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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...
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Other Nuclides: 31P, 19F, 15N NMR01:16

Other Nuclides: 31P, 19F, 15N NMR

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Many organic, inorganic, and biological molecules contain spin-half nuclei such as nitrogen-15, fluorine-19, and phosphorus-31. As a result, NMR studies of these nuclei have found extensive applications in chemical and biological research.
While fluorine-19 and phosphorous-31 have high natural abundances (100%) and positive gyromagnetic ratios, nitrogen-15 has a low natural abundance and a negative gyromagnetic ratio. However, nitrogen-15 is still preferred over nitrogen-14 (which has a...
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NMR Spectroscopy: Chemical Shift Overview01:15

NMR Spectroscopy: Chemical Shift Overview

3.6K
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...
3.6K
¹H NMR Chemical Shift Equivalence: Homotopic and Heterotopic Protons01:03

¹H NMR Chemical Shift Equivalence: Homotopic and Heterotopic Protons

3.9K
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...
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Carbon-13 (¹³C) NMR: Overview01:10

Carbon-13 (¹³C) NMR: Overview

6.6K
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...
6.6K
¹H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons00:58

¹H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons

3.7K
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...
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Atomic Scale Structural Studies of Macromolecular Assemblies by Solid-state Nuclear Magnetic Resonance Spectroscopy
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Atomic Scale Structural Studies of Macromolecular Assemblies by Solid-state Nuclear Magnetic Resonance Spectroscopy

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15N chemical shift referencing in solid state NMR.

Philippe Bertani1, Jésus Raya1, Burkhard Bechinger1

  • 1Université de Strasbourg/CNRS, UMR7177, Institut de Chimie de Strasbourg, 1 rue Blaise Pascal, 67070 Strasbourg, France.

Solid State Nuclear Magnetic Resonance
|April 22, 2014
PubMed
Summary
This summary is machine-generated.

Accurate chemical shift referencing in solid-state NMR is crucial for precise structure determination. Ammonium chloride-15N ((15)NH4Cl) is proposed as a reliable external standard for calibrating nitrogen-15 chemical shifts, improving data consistency.

Keywords:
(15)N NMRChemical shift referencingReference compoundSolid stateStandard

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Area of Science:

  • Solid-state Nuclear Magnetic Resonance (NMR) Spectroscopy
  • Materials Science
  • Structural Biology

Background:

  • Solid-state NMR spectroscopy enables high-resolution structure determination of diverse materials.
  • Inconsistent chemical shift referencing, particularly for nitrogen-15 ((15)N), limits the precision of structural calculations.
  • Existing (15)N chemical shift standards vary, leading to significant discrepancies (up to 22 ppm) in published data.

Purpose of the Study:

  • To evaluate methods for proton-decoupled (15)N solid-state NMR spectroscopy chemical shift referencing.
  • To report (15)N chemical shifts for commonly used reference compounds.
  • To establish a reliable external standard for calibrating (15)N chemical shifts.

Main Methods:

  • Evaluation of various chemical shift referencing methods in proton-decoupled (15)N solid-state NMR.
  • Determination of (15)N chemical shifts for selected reference compounds.
  • Assessment of ammonium chloride-15N ((15)NH4Cl) as an external standard for magic angle spinning and static NMR experiments.

Main Results:

  • Significant variations (up to 22 ppm) were observed in published (15)N chemical shift data due to differing standards.
  • (15)NH4Cl in powdered form provides a suitable external reference standard for (15)N chemical shift calibration.
  • Ammonium chloride-15N ((15)NH4Cl) yields narrow NMR lines and is compatible with cross-polarization experiments.
  • The temperature dependence of the (15)NH4Cl chemical shift was characterized.

Conclusions:

  • Standardization of (15)N chemical shift referencing is essential for improving the accuracy and comparability of solid-state NMR structure determination.
  • (15)NH4Cl is a robust and recommended external standard for (15)N chemical shift calibration in solid-state NMR.
  • The reported data facilitate recalibration of existing datasets and ensure consistency in future research.