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相关概念视频

Diamagnetic Shielding of Nuclei: Local Diamagnetic Current01:14

Diamagnetic Shielding of Nuclei: Local Diamagnetic Current

848
An applied magnetic field causes the electrons present in the molecule to circulate, setting up a local diamagnetic current within the molecule. The local diamagnetic current arising from circulating sigma-bonding electrons induces a magnetic field, Blocal that opposes the applied magnetic field, B0. The effective magnetic field experienced by these nuclei is given by the difference between the applied and local magnetic fields in a phenomenon called local diamagnetic shielding. Essentially,...
848
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
π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

1.2K
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
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

1.0K
Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are...
1.0K
NMR Spectroscopy of Aromatic Compounds01:14

NMR Spectroscopy of Aromatic Compounds

4.7K
Aromatic compounds can be identified or analyzed using proton NMR and carbon‐13 NMR. Typically, aromatic hydrogens or hydrogens directly bonded to the aromatic rings are strongly deshielded by the aromatic ring current. Therefore, they absorb in the range of 6.5–8.0 ppm in proton NMR spectra. For instance, aromatic hydrogens directly bonded to the benzene ring absorb at 7.3 ppm. However, aromatic hydrogens of larger rings absorb farther upfield or downfield than the ideal range.
4.7K
NMR Spectroscopy: Chemical Shift Overview01:15

NMR Spectroscopy: Chemical Shift Overview

1.5K
The position of the absorption signal of a sample is reported relative to the position of the signal of tetramethylsilane (TMS), which is added as an internal reference while recording spectra. The difference between the absorption frequencies of the sample and TMS (in Hz) is divided by the spectrometer operating frequency (in MHz) to obtain a dimensionless quantity called the chemical shift. It is reported on the δ (delta) scale and expressed in parts per million.
For instance, the proton...
1.5K

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Line Shape Analysis of Dynamic NMR Spectra for Characterizing Coordination Sphere Rearrangements at a Chiral Rhenium Polyhydride Complex
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精确的NMR屏蔽与σ-功能.

Steffen Fauser1, Viktoria Drontschenko2, Christian Ochsenfeld2,3

  • 1Lehrstuhl für Theoretische Chemie, Universität Erlangen-Nürnberg, Egerlandstr. 3, D-91058 Erlangen, Germany.

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概括
此摘要是机器生成的。

新的西格玛功能器为核磁共振 (NMR) 屏蔽提供了高度准确的预测. 这些先进的密度功能方法在计算反应能量方面也出色,证明了它们的普遍适用性.

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科学领域:

  • 计算化学计算化学
  • 量子化学 是一个量子化学.
  • 密度函数理论 密度函数理论

背景情况:

  • 密度函数理论 (DFT) 的近期进展引入了第五层相关函数,称为σ-函数.
  • σ-函数在计算上与随机相近似,但在主要组化学中为能量提供更高的精度.
  • 这些函数在计算反应和过渡状态能量方面表现优于双混合函数方面表现有希望.

研究的目的:

  • 系统地评估 σ 函数在描述核磁共振 (NMR) 屏蔽时的准确性.
  • 确定仅在能量数据上进行优化的s-函数能否准确预测像NMR屏蔽这样的响应特性.
  • 为了比较 σ 函数的性能与其他 DFT 方法,包括双混合函数,用于 NMR 屏蔽计算.

主要方法:

  • 为计算NMR屏蔽的s-函数的实施和应用.
  • 使用Kohn-Sham计算与输入轨道和自身值的Perdew-Burke-Ernzerhof (PBE) 交换相关函数.
  • 在各种DFT方法中对NMR屏蔽结果进行系统的调查和比较.

主要成果:

  • σ-功能产生的高精度的NMR屏蔽,尽管不是明确优化这些属性.
  • 当与PBE轨道相结合时,s-functional在所有测试的DFT方法中产生了最准确的NMR屏蔽.
  • 即使与采用双混合功能方法相比,也可以达到这种准确性.

结论:

  • σ-函数证明了普遍适用性,准确地预测了能量和响应特性,就像NMR屏蔽一样.
  • 它们能够为能源和财产计算提供可靠的结果,这使得它们成为多功能"万能"功能器.
  • 这些发现突显了s-functionals作为各种化学应用中的强大和精确工具的潜力.