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Spin–Spin Coupling Constant: Overview01:08

Spin–Spin Coupling Constant: Overview

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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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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.
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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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Atomic Nuclei: Nuclear Spin State Overview01:03

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NMR-active nuclei have energy levels called 'spin states' that are associated with the orientations of their nuclear magnetic moments. In the absence of a magnetic field, the nuclear magnetic moments are randomly oriented, and the spin states are degenerate. When an external magnetic field is applied, the spin states have only 2 + 1 orientations available to them. A proton with = ½ has two available orientations. Similarly, for a quadrupolar nucleus with a nuclear spin value of one, the...
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Coordination compounds and complexes exhibit different colors, geometries, and magnetic behavior, depending on the metal atom/ion and ligands from which they are composed. In an attempt to explain the bonding and structure of coordination complexes, Linus Pauling proposed the valence bond theory, or VBT, using the concepts of hybridization and the overlapping of the atomic orbitals. According to VBT, the central metal atom or ion (Lewis acid) hybridizes to provide empty orbitals of suitable...
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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.
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Spin generation via bulk spin current in three-dimensional topological insulators.

Xingyue Peng1, Yiming Yang1, Rajiv R P Singh1

  • 1Department of Physics, University of California, One Shields Avenue, California 95616, USA.

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|March 3, 2016
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We discovered a new spin generation mechanism in 3D topological insulators, utilizing the bulk to enhance spin transport and conductivity. This bulk-mediated process is unique to topological insulators and absent in 2D systems like graphene.

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Area of Science:

  • Condensed matter physics
  • Spintronics
  • Topological materials

Background:

  • Current models of spin generation in 3D topological insulators (TIs) focus on surface phenomena due to momentum-spin locking.
  • The role of the bulk in spin generation within TIs has been largely overlooked.

Purpose of the Study:

  • To propose and investigate a novel spin generation mechanism in 3D TIs that explicitly involves the bulk.
  • To demonstrate how an external electric field can induce spin transport through the bulk, enhancing spin generation and surface conductivity.

Main Methods:

  • Theoretical modeling of spin generation via bulk transport in 3D TIs.
  • Numerical simulations to confirm the proposed mechanism.
  • Investigating the influence of surface disorder and the Dyakonov-Perel mechanism on spin relaxation.

Main Results:

  • An external electric field creates a transverse pure spin current through the bulk, connecting top and bottom surfaces.
  • Surface disorder enhances spin relaxation time via the Dyakonov-Perel mechanism, boosting spin generation efficiency and surface conductivity.
  • The mechanism is confirmed to originate from the topological connection between surfaces and is absent in 2D systems like graphene.

Conclusions:

  • The bulk of 3D TIs plays a crucial role in spin generation, offering a new pathway beyond surface-based mechanisms.
  • This bulk-mediated spin generation mechanism significantly enhances spin generation efficiency and surface conductivity.
  • The unique topological properties of 3D TIs are essential for this phenomenon, distinguishing them from other 2D materials.