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Related Concept Videos

¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

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 slanted or...
Interpreting ¹H NMR Signal Splitting: The (n + 1) Rule01:10

Interpreting ¹H NMR Signal Splitting: The (n + 1) Rule

In the AX proton spin system, proton A can sense the two spin states of a coupled proton X, resulting in a doublet NMR signal with two peaks of equal (1:1) intensity. When proton A is coupled to two equivalent protons (AX2 spin system), the spin states of each X can be aligned with or against the external field, creating three possible scenarios. This results in a 1:2:1  triplet signal, where the central peak corresponds to the chemical shift of A and is twice as large or intense as the others.
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,...
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...

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Site Directed Spin Labeling and EPR Spectroscopic Studies of Pentameric Ligand-Gated Ion Channels
11:19

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Exact and numerical results for a dimerized coupled spin- 1/2 chain

Martins1, Nienhuis

  • 1School of Natural Sciences, Institute for Advanced Study, Olden Lane, Princeton, New Jersey 08540 and Departamento de Fisica, Universidade Federal de Sao Carlos, Caixa Postal 676, 13565-905, Sao Carlos, Brazil.

Physical Review Letters
|December 2, 2000
PubMed
Summary

We found exact solutions for coupled spin-1/2 chains, revealing a small gap in dimerized chains and evidence of a spontaneously dimerized ground state in homogeneous chains. These findings offer critical benchmarks for theoretical physics models.

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Area of Science:

  • Condensed Matter Physics
  • Quantum Many-Body Systems
  • Theoretical Physics

Background:

  • Coupled spin chains are fundamental models in condensed matter physics.
  • Understanding the effects of interactions and dimerization is crucial for predicting material properties.

Purpose of the Study:

  • To establish exact results for coupled spin-1/2 chains under specific conditions.
  • To provide benchmark cases for numerical and analytical techniques.
  • To investigate the phase diagram and ground state properties.

Main Methods:

  • Analytical solutions for specific parameter values (delta = 1/2, V = -2 and V = -4).
  • Analysis of energy gaps and ground state properties.
  • Utilizing numerical diagonalization and bosonization techniques.

Main Results:

  • An exact result for a dimerized spin-1/2 chain at delta = 1/2 and V = -2, exhibiting a small but finite energy gap.
  • Evidence for a spontaneously dimerized ground state in a homogeneous chain at V = -4.
  • Indications of potential gapless phases in the regime 0 <= V < -2 due to the interplay of dimerization and interaction.

Conclusions:

  • The identified exact solutions serve as valuable test cases for computational and approximate methods.
  • The study sheds light on the complex phase diagram of interacting and dimerized spin chains.
  • Further research into the interplay of dimerization and interaction is warranted to fully characterize gapless phases.