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Related Concept Videos

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...
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: 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,...
¹³C NMR: ¹H–¹³C Decoupling01:04

¹³C NMR: ¹H–¹³C Decoupling

The probability of having two carbon-13 atoms next to each other is negligible because of the low natural abundance of carbon-13. Consequently, peak splitting due to carbon-carbon spin-spin coupling is not observed in spectra. However, protons up to three sigma bonds away split the carbon signal according to the n+1 rule, resulting in complicated spectra.
A broadband decoupling technique is used to simplify these complex, sometimes overlapping, signals. Broadband decoupling relies on 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...
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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Related Experiment Video

Updated: May 28, 2026

Setting Limits on Supersymmetry Using Simplified Models
07:46

Setting Limits on Supersymmetry Using Simplified Models

Published on: November 15, 2013

Constraints on the fundamental string coupling from B-mode experiments.

A Avgoustidis1, E J Copeland, A Moss

  • 1Centre for Theoretical Cosmology, DAMTP, CMS, Wilberforce Road, Cambridge CB3 0WA, United Kingdom.

Physical Review Letters
|October 27, 2011
PubMed
Summary
This summary is machine-generated.

Cosmic superstrings with multiple tensions and Y junctions leave imprints on the cosmic microwave background (CMB). The string coupling constant g(s) influences network density, potentially shifting the B-mode peak for direct constraint.

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Area of Science:

  • Cosmology and Astrophysics
  • String Theory
  • Cosmic Microwave Background (CMB) Physics

Background:

  • Cosmic superstrings are hypothetical topological defects formed in the early universe.
  • Their observational signatures, particularly on the CMB, are crucial for testing fundamental physics.
  • The properties of superstring networks depend significantly on the string coupling constant g(s).

Purpose of the Study:

  • To investigate the imprints of cosmic superstring networks with diverse string tensions and Y junctions on CMB temperature and polarization spectra.
  • To analyze the impact of the string coupling constant g(s) on the network's number and energy densities.
  • To establish direct constraints on g(s) and fundamental string tension μ(F) using CMB data.

Main Methods:

  • Analysis of CMB temperature and polarization spectra for signatures of superstring networks.
  • Modeling of superstring network evolution in different regimes of the string coupling constant g(s) (g(s) ~ 1 and g(s) ≪ 1).
  • Forecasting joint constraints on g(s) and μ(F) using simulated data from future CMB polarization experiments.

Main Results:

  • The dominance of different string types in scaling networks varies with g(s), affecting network densities.
  • An observable shift in the B-mode peak position is identified as a distinct signal.
  • This shift provides a direct constraint on g(s), and such a detectable shift is achievable with planned experiments.

Conclusions:

  • Cosmic superstring networks with multiple tensions and Y junctions offer testable predictions for CMB polarization.
  • The B-mode peak shift serves as a unique probe for the string coupling constant g(s).
  • Future CMB experiments are poised to constrain fundamental string theory parameters.