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相关概念视频

¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

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...
¹H NMR Chemical Shift Equivalence: Homotopic and Heterotopic Protons01:03

¹H NMR Chemical Shift Equivalence: Homotopic and Heterotopic Protons

Protons in identical electronic environments within a molecule are chemically equivalent and have the same chemical shift. The replacement test is a useful tool to identify chemical equivalence and predict NMR spectra. A substituent replaces each of the protons being examined and the resulting molecules are compared. If the same molecule is obtained, the protons are equivalent or homotopic. Replacement of any hydrogens in ethane by chlorine yields chloroethane because all six protons are...
¹H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons00:58

¹H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons

Replacing each alpha-hydrogen in chloroethane by bromine (or a different functional group) yields a pair of enantiomers. Such protons are called prochiral or enantiotopic and are related by a mirror plane. Enantiotopic protons are chemically equivalent in an achiral environment. Because most proton NMR spectra are recorded using achiral solvents, enantiotopic hydrogens yield a single signal.
In chiral compounds such as 2-butanol, replacing the methylene hydrogens at C3 produces a pair of...
NMR Spectroscopy: Spin–Spin Coupling01:08

NMR Spectroscopy: Spin–Spin Coupling

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

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

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.

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相关实验视频

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Spin Saturation Transfer Difference NMR (SSTD NMR): A New Tool to Obtain Kinetic Parameters of Chemical Exchange Processes
11:44

Spin Saturation Transfer Difference NMR (SSTD NMR): A New Tool to Obtain Kinetic Parameters of Chemical Exchange Processes

Published on: November 12, 2016

在十甲基氨基烯离子体中的 anisotropic NMR相互作用张量.

Robert W Schurko1, Ivan Hung, Charles L B Macdonald

  • 1Department of Chemistry and Biochemistry, University of Windsor, Windsor, Ontario, Canada N9B 3P4. rschurko@uwindsor.ca

Journal of the American Chemical Society
|October 31, 2002
PubMed
概括
此摘要是机器生成的。

双 (pentamethylcyclopentadienyl) 离子,[Cp(2) Al](+),具有很高的对称性,类似于铁. 核磁共振和计算方法揭示了它的结构,动态环旋转和异型核磁共振相互作用,证实了它的独特特性.

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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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科学领域:

  • 有机金属化学 有机金属化学
  • 固态核磁共振 (NMR) 光谱学 固态核磁共振 (NMR) 光谱
  • 计算化学计算化学

背景情况:

  • 主组金属,如二甲 (pentamethylcyclopentadienyl) 酸 ([Cp(2) Al] ((+)),在结构上类似于过渡金属金属,如铁.
  • 了解这些化合物的分子结构和电子特性对于推进主要组化学至关重要.
  • 无极性NMR相互作用张量器提供了对当地的电子环境和分子对称性的详细见解.

研究的目的:

  • 综合描述 bis(pentamethylcyclopentadienyl) 离子 ([Cp(2) Al] ((+)) 的分子结构和异构性NMR相互作用张量.
  • 为了研究动态行为,特别是环旋转,在[Cp(2) Al](+) 电位内.
  • 将实验性NMR数据与理论计算相关联,以了解观察到的NMR相互作用的起源.

主要方法:

  • 固态核磁共振 (NMR) 实验,包括27Al[(1) H]神奇角旋转 (MAS) 和静态NMR,以及13C CPMAS NMR.
  • 单晶X射线衍射用于[Cp(2) Al][AlCl(4) ]的结构确定.
  • 最初的自一致场 (SCF) 和混合密度函数理论 (DFT) 计算,以及NMR粉末模式的分析和数值模拟.

主要成果:

  • 确定了[Cp(2) Al][AlCl(4) ]的单晶X射线衍射结构.
  • 固态27Al NMR揭示了轴对称的化学屏蔽和电场梯度张量,具有很大的化学屏蔽跨度 (Omega = 83(3) ppm) 和一个小的核四极合常量 (C(Q) (((27) Al) = 0.86(10) MHz).
  • 13C NMR 实验和计算表明,甲基cyclopentadienyl 环的动态旋转具有较低的内部旋转屏障.

结论:

  • 双 (pentamethylcyclopentadienyl) 离子 ([Cp(2) Al](+)) 具有高分子对称性,与其与铁素的结构相似.
  • 实验和理论数据非常一致,为NMR相互作用的分子起源提供了洞察力.
  • 观察到的NMR参数,包括大化学屏蔽跨度和小四极合常数,通过[Cp(2) Al](+) 离子的高对称性得到合理化.