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Related Concept Videos

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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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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Applications Of NMR In Biology01:25

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Nuclear magnetic resonance (NMR) spectroscopy is a very valuable analytical technique for researchers. It has been used for more than 50 years as an analytical tool. F. Bloch and E. Purcell formulated NMR in 1946 and won the 1952 Nobel Prize in Physics  for their work. Biological macromolecules such as proteins, nucleic acids, lipids, and organic molecules including pharmaceutical compounds, can be studied using this versatile tool that exploits the magnetic properties of certain nuclei.
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The Pople nomenclature system classifies spin systems based on the difference between their chemical shifts. Coupled spins are denoted by capital letters with subscripts indicating the number of equivalent nuclei. When the coupled nuclei have well-separated chemical shifts, they are assigned letters that are far apart in the alphabet, such as A and X. When the difference in chemical shifts is small, coupled nuclei are named using adjacent letters of the alphabet (AB, MN, or XY).
A proton...
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NMR spectrometers consist of a strong magnet, a radiofrequency transmitter, and a detector attached to a computer console for recording spectra of samples containing NMR-active nuclei. In first-generation NMR instruments called continuous-wave spectrometers, the resonance frequencies of the nuclei are determined by frequency-sweep or field-sweep methods. The magnetic field strength is fixed and the rf signal is swept in the former, while the radiofrequency signal is fixed and the magnetic field...
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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.
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Quantitative 31P NMR Analysis of Lignins and Tannins
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Optimized slice-selective 1H NMR experiments combined with highly accurate quantitative 13C NMR using an internal

Tangi Jézéquel1, Virginie Silvestre1, Katy Dinis1

  • 1Université de Nantes, CNRS, CEISAM UMR 6230, Nantes, France.

Journal of Magnetic Resonance (San Diego, Calif. : 1997)
|February 16, 2018
PubMed
Summary

New NMR methods, Multi-WET and Profiled-WET, enable accurate 13C isotopic analysis. These techniques improve Position-specific Isotope Analysis (PSIA) by overcoming limitations with short relaxation times, reducing experiment duration.

Keywords:
AccuracyFrequency-swept pulseIsotopic analysisQuantitative NMRRadiation damping

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Area of Science:

  • Analytical Chemistry
  • Spectroscopy
  • Nuclear Magnetic Resonance (NMR)

Background:

  • Isotope Ratio Monitoring by 13C NMR (irm-13C NMR) is crucial for tracing biochemical pathways via Position-specific Isotope Analysis (PSIA).
  • Traditional PSIA combines irm-MS and 13C NMR, but requires separate measurements and is not always feasible.
  • Existing NMR-only methods using the DWET sequence are limited by short T1 relaxation times, especially with added relaxing agents.

Purpose of the Study:

  • To develop advanced NMR techniques for highly accurate (1‰) 13C isotopic analysis using only NMR.
  • To overcome the limitations of the DWET sequence regarding short T1 relaxation times in irm-13C NMR.
  • To enhance the robustness and efficiency of NMR-based PSIA.

Main Methods:

  • Development and optimization of two new 1H NMR pulse sequences: Multi-WET and Profiled-WET.
  • Integration of these sequences with 13C isotopic NMR measurements for quantitative analysis.
  • Evaluation of the new methods' accuracy and robustness using vanillin as a test compound.

Main Results:

  • Multi-WET and Profiled-WET achieve the required 1‰ accuracy for 13C isotopic analysis.
  • The new sequences demonstrate improved immunity to T1 variations compared to the original DWET method.
  • Enhanced robustness to pulse miscalibrations was observed for both novel sequences.

Conclusions:

  • Multi-WET and Profiled-WET significantly advance irm-13C NMR by enabling accurate isotopic analysis with high concentrations of relaxing agents.
  • These methods reduce overall experiment time, making PSIA more accessible and efficient.
  • The developed techniques offer a more versatile and reliable NMR-only approach for isotopic analysis.