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相关概念视频

Two-Dimensional (2D) NMR: Overview01:12

Two-Dimensional (2D) NMR: Overview

632
The 1D NMR spectrum of large and complex molecules like natural products has complicated splitting patterns and overlapping signals, which can be easily interpreted using 2-dimensional (2D) NMR. Unlike 1D NMR, 2D NMR has two frequency axes that provide the coupling information between the nucleus A and nucleus B in a molecule. The process from which 2D spectra are obtained has four steps.
The first step is the preparation period, during which nucleus A is excited with a radiofrequency pulse....
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Chemical Shift: Internal References and Solvent Effects01:17

Chemical Shift: Internal References and Solvent Effects

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In an NMR sample, precise measurement of the absolute absorption frequencies of nuclei is difficult. A standard internal reference compound is added, and the frequency difference between the reference signal and sample signals is measured.
The internal reference compound generally used in NMR spectroscopy is tetramethylsilane (TMS). TMS is preferred because it is chemically inert, soluble in NMR solvents, and easily removable. Also, the highly shielded methyl protons in TMS yield an intense...
620
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: Chemical Shift Overview01:15

NMR Spectroscopy: Chemical Shift Overview

1.4K
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...
1.4K
¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR01:15

¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR

1.1K
The axial and equatorial protons in cyclohexane can be distinguished by performing a variable-temperature NMR experiment. In this process, except for one proton, the remaining eleven protons are replaced by deuterium. The deuterium substitution avoids the possible peak splitting caused by the spin-spin coupling between the adjacent protons. The remaining proton flips between the axial and equatorial positions.
1.1K
NMR Spectrometers: Resolution and Error Correction01:14

NMR Spectrometers: Resolution and Error Correction

678
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...
678

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Spin Saturation Transfer Difference NMR SSTD NMR: A New Tool to Obtain Kinetic Parameters of Chemical Exchange Processes
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EFG罗塞塔石:在DFT计算和固态NMR实验之间进行翻译.

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  • 1Fritz-Haber Institute of the Max Planck Society, Berlin, Germany.

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概括

本研究提供了使用第一原理模拟来解释高性能材料中的四极体固态NMR数据的指导方针. 优化计算方法增强了理论和实验之间的协同作用,以实现准确的材料分析.

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科学领域:

  • 计算材料科学科学 计算材料科学
  • 固态核磁共振 (SS-NMR) 光谱学
  • 量子化学和密度函数理论 (DFT)

背景情况:

  • 高性能材料 (HPM) 由于动态效应或无形相,经常表现出复杂的SS-NMR光谱.
  • 从HPM中准确解释SS-NMR光谱需要可靠的第一原则计算方法.
  • 四极核,如7Li和27Al,具有独特的相互作用 (四极合),提供宝贵的结构和对称性信息超出化学屏蔽异构性 (CSA).

研究的目的:

  • 通过使用平面波DFT,对四极核的电场梯度 (EFG) 计算进行基准测试和优化模拟策略.
  • 调查材料结构和模拟参数对EFG张量计算的影响.
  • 制定实用指导方针,将第一原则模拟与HPM的实验SS-NMR数据相结合.

主要方法:

  • 使用平面波DFT进行EFG张量计算的各种模拟策略的基准测试.
  • 对物理近似和模拟参数对结果影响的系统研究,特别是对轻核 (例如,7Li) 的影响.
  • 综合文献调查和创建7Li四极合器的参考尺度.

主要成果:

  • 模拟参数的选择显著影响了EFG计算的准确性和可转移性,特别是对于轻四极核.
  • 证明了模拟策略对于将DFT预测与实验SS-NMR数据相关的关键重要性.
  • 为7Li四极合器提供了第一个全面的参考尺度和文献调查.

结论:

  • 制定了选择最佳模拟策略的实际指导方针,以最大限度地提高DFT计算和实验SS-NMR之间的协同作用.
  • 通过提高第一原则模拟的可靠性,提高了HPM复杂SS-NMR光谱的解释能力.
  • 通过提供理论与实验数据相关性的强有力的框架,促进了对HPM的进一步研究.