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Related Concept Videos

Chemical Shift: Internal References and Solvent Effects01:17

Chemical Shift: Internal References and Solvent Effects

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...
¹³C NMR: Distortionless Enhancement by Polarization Transfer (DEPT)01:20

¹³C NMR: Distortionless Enhancement by Polarization Transfer (DEPT)

When proton-coupled carbon-13 spectra are simplified by a broadband proton decoupling technique, structural information about the coupled protons is lost. Distortionless enhancement by polarization transfer (DEPT) is a technique that provides information on the number of hydrogens attached to each carbon in a molecule. While the DEPT experiment utilizes complex pulse sequences, the pulse delay and flip angle are specifically manipulated. The resulting signals have different phases depending on...
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

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 slanted or...
¹³C NMR: ¹H–¹³C Decoupling01:04

¹³C NMR: ¹H–¹³C Decoupling

The probability of having two carbon-13 atoms next to each other is negligible because of the low natural abundance of carbon-13. Consequently, peak splitting due to carbon-carbon spin-spin coupling is not observed in spectra. However, protons up to three sigma bonds away split the carbon signal according to the n+1 rule, resulting in complicated spectra.
A broadband decoupling technique is used to simplify these complex, sometimes overlapping, signals. Broadband decoupling relies on a...
Carbon-13 (¹³C) NMR: Overview01:10

Carbon-13 (¹³C) NMR: Overview

Carbon-13 is a naturally occurring NMR-active isotope of carbon with a low natural abundance of 1.1%. In contrast, carbon-12 is the most abundant isotope of carbon with zero nuclear spin. Therefore, it is NMR inactive. The gyromagnetic ratio of carbon-13 is smaller than that of protons. As a result, carbon-13 resonance is about 6000 times weaker than proton resonance. For a given magnetic field strength, the resonance frequency of carbon-13 is about one-fourth of the resonance frequency for...
NMR Spectroscopy: Chemical Shift Overview01:15

NMR Spectroscopy: Chemical Shift Overview

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...

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Atomic Scale Structural Studies of Macromolecular Assemblies by Solid-state Nuclear Magnetic Resonance Spectroscopy
14:55

Atomic Scale Structural Studies of Macromolecular Assemblies by Solid-state Nuclear Magnetic Resonance Spectroscopy

Published on: September 17, 2017

13C INEPT diffusion-ordered NMR spectroscopy (DOSY) with internal references.

Deyu Li1, Russell Hopson, Weibin Li

  • 1Department of Chemistry, Brown University, Providence, Rhode Island 02912, USA.

Organic Letters
|February 7, 2008
PubMed
Summary

This study introduces 13C INEPT Diffusion-Ordered NMR Spectroscopy (DOSY) to analyze the aggregation of THF-solvated LDA dimeric complexes. The method successfully identified six distinct components, validating the aggregation state analysis.

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Area of Science:

  • Analytical Chemistry
  • Physical Chemistry
  • Organic Chemistry

Background:

  • Studying the aggregation state of organometallic complexes is crucial for understanding their reactivity.
  • Lithium diisopropylamide (LDA) is a widely used strong base in organic synthesis.
  • Accurate determination of aggregation states requires advanced spectroscopic techniques.

Purpose of the Study:

  • To develop and validate a 13C INEPT Diffusion-Ordered NMR Spectroscopy (DOSY) method.
  • To investigate the aggregation state of tetrahydrofuran (THF)-solvated LDA dimeric complex.
  • To establish a reliable method for analyzing complex aggregation phenomena.

Main Methods:

  • Utilized 13C INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) NMR.
  • Employed Diffusion-Ordered NMR Spectroscopy (DOSY) with an internal reference system.
  • Analyzed THF-solvated LDA dimeric complex.

Main Results:

  • Successfully identified six distinct components in the diffusion dimension.
  • Demonstrated excellent agreement between DOSY-generated 13C INEPT spectrum slices and individual INEPT spectra.
  • Achieved a high correlation coefficient (r = 0.9985) in the linear least-squares fit between log D and log FW.

Conclusions:

  • The developed 13C INEPT DOSY method is highly effective for studying aggregation states.
  • The technique provides accurate and reliable data for complex molecular systems.
  • This method offers a robust approach for characterizing the aggregation behavior of organometallic species.