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Related Concept Videos

Spin–Spin Coupling Constant: Overview01:08

Spin–Spin Coupling Constant: Overview

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In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal from the methyl protons is split into three peaks with 1:2:1 relative intensities. The methylene protons appear as a quartet, with the relative intensities of 1:3:3:1.
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must...
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¹H NMR: Long-Range Coupling01:27

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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...
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¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

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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.
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...
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Aromatic Hydrocarbon Cations: Structural Overview01:18

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Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
Removing one hydrogen from the intervening CH2 group...
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Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.
The central atom need not be NMR-active because its electrons are affected by the electron polarization of the spin-active atoms. However, spin information is transmitted less effectively than in one-bond coupling, and 2J values are usually weaker than 1J values. The energy of...
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Spin–Spin Coupling: One-Bond Coupling01:17

Spin–Spin Coupling: One-Bond Coupling

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Coupling interactions are strongest between NMR-active nuclei bonded to each other, where spin information can be transmitted directly through the pair of bonding electrons. While nuclei polarize their electrons to the opposite spins, the bonding electron pair has opposite spins. Configurations with antiparallel nuclear spins are expected to be lower in energy. When coupling makes antiparallel states more favorable, J is considered to have a positive value. The one-bond coupling constant, 1J,...
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Anharmonic coupling between fundamental modes in tetramethylurea.

Yuanzheng Chen1, Zhiwei Men2, Juntao Li2

  • 1State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People's Republic of China.

The Journal of Chemical Physics
|May 3, 2014
PubMed
Summary

High pressure Raman spectroscopy revealed phase transitions in tetramethylurea at 0.2 GPa and 7.4 GPa. A Fermi resonance phenomenon was observed, showing an exponential intensity-frequency difference relationship.

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Area of Science:

  • Solid-state chemistry
  • Spectroscopy
  • Materials science

Background:

  • Tetramethylurea (TMU) is a molecule with potential applications in materials science.
  • Understanding its behavior under extreme conditions, such as high pressure, is crucial for exploring new material properties.
  • Raman spectroscopy is a powerful technique for probing molecular vibrations and structural changes.

Purpose of the Study:

  • To investigate the phase transitions of tetramethylurea under high pressure.
  • To explore the pressure-induced spectral changes and identify novel phenomena.
  • To analyze the relationship between pressure, phase transitions, and spectral characteristics, specifically Fermi resonance.

Main Methods:

  • In situ high-pressure Raman spectroscopy was employed.
  • Measurements were conducted up to 25 GPa.
  • Phase transitions were identified by analyzing spectral changes.

Main Results:

  • Liquid-solid and solid-solid phase transitions were observed at 0.2 GPa and 7.4 GPa, respectively.
  • An unprecedented Fermi resonance between fundamental modes was detected.
  • An exponential relationship was found between the intensity and frequency difference of the Fermi resonance.

Conclusions:

  • High pressure significantly alters the structural and vibrational properties of tetramethylurea.
  • Fermi resonance in tetramethylurea exhibits a unique pressure-dependent behavior.
  • Pressure serves as a valuable tool for investigating Fermi resonance parameters and their correlations.