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相关概念视频

Radical Reactivity: Intramolecular vs Intermolecular01:33

Radical Reactivity: Intramolecular vs Intermolecular

Radical reactions can occur either intermolecularly or intramolecularly. In an intermolecular radical reaction, a nucleophilic radical adds to an electrophilic alkene or vice versa. In such reactions, the radical and generally the alkene, which is also called the radical trap, are two different molecules. Additionally, for such intermolecular reactions to occur, the radical trap must be active, present in an excess concentration, and the radical starting material must have a weak carbon–halogen...
Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal Electrocyclic Reactions: Stereochemistry

The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
Thermal and Photochemical Electrocyclic Reactions: Overview01:26

Thermal and Photochemical Electrocyclic Reactions: Overview

Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
Woodward–Hoffmann Selection Rules and Microscopic Reversibility01:34

Woodward–Hoffmann Selection Rules and Microscopic Reversibility

Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...
ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH301:11

ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH3

All ortho–para directors, excluding halogens, are activating groups. These groups donate electrons to the ring, making the ring carbons electron-rich. Consequently, the reactivity of the aromatic ring towards electrophilic substitution increases. For instance, the nitration of anisole is about 10,000 times faster than the nitration of benzene. The electron-donating effect of the methoxy group in anisole activates the ortho and para positions on the ring and stabilizes the corresponding...

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Direct Imaging of Laser-driven Ultrafast Molecular Rotation
10:52

Direct Imaging of Laser-driven Ultrafast Molecular Rotation

Published on: February 4, 2017

具有旋转触发器的分子内部电子排列.

Shoko Kume1, Kuniharu Nomoto, Tetsuro Kusamoto

  • 1Department of Chemistry, Graduate School of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. kume@chem.s.u-tokyo.ac.jp

Journal of the American Chemical Society
|September 25, 2009
PubMed
概括

单个分子系统中的电子转移是由分子旋转控制的. 这项研究揭示了铁结合的铜复合体如何表现出同步运动和电子迁移,随着温度的变化而切换氧化还原活性.

科学领域:

  • 超分子化学 超分子化学
  • 分子电子学分子电子学
  • 协调化学 协调化学

背景情况:

  • 单分子系统可以精确控制化学和物理过程.
  • 电子转移是许多生物和化学系统的基础.
  • 分子运动可以影响电子属性.

研究的目的:

  • 构建和描述一个单分子系统,其中电子转移是由分子运动触发的.
  • 为了研究分子构成,氧化还原活性和温度之间的相互作用.
  • 阐明同步运动和电子迁移的机制.

主要方法:

  • 用X射线晶体学进行异构体的结构特征.
  • 质子核磁共振 ((1) H NMR) 谱学用于研究同位素相互转换.
  • 电化学测量以确定氧化还原潜力.
  • 电子偏磁共振 (EPR) 和UV-Vis吸收光谱学用于监测氧化和光谱变化.

主要成果:

  • 合成了一种具有两个不同的协调构造的铁结合铜复合体.
  • 异构体通过室温的皮里米丁旋转相互转换,冷低于233 K.
  • 氧化状态会影响同位素的稳定性,导致自发的同位素化.

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Fluorescence Lifetime Imaging of Molecular Rotors in Living Cells

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Direct Imaging of Laser-driven Ultrafast Molecular Rotation
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Coulomb Explosion Imaging as a Tool to Distinguish Between Stereoisomers
08:51

Coulomb Explosion Imaging as a Tool to Distinguish Between Stereoisomers

Published on: August 18, 2017

Fluorescence Lifetime Imaging of Molecular Rotors in Living Cells
09:45

Fluorescence Lifetime Imaging of Molecular Rotors in Living Cells

Published on: February 9, 2012

  • 电子转移特征 (铜与铁素) 根据温度和分子运动进行交替.
  • 观察到同步运动和电子迁移作为一阶段的光谱转换.

    • 结论:

    • 分子旋转运动可以有效地触发和控制单分子系统中的电子转移.
    • 该系统展示了氧化还原中心的温度依赖的切换.
    • 这项工作为设计基于构造变化的可调节电子属性的分子设备提供了洞察力.