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Related Concept Videos

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.
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...
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...
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...

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15N CPMG Relaxation Dispersion for the Investigation of Protein Conformational Dynamics on the &#181;s-ms Timescale
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Quinuclidine complex with alpha-cyclodextrin: a diffusion and 13C NMR relaxation study.

Sahar Nikkhou Aski1, Jozef Kowalewski

  • 1Department of Physical, Inorganic and Structural Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden.

Magnetic Resonance in Chemistry : MRC
|February 1, 2008
PubMed
Summary

Quinuclidine and alpha-cyclodextrin form a 1:1 inclusion complex in solution. NMR studies reveal rapid guest-host exchange and altered reorientational dynamics for quinuclidine upon complexation.

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Area of Science:

  • Supramolecular Chemistry
  • Physical Chemistry
  • Analytical Chemistry

Background:

  • Cyclodextrins are widely used host molecules for forming inclusion complexes.
  • Understanding the dynamics of guest-host interactions is crucial for applications in drug delivery and molecular recognition.

Purpose of the Study:

  • To investigate the stability and dynamics of the quinuclidine-alpha-cyclodextrin inclusion complex in solution.
  • To determine the association constant and analyze the reorientational dynamics of both guest and host molecules.

Main Methods:

  • Nuclear Magnetic Resonance (NMR) spectroscopy was employed.
  • Measurements included translational diffusion coefficients and carbon-13 NMR relaxation at two magnetic fields.

Main Results:

  • A 1:1 stoichiometry was established with an association constant of 35 +/- 3 M(-1).
  • Rapid exchange between the host cavity and bulk solution was observed.
  • Reorientational dynamics of quinuclidine were analyzed, showing increased anisotropy in the bound form.

Conclusions:

  • The quinuclidine-alpha-cyclodextrin complex exhibits moderate stability in solution.
  • NMR relaxation data provide insights into the motional characteristics of guest molecules within cyclodextrin hosts.
  • The study highlights the utility of NMR for characterizing host-guest complex dynamics.