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Related Concept Videos

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

969
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...
969
¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR01:15

¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR

1.3K
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.
1.3K
NMR Spectroscopy of Benzene Derivatives01:34

NMR Spectroscopy of Benzene Derivatives

9.4K
Simple unsubstituted benzene has six aromatic protons, all chemically equivalent. Therefore, benzene exhibits only a singlet peak at δ 7.3 ppm in the 1H NMR spectrum. The observed shift is far downfield because the aromatic ring current strongly deshields the protons. Any substitution on the benzene ring makes the aromatic protons nonequivalent, and the protons split each other. The peak is, therefore, no longer a singlet and the splitting pattern and their associated coupling...
9.4K
¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

2.1K
The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in spectra, the presence of multiple bonds along the coupling pathway can result in observable long-range coupling.
In alkenes, spin information is communicated via σ–π overlap, as seen in allylic (four-bond) and homoallylic (five-bond) couplings. These coupling interactions are stronger when the σ bond is parallel to the alkene...
2.1K
¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

1.4K
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.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied...
1.4K
π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

1.4K
In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter of the molecule. This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside. The protons in benzene are deshielded and exhibit high chemical shifts in the range 6.5–8.5 ppm. The shielding effect at the center of the ring is evident in complex aromatic molecules, such as...
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Dynamics in Flexible Pillar[n]arenes Probed by Solid-State NMR.

Ashlea R Hughes1, Ming Liu1,2, Subhradip Paul3

  • 1Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, United Kingdom.

The Journal of Physical Chemistry. C, Nanomaterials and Interfaces
|July 9, 2021
PubMed
Summary
This summary is machine-generated.

Pillar[n]arenes exhibit dynamic behavior crucial for molecular separations. Solid-state NMR reveals distinct motion between the core, flexible groups, and guest molecules, highlighting strong host-guest interactions.

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Area of Science:

  • Supramolecular Chemistry
  • Materials Science
  • Solid-State NMR Spectroscopy

Background:

  • Pillar[n]arenes are supramolecular assemblies with inherent molecular flexibility.
  • This flexibility enables technologically important molecular separation applications.

Purpose of the Study:

  • To investigate the dynamical behavior of perethylated pillar[n]arenes (n=5, 6) and their xylene adducts.
  • To understand the relationship between molecular dynamics and host-guest interactions in pillararene systems.

Main Methods:

  • Variable-temperature solid-state Nuclear Magnetic Resonance (NMR) experiments were conducted.
  • Measurements included site-selective 13C 1H heteronuclear dipolar couplings and 1H/13C spin-lattice relaxation times.
  • Order parameters and correlation times were extracted to analyze molecular motion.

Main Results:

  • Fast motional regimes were observed for pillar[n]arenes at room temperature.
  • Significant differences in dynamics were found between the pillararene core, ethoxy groups, and adsorbed xylene guest.
  • Unusual 13C 1H heteronuclear dipolar couplings indicated strong host-guest interactions for p-xylene in pillar[6]arene.

Conclusions:

  • The study elucidates the distinct motional characteristics within pillararene structures and their guest molecules.
  • Observed dynamics are critical for understanding and optimizing molecular separation capabilities.
  • Strong host-guest interactions were confirmed, pinpointing the location of p-xylene within the pillararene host.