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Related Concept Videos

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...
NMR Spectrometers: Resolution and Error Correction01:14

NMR Spectrometers: Resolution and Error Correction

When magnetic nuclei in a sample achieve resonance and undergo relaxation, the signal detected in NMR is an approximately exponential free induction decay. Fourier transform of an exponential decay yields a Lorentzian peak in the frequency domain. Lorentzian peaks in an NMR spectrum are defined by their amplitude, full width at half maximum, and position, where the peak width is governed by the spin-spin relaxation time alone. In real experiments, however, the applied magnetic field is rendered...
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...
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...
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.
Two-Dimensional (2D) NMR: Overview01:12

Two-Dimensional (2D) NMR: Overview

The 1D NMR spectrum of large and complex molecules like natural products has complicated splitting patterns and overlapping signals, which can be easily interpreted using 2-dimensional (2D) NMR. Unlike 1D NMR, 2D NMR has two frequency axes that provide the coupling information between the nucleus A and nucleus B in a molecule. The process from which 2D spectra are obtained has four steps.
The first step is the preparation period, during which nucleus A is excited with a radiofrequency pulse.

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NMR 15N Relaxation Experiments for the Investigation of Picosecond to Nanoseconds Structural Dynamics of Proteins
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Cautionary note: linewidth effect in dynamic NMR.

J A Weil1, B C S Wong, J Yu

  • 1Dept. of Chemistry, University of Saskatchewan, Saskatoon, Canada. ohn.weil@usask.ca

Spectrochimica Acta. Part A, Molecular and Biomolecular Spectroscopy
|August 6, 2010
PubMed
Summary
This summary is machine-generated.

Nuclear Magnetic Resonance (NMR) spectroscopy requires estimating inherent line width (w) for kinetic studies. A new model function w(T) was developed to analyze activation energy variations, revealing a wide range of possible values.

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Area of Science:

  • Physical Chemistry
  • Spectroscopy
  • Chemical Kinetics

Background:

  • Nuclear Magnetic Resonance (NMR) spectroscopy is crucial for studying molecular dynamics.
  • Kinetic processes in solute molecules broaden spectral lines, obscuring inherent width (w).
  • Accurate determination of inherent line width is essential for kinetic analysis.

Purpose of the Study:

  • To develop a self-consistent model function w(T) to estimate inherent line width.
  • To evaluate the variation in Arrhenius activation energy using the parameters within w(T).
  • To assess the impact of line-width parameters on kinetic data interpretation.

Main Methods:

  • Development of a self-consistent model function w(T).
  • Application of the model to NMR spectral data.
  • Analysis of spectral fits and derived kinetic parameters.

Main Results:

  • The model function w(T) successfully estimates inherent line width.
  • A significant number of line-width parameters yield excellent spectral fits.
  • These fits result in a considerable range of possible Arrhenius activation energies.

Conclusions:

  • The developed model provides a method to account for line-width broadening in NMR kinetic studies.
  • The variability in derived activation energies highlights the importance of accurate line-width estimation.
  • Further refinement of the model may lead to more precise kinetic parameter determination.