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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

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

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

NMR Spectroscopy: Spin–Spin Coupling

3.6K
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...
3.6K
Ferromagnetism01:31

Ferromagnetism

3.4K
Materials like iron, nickel, and cobalt consist of magnetic domains, within which the magnetic dipoles are arranged parallel to each other. The magnetic dipoles are rigidly aligned in the same direction within a domain by quantum mechanical coupling among the atoms. This coupling is so strong that even thermal agitation at room temperature cannot break it. The result is that each domain has a net dipole moment. However, some materials have weaker coupling, and are ferromagnetic at lower...
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Spin–Spin Coupling: One-Bond Coupling01:17

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1.6K
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,...
1.6K
¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

2.9K
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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Visualizing Uniaxial-strain Manipulation of Antiferromagnetic Domains in Fe1+YTe Using a Spin-polarized Scanning Tunneling Microscope
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The Antiferromagnetic Spin Coupling in Non-Kekulé Acenes-Impressive Polyradical Character Revealed by High-Level

Mario Vazdar1, Mirjana Eckert-Maksić2, Hans Lischka3,4,5

  • 1Division of Organic Chemistry and Biochemistry, Rudjer Bosković Institute, P.O.B. 180, HR-10002, Zagreb, Croatia. mario.vazdar@irb.hr.

Chemphyschem : a European Journal of Chemical Physics and Physical Chemistry
|March 19, 2016
PubMed
Summary

New polyradical molecules, dimethylenepolycyclobutadienes, exhibit unique electronic properties. These non-Kekulé acene analogues show antiferromagnetic coupling and significant polyradical character, challenging Hund

Keywords:
ab initio calculationsnon-kekulé structurespi interactionspolyradicals

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Area of Science:

  • Computational Chemistry
  • Organic Chemistry
  • Materials Science

Background:

  • Acenes and polycyclobutadienes (CBDs) are fundamental π-conjugated systems.
  • Non-Kekulé structures offer unique electronic and magnetic properties.
  • Hund's rule governs electron spin coupling in stable molecules.

Purpose of the Study:

  • Investigate the electronic and magnetic properties of non-Kekulé acene analogues.
  • Explore the behavior of dimethylenepolycyclobutadienes with varying lengths.
  • Characterize the polyradical nature and spin coupling in these systems.

Main Methods:

  • Complete active space self-consistent field (CASSCF) calculations.
  • Multireference configuration interaction with singles and doubles (MR-CISD(Q)) calculations.
  • Multireference averaged quadratic coupled-cluster (MR-AQCC) calculations.

Main Results:

  • Predicted stable states are triplet (odd CBD units) or singlet (even CBD units) due to antiferromagnetic coupling.
  • Demonstrated significant polyradical character, increasing with molecular size.
  • Observed over eleven unpaired electrons in the singlet state of the largest molecule (eight CBD units).

Conclusions:

  • Dimethylenepolycyclobutadienes exhibit exceptional polyradical properties.
  • Antiferromagnetic spin coupling violates Hund's rule in larger oligomers.
  • Small energy gaps between singlet and higher multiplicity states highlight unique electronic behavior.