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相关概念视频

Spin–Spin Coupling Constant: Overview01:08

Spin–Spin Coupling Constant: Overview

932
In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal from the methyl protons is split into three peaks with 1:2:1 relative intensities. The methylene protons appear as a quartet, with the relative intensities of 1:3:3:1.
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must...
932
Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)

1.1K
Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred via electron spin interactions between adjacent C‑H bond orbitals. This generally favors the antiparallel arrangement of spins, so 3J values are usually positive.
The extent of coupling depends on the C‑C bond length, the two H‑C‑C angles, any electron-withdrawing substituents, and the dihedral angle between the...
1.1K
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
Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)

1.0K
Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.
The central atom need not be NMR-active because its electrons are affected by the electron polarization of the spin-active atoms. However, spin information is transmitted less effectively than in one-bond coupling, and 2J values are usually weaker than 1J values. The energy of...
1.0K
NMR Spectrometers: Resolution and Error Correction01:14

NMR Spectrometers: Resolution and Error Correction

700
When magnetic nuclei in a sample achieve resonance and undergo relaxation, the signal detected in NMR is an approximately exponential free induction decay. Fourier transform of an exponential decay yields a Lorentzian peak in the frequency domain. Lorentzian peaks in an NMR spectrum are defined by their amplitude, full width at half maximum, and position, where the peak width is governed by the spin-spin relaxation time alone. In real experiments, however, the applied magnetic field is rendered...
700
NMR Spectroscopy: Spin–Spin Coupling01:08

NMR Spectroscopy: Spin–Spin Coupling

1.4K
The spin state of an NMR-active nucleus can have a slight effect on its immediate electronic environment. This effect propagates through the intervening bonds and affects the electronic environments of NMR-active nuclei up to three bonds away; occasionally, even farther. This phenomenon is called spin–spin coupling or J-coupling. Coupling interactions are mutual and result in small changes in the absorption frequencies of both nuclei involved. While nuclei of the same element are involved...
1.4K

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Excitonic Hamiltonians for Calculating Optical Absorption Spectra and Optoelectronic Properties of Molecular Aggregates and Solids
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非对线自旋密度函数理论的自我相互作用校正方案.

Nicolas Tancogne-Dejean1,2, Martin Lüders1, Carsten A Ullrich3

  • 1Max Planck Institute for the Structure and Dynamics of Matter and Center for Free-Electron Laser Science, Luruper Chaussee 149, 22761 Hamburg, Germany.

The Journal of chemical physics
|December 12, 2023
PubMed
概括

我们将密度函数理论的自我相互作用校正 (SIC) 方法扩展到非对线磁系统. 平均密度SIC提高了电离能,而Perdew-Zunger SIC则引入了交换相关扭矩.

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科学领域:

  • 计算化学是一种计算化学.
  • 量子力学就是量子力学.
  • 材料科学是一种材料科学.

背景情况:

  • 密度函数理论 (DFT) 是一种用于电子结构计算的强大的量子力学方法.
  • 自相互作用误差 (SIE) 是标准DFT近似的已知限制,影响某些属性的准确性.
  • 非线性磁力包括与特定轴不对齐的磁矩,在分子磁铁和集群中很常见.

研究的目的:

  • 扩展已建立的自我相互作用校正 (SIC) 方案,特别是Perdew-Zunger (PZ-SIC) 和平均密度SIC (AD-SIC),以处理具有非对线磁性的系统.
  • 评估这些扩展的SIC方案的性能与分子和金属集群系统的局部自旋密度近似值 (LSDA) 相结合.
  • 分析由扩展的PZ-SIC产生的交换相关磁场的行为.

主要方法:

  • 在DFT框架内对非对线磁系统实施通用的Perdew-Zunger SIC和平均密度SIC.
  • 应用这些扩展的SIC方法,结合局部自旋密度近似 (LSDA),以计算所选分子和金属的电子性质.
  • 对交换相关性磁场及其与局部总磁化对齐的研究.

主要成果:

  • 扩展的AD-SIC方案有效地提高了计算的电离能,与对直线系统的发现一致.
  • 扩展的AD-SIC方案显示了精确预测微妙属性的局限性,例如极性分子的双极时刻.
  • 由扩展的PZ-SIC产生的交换相关磁场被发现与局部总磁化不一致,导致交换相关扭矩.

结论:

  • 开发的SIC方案为DFT中的非线性磁性处理提供了可行的方法.
  • AD-SIC对改善电离能有好处,但对其他特性需要进一步精细化.
  • 在PZ-SIC中交换相关磁场的错位引入了扭矩,为研究磁现象提供了新的途径.