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

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

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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...
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¹³C NMR: ¹H–¹³C Decoupling01:04

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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.
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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.
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¹H NMR of Labile Protons: Deuterium (²H) Substitution00:48

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This lesson illustrates the role of deuterium substitution in simplifying the NMR spectrum of compounds comprising labile protons. One method employed is the use of deuterium. Amongst the three isotopes of hydrogen, deuterium (2H) has a nucleus composed of one proton and one neutron. When the D2O solvent is added to a pure dry ethanol solution, its labile proton is substituted with deuterium.
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Carbon-13 (¹³C) NMR: Overview01:10

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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...
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Mass Spectrometry: Isotope Effect01:13

Mass Spectrometry: Isotope Effect

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Most elements exist in nature as a mixture of isotopes. The isotopes differ in weight due to their respective number of neutrons. The molecular weight of a molecule is different depending on the specific isotope of its elements involved. As a result, the mass spectrum of the molecule exhibits peaks from the same fragment at multiple positions. The positions of these mass signals depend on the mass differences between isotopes. Furthermore, the intensity of these signals is dependent on the...
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Isotopically enriched 13C diffusion-ordered NMR spectroscopy: analysis of methyllithium.

Chicheung Su1, Russell Hopson, Paul G Williard

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

The Journal of Organic Chemistry
|October 19, 2013
PubMed
Summary

This study introduces isotopic-labeled carbon-13 diffusion-ordered NMR spectroscopy (DOSY) for analyzing molecular aggregation. The method reveals methyllithium forms tetramers in ether and dimers with diamines.

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Area of Science:

  • Analytical Chemistry
  • Organic Chemistry
  • Physical Chemistry

Background:

  • Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful tool for chemical analysis.
  • Diffusion-ordered NMR spectroscopy (DOSY) is used to determine diffusion coefficients and infer molecular size.
  • Characterizing the aggregation state of organometallic compounds like methyllithium is crucial for understanding their reactivity.

Purpose of the Study:

  • To develop and apply isotopic-labeled carbon-13 diffusion-ordered NMR spectroscopy (DOSY) with diffusion coefficient-formula weight (D-FW) analysis.
  • To characterize the aggregation state of methyllithium aggregates and their complexes with diamines.

Main Methods:

  • Development of (13)C-labeled internal standards using (13)C-labeled benzene and iodomethane.
  • Application of DOSY NMR combined with D-FW analysis.
  • Investigation of methyllithium in diethyl ether and in the presence of chelating diamines.

Main Results:

  • Isotopic labeling significantly expands the applicability of DOSY D-FW analysis.
  • Methyllithium exists as a tetrasolvated tetramer in diethyl ether.
  • Methyllithium forms exclusively bis-solvated dimers when complexed with diamines.

Conclusions:

  • Isotopic-labeled (13)C DOSY-NMR with D-FW analysis is a robust method for studying aggregation states.
  • The aggregation behavior of methyllithium is highly dependent on the solvent and coordinating ligands.