存在一定的线性相关关系．有机质腐殖质化程度较深的样品比原土壤具有更大的吸附容量,其吸附等温线表现出更为明显的非线性,而且具有更明显的解吸滞后现象．说明土壤中一些结构紧密和含极性官能团较少的有机质是引起菲的非线性吸附过程和解吸滞后现象的主要原因。     

A Quantitative Study of Solvent Effects on Keto-Enol Tautomeric Equilibria

Introduction Organic compounds usually have labile bonds. It is well known that thousands of reactions are carried out in solutions, only a few in the gas-phase. Most chemists are now familiar with the fact that solvent may have a strong influence on reaction rate and equilibrium, so in the last decade, resarching the influence of solvent effects on reaction rate and reaction equilibrium was a very important and     

分子轨道法在C_6脂肪酮异构体电离碎裂过程中的应用

本文利用自编的成键轨道线性组合分子轨道法计算程序对四个C_6脂肪酮的C—C骨架键断裂几率进行理论计算。预测了酮的质谱碎裂,与实验谱图比较符合。     

杂化轨道理论中等性杂化和不等性杂化的讨论

采用计算量子化学方法讨论了杂化轨道理论中等性杂化和不等性杂化,指出了不是轨道电子数而是杂化轨道成分是确定等性杂化和不等性杂化的判据。     

铌团簇和配合物的多面体分子轨道理论研究

应用多面体理论方法研究铌纯金属团簇和配合物的电子结构成键性质,并将理论预测结果用密度泛函方法验证.证实多面体理论是简便预测过渡金属簇,尤其是簇骨架电子结构的有效方法.铌的小团簇总是以低自旋密堆积结构为稳定构型,当体系的价电子数满足轨道成键数时,用该方法可较准确地推断成键性质(如Nb₄和[Nb₆Cl₁₂]⁺⁴);而对于不满足成键数的体系(如Nb₅和Nb₆),则可利用该理论分析畸变趋势.     

Pure and Binary Gas Adsorption Equilibria and Kinetics of Methane and Nitrogen on 4A Zeolite by Isotope Exchange Technique

The Isotope Exchange Technique (IET) was used to simultaneously measure pure and binary gas adsorption equilibria and kinetics (self-diffusivities) of CH₄ and N₂ on pelletized 4A zeolite. The experiment was carried out isothermally without disturbing the adsorbed phase. CH₄ was selectively adsorbed over N₂ by the zeolite because of its higher polarizability. The multi-site Langmuir model described the pure gas and binary adsorption equilibria fairly well at three different temperatures. The selectivity of adsorption of CH₄ over N₂ increased with increasing pressure at constant gas phase composition and temperature. This curious behavior was caused by the differences in the sizes of the adsorbates. The diffusion of CH₄ and N₂ into the zeolite was an activated process and the Fickian diffusion model described the uptake of both pure gases and their mixtures. The self-diffusivity of N₂ was an order of magnitude larger than that for CH₄. The pure gas self-diffusivities for both components were constants over a large range of surface coverages (0 < < 0.5). The self-diffusivities of CH₄ and N₂ from their binary mixtures were not affected by the presence of each other, compared to their pure gas self-diffusivities at identical surface coverages.     

Effects of Alkali Metal Ion Cationization on Fragmentation Pathways of Triazole-Epothilone

The collisionally activated dissociation mass spectra of the protonated and alkali metal cationized ions of a triazole-epothilone analogue were studied in a Fourier transform ion cyclotron resonance mass spectrometer. The fragmentation pathway of the protonated ion was characterized by the loss of the unit of C₃H₄O₃. However, another fragmentation pathway with the loss of C₃H₂O₂ was identified for the complex ions with Na⁺, K⁺, Rb⁺, and Cs⁺. The branching ratio of the second pathway increases with the increment of the size of alkali metal ions. Theoretical calculations based on density functional theory (DFT) method show the difference in the binding position of the proton and the metal ions. With the increase of the radii of the metal ions, progressive changes in the macrocycle of the compound are induced, which cause the corresponding change in their fragmentation pathways. It has also been found that the interaction energy between the compound and the metal ion decreases with increase in the size of the latter. This is consistent with the experimental results, which show that cesiated complexes readily eject Cs⁺ when subject to collisions.     

基于密度泛函理论的橘霉素衍生物质谱裂解行为研究

采用电喷雾离子阱质谱(ESI-IT-MSn),研究了橘霉素衍生物正离子模式下的裂解行为,并结合密度泛函理论(DFT)证明裂解途径的可靠性,为同类化合物的准确鉴定提供实验和理论基础。核磁共振仪对海莲内生真菌代谢产物分离得到的橘霉素衍生物结构进行确认,电喷雾离子阱质谱正离子模式扫描,测定化合物氢氘交换产物多级裂解质谱图,并结合键断裂能和Mulliken电荷分布,进一步验证裂解规律。实验结果表明,分子结构中-COOH和CH3COO-在一级裂解过程中易脱除CO2和COCH2,CID-MS2观测到丢失H2O/HDO、H2O和-CH3或CHCO/CDCO的碎片离子峰,且丰度依次降低,理论计算也证明,这些碎片离子的总能量依次增大,稳定性降低,键断裂能逐渐增大。该结果丰富了橘霉素衍生物的电喷雾质谱裂解规律,有助于橘霉素衍生物结构的准确鉴定,为该类化合物的检测和痕量分析提供了更多支持。     

Photoinduced Remote Functionalization of Amides and Amines Using Electrophilic Nitrogen Radicals

The selective functionalization of C(sp³)?H bonds at distal positions to functional groups is a challenging task in synthetic chemistry. Reported here is a photoinduced radical cascade strategy for the divergent functionalization of amides and protected amines. The process is based on the oxidative generation of electrophilic amidyl radicals and their subsequent transposition by 1,5‐H‐atom transfer, resulting in remote fluorination, chlorination and, for the first time, thioetherification, cyanation, and alkynylation. The process is tolerant of most common functional groups and delivers useful building blocks that can be further elaborated. The utility of this strategy is demonstrated through the late‐stage functionalization of amino acids and a dipeptide.     

计算化学在分子轨道教学中的应用

分子轨道理论是理解分子电子结构与微观性质的重要理论之一,也是本科生与研究生结构化学教学中的重点与难点。学生对原子轨道组合形成分子轨道、分子轨道能级交叉混合等知识的理解缺乏形象直观、定量的认识。本文通过基于量子化学或密度泛函理论的Gaussian 03计算软件,计算、绘制并分析了F_2、O_2、N_2、HF、CO等的分子轨道能级图,将学生较难理解的内容定量、直观地呈现出来,形象地解释了分子轨道成键原则与电子填充原则等分子轨道理论中的重难点,加深了学生对分子轨道理论的理解,特别是sp轨道混杂导致的σ_(2p_z)与π_(2p)轨道能级交叉这一难点,激发了学生学习的主动性和积极性,提高了教学质量。在此基础上,利用分子轨道理论分析了CO_2的电子结构,使学生学会应用分子轨道理论解决实际问题,巩固了相关课堂理论知识。