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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 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: 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...
¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

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 first.
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,...
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...

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Measuring Magnetically-Tuned Ferroelectric Polarization in Liquid Crystals
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Magnetoelectric coupling in 4,4'-stilbenedinitrene.

Ö Günaydın-Şen1, P Chen, J Fosso-Tande

  • 1Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, USA.

The Journal of Chemical Physics
|June 8, 2013
PubMed
Summary

Researchers studied the optical properties of 4,4′-stilbenedinitrene using low temperatures and high magnetic fields. They found these conditions allow control over the molecule's optical contrast by manipulating its singlet-triplet equilibrium.

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Area of Science:

  • Physical Chemistry
  • Quantum Chemistry
  • Materials Science

Background:

  • Organic biradicals are crucial in understanding charge-spin interactions.
  • Controlling the singlet-triplet equilibrium is key to modulating molecular properties.

Purpose of the Study:

  • To investigate the optical properties of 4,4′-stilbenedinitrene under varying physical conditions.
  • To correlate optical behavior with the singlet-triplet spin state equilibrium.
  • To explore the influence of external stimuli on organic biradical properties.

Main Methods:

  • Experimental measurements of optical properties at low temperatures and high magnetic fields.
  • First-principles calculations for theoretical validation.
  • Analysis of magneto-optical response using population and Beer's law frameworks.

Main Results:

  • Optical contrast was manipulated by controlling the singlet-triplet equilibrium, reaching -2.5 × 10(2) cm(-1) at 555 nm and 35 T.
  • The singlet-triplet spin gap was determined from magneto-optical response analysis.
  • Specific absorption features were identified as originating from singlet or triplet state excitations.

Conclusions:

  • External tuning parameters (temperature, magnetic field) effectively control the singlet-triplet equilibrium in 4,4′-stilbenedinitrene.
  • Understanding charge-spin coupling in open-shell molecules is enhanced.
  • Chemical structure modifications can tune charge-spin interactions in organic biradicals.