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

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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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¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

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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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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.
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NMR Spectroscopy: Spin–Spin Coupling01:08

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The spin state of an NMR-active nucleus can have a slight effect on its immediate electronic environment. This effect propagates through the intervening bonds and affects the electronic environments of NMR-active nuclei up to three bonds away; occasionally, even farther. This phenomenon is called spin–spin coupling or J-coupling. Coupling interactions are mutual and result in small changes in the absorption frequencies of both nuclei involved. While nuclei of the same element are involved...
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¹H NMR: Interpreting Distorted and Overlapping Signals01:02

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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...
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Intra- and Inter-Molecular Spin Coupling in Phenalenyl Dimeric Systems.

Maria C Buta1, Bogdan Frecus1, Mirela Enache1

  • 1Institute of Physical Chemistry, Splaiul Independentei 202, 060021 Bucharest, Romania.

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|August 6, 2021
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Summary

We studied phenalenyl radical dimers, revealing novel intermolecular binding patterns. Our findings offer insights into supramolecular assembly and potential applications in organic magnetism and spintronics.

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Area of Science:

  • Materials Science
  • Computational Chemistry
  • Organic Chemistry

Background:

  • Phenalenyl is a triangular aromatic molecule with unpaired electrons.
  • Its derivatives form stacked radical crystal structures, relevant for supramolecular assembly.
  • Understanding intermolecular binding is crucial for designing radical-based materials.

Purpose of the Study:

  • Investigate intermolecular binding in phenalenyl dimers using advanced computational methods.
  • Elucidate the interplay of exchange coupling and van der Waals forces in radical interactions.
  • Develop phenomenological models to interpret and predict binding behaviors.

Main Methods:

  • Comparative study using wave function-based and density functional theories.
  • Calculation of potential energy surfaces for varying inter-planar separation and rotation.
  • Development and calibration of phenomenological models based on orbital overlap and valence bond theory.

Main Results:

  • Discovered a novel binding pattern in phenalenyl dimers not previously reported.
  • Successfully interpreted the binding dependence using a transparent phenomenological model.
  • Developed a simplified phenomenological valence bond model calibrated with ab initio data.

Conclusions:

  • The study provides theoretical insights into the supramolecular assembly of phenalenyl radical systems.
  • The findings are relevant for methodological advancements in computational chemistry.
  • Phenalenyl dimers show promise for applications in spintronics and organic magnetism.