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Related Concept Videos

Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)

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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: Three-Bond Coupling (Vicinal Coupling)01:22

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Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred via electron spin interactions between adjacent C‑H bond orbitals. This generally favors the antiparallel arrangement of spins, so 3J values are usually positive.
The extent of coupling depends on the C‑C bond length, the two H‑C‑C angles, any electron-withdrawing substituents, and the dihedral angle between the...
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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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Spin–Spin Coupling Constant: Overview01:08

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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.
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Aryldiazonium Salts to Azo Dyes: Diazo Coupling01:11

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The reaction of weakly electrophilic aryldiazonium (also called arenediazonium) salts with highly activated aromatic compounds leads to the formation of products with an —N=N— link, called an azo linkage. This reaction, presented in Figure 1, is known as diazo coupling and occurs without the loss of the nitrogen atoms of the aryldiazonium salt. Highly activated aromatic compounds such as phenols or arylamines favor the diazo coupling reaction. The coupling generally occurs at the...
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Nonalternating π-System Mediated Spin Coupling in Azulene Nitronyl Nitroxide Diradicals.

Di Wang1, Chengfang Shi2, Martin Baumgarten3

  • 1Anhui Key Laboratory of Advanced Building Materials, School of Materials Science and Chemical Engineering, Anhui Jianzhu University, Hefei 230601, China.

The Journal of Organic Chemistry
|August 13, 2024
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Researchers explored spin coupling in naphthalene (Na) and azulene (Az) bridged compounds. Both compounds showed antiferromagnetic coupling, with azulene exhibiting a preferred short coupling path, offering insights into molecular magnetism.

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Area of Science:

  • Molecular Magnetism
  • Quantum Chemistry
  • Supramolecular Chemistry

Background:

  • Understanding spin coupling in molecular bridges is crucial for designing advanced magnetic materials.
  • Alternating and nonalternating π-systems exhibit distinct electronic properties influencing magnetic interactions.

Purpose of the Study:

  • To investigate and compare spin coupling mechanisms in isoelectronic compounds with naphthalene and azulene bridges.
  • To elucidate the preferred spin coupling pathways in nonalternating π-systems like azulene.

Main Methods:

  • Synthesis of isoelectronic compounds 2,6-Na-NN and 2,6-Az-NN.
  • Variable-temperature Electron Paramagnetic Resonance (VT-EPR) and SQUID magnetometry.
  • Density Functional Theory (DFT) computations.

Main Results:

  • Both 2,6-Na-NN and 2,6-Az-NN displayed antiferromagnetic coupling (J = -22.3 cm⁻¹ and J = -30.1 cm⁻¹, respectively).
  • DFT calculations confirmed negative coupling constants and delocalization of spin densities into the molecular bridges.
  • The study identified a preferred short coupling path for the 2,6-Az-NN compound.

Conclusions:

  • Azulene-based molecular bridges facilitate efficient antiferromagnetic spin coupling.
  • Short spin-transfer pathways are favored in nonalternating π-systems like azulene.
  • The findings provide a foundation for designing novel molecular magnetic materials with tailored properties.