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Related Concept Videos

¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

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

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

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...
Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)

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 involved orbitals. The...
NMR Spectroscopy: Spin–Spin Coupling01:08

NMR Spectroscopy: Spin–Spin Coupling

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 in...
Spin–Spin Coupling Constant: Overview01:08

Spin–Spin Coupling Constant: Overview

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 have a...
Spin–Spin Coupling: One-Bond Coupling01:17

Spin–Spin Coupling: One-Bond Coupling

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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Optimizing Magnetic Force Microscopy Resolution and Sensitivity to Visualize Nanoscale Magnetic Domains
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Polyacene spacers in intramolecular magnetic coupling.

Md Ehesan Ali1, Sambhu N Datta

  • 1Department of Chemistry, Indian Institute of Technology-Bombay, Powai, Mumbai-400076, India.

The Journal of Physical Chemistry. A
|December 8, 2006
PubMed
Summary

We predict magnetic coupling in nitronyl nitroxide diradicals using DFT. Coupling constants vary with polyacene coupler structure, with larger values for linear couplers and increasing ferromagnetic coupling from tetracene onward.

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Area of Science:

  • Quantum Chemistry
  • Materials Science
  • Organic Electronics

Background:

  • Nitronyl nitroxide diradicals are key organic magnetic materials.
  • Understanding intramolecular magnetic exchange coupling is crucial for designing new magnetic molecules.

Purpose of the Study:

  • Predict magnetic exchange coupling constants (J) for eleven nitronyl nitroxide diradicals with polyacene couplers.
  • Investigate the influence of linear and angular polyacene structures on magnetic coupling.
  • Develop an empirical formula to predict J values.

Main Methods:

  • Broken-symmetry density functional treatment (DFT) to calculate J.
  • Calculation of Nuclear Independent Chemical Shift (NICS) values.
  • Analysis of Wiberg bond order (BO) and angle of twist (phi).

Main Results:

  • J decreases with fused rings for small linear acenes, then increases from anthracene due to enhanced diradical character.
  • Bent couplers yield smaller J values than linear couplers of similar size.
  • NICS values are lower in diradicals compared to parent polyacenes.
  • An empirical formula combining BO, phi, and NICS accurately predicts J.

Conclusions:

  • The diradical character of linear acene couplers significantly impacts magnetic coupling.
  • The developed empirical formula provides a reliable method for predicting J.
  • Results offer insights for designing organic materials with tailored magnetic properties.