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相关概念视频

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

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In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter of the molecule. This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside. The protons in benzene are deshielded and exhibit high chemical shifts in the range 6.5–8.5 ppm. The shielding effect at the center of the ring is evident in complex aromatic molecules, such as...
1.2K
Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal Electrocyclic Reactions: Stereochemistry

2.0K
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.
2.0K
Stereoisomerism of Cyclic Compounds02:33

Stereoisomerism of Cyclic Compounds

8.8K
In this lesson, we delve into the role of ring conformation and its stability, which determines the spatial arrangement and, consequently, the molecular symmetry and stereoisomerism of cyclic compounds. 1,2-Dimethylcyclohexane is used as a case study to evaluate the possible number of stereoisomers. Here, given the multiple (n = 2) chiral centers, there are 2n = 4 possible configurations that lack a plane of symmetry, as the ring skeleton exists in a non-planar chair conformation. In addition,...
8.8K
Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

1.8K
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
1.8K
π Electron Effects on Chemical Shift: Overview01:27

π Electron Effects on Chemical Shift: Overview

1.1K
An applied magnetic field causes loosely bound π-electrons in organic molecules to circulate, producing a local or induced diamagnetic field over a large spatial volume. As the molecules tumble in solution, the field generated by π-electrons in spherical substituents results in a zero net field. However, the net field generated by π-electrons in non-spherical substituents is not zero. The effect of this induced field depends on the orientation of the molecule with respect to B0,...
1.1K
¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

831
At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...
831

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在环形分子中逆光诱导的环流.

Krishna Reddy Nandipati1,2, Sudip Sasmal1, Oriol Vendrell1,3

  • 1Theoretische Chemie, Physikalisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 229, 69120 Heidelberg, Germany.

The journal of physical chemistry letters
|May 2, 2024
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概括

环形分子在兴奋状态下表现出反向电子环流,纯粹由电子结构驱动,而不是振动. 这一发现为分子电子行为提供了新的见解.

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科学领域:

  • 量子化学 是一个量子化学.
  • 分子物理学 分子物理学
  • 电子结构理论 电子结构理论

背景情况:

  • 永久电子环流存在于退化的兴奋电子状态 (ΓE).
  • 之前的研究已经确定了电子电流对抗光极化的逆电流多元体,归因于振动合.
  • 纯电子结构在支这些电流中的作用尚未得到充分探索.

研究的目的:

  • 为了研究基于电子结构的环状分子中逆电流流体的存在.
  • 阐明导致逆电流形成的基础电子机制.
  • 为识别这些分子中的多样体提供一个计算框架.

主要方法:

  • 使用具有循环对称性的紧密结合模型进行理论建模.
  • 和sym-triazine的初始电子结构计算.
  • 电子轨道角动量和电流生成的分析.

主要成果:

  • 环形分子系统以纯电子结构为基础,独立于振动合,表现出逆流多元体.
  • 紧密结合模型和ab initio计算证实了逆电流的电子起源.
  • 提出了一种方法,使用量子化学来区分正规和逆流变频器.

结论:

  • 反向电子环流可以来自分子的内在电子特性,特别是在环系统中.
  • 这一发现扩大了对激发状态下的电子动态的理解,超越了振动效应.
  • 开发的框架有助于识别和研究各种分子系统中的逆流现象.