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¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

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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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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: One-Bond Coupling01:17

Spin–Spin Coupling: One-Bond Coupling

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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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NMR Spectroscopy: Spin–Spin Coupling01:08

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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...
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Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

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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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Atomic Nuclei: Larmor Precession Frequency01:11

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The earth's gravitational field produces a 'twisting force' perpendicular to the angular momentum of a spinning mass (such as a spinning top) that causes the mass to 'wobble' around the gravitational field axis in a phenomenon called precession. Similarly, the magnetic moment (μ) of a spinning nucleus precesses due to an external magnetic field directed along the z-axis. The precession of the magnetic moment vector about the magnetic field is called Larmor precession,...
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Revisiting the bound on axion-photon coupling from globular clusters.

Adrian Ayala1, Inma Domínguez1, Maurizio Giannotti2

  • 1Universidad de Granada, 18071 Granada, Spain.

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Researchers set a new strong limit on axion-photon coupling using globular clusters. This finding significantly improves previous constraints on axion properties in astrophysics.

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Area of Science:

  • Astrophysics
  • Particle Physics
  • Cosmology

Background:

  • Axions are hypothetical particles that may solve the strong CP problem in particle physics.
  • Axion production via photon conversion in stellar cores can affect stellar evolution.
  • The R parameter, a ratio of stellar populations in globular clusters, is sensitive to axion energy loss.

Purpose of the Study:

  • To derive a strong bound on the axion-photon coupling constant, g(aγ).
  • To investigate the impact of helium mass fraction (Y) on axion coupling constraints.
  • To establish the most stringent limit on axion-photon coupling in a broad mass range.

Main Methods:

  • Analysis of the R parameter from a sample of 39 Galactic Globular Clusters.
  • Comparison of observed R values with state-of-the-art stellar evolution models.
  • Incorporation of recent helium abundance data from H II regions as a benchmark.

Main Results:

  • The derived axion-photon coupling strength g(aγ) is highly dependent on the adopted helium mass fraction Y.
  • An upper bound of g(aγ) < 0.66 × 10⁻¹⁰ GeV⁻¹ at 95% confidence level was obtained.
  • This represents a significant improvement over previous experimental and observational constraints.

Conclusions:

  • Globular clusters provide a powerful astrophysical probe for constraining fundamental particle physics parameters.
  • The derived limit on axion-photon coupling is the strongest to date across a wide mass range.
  • Future studies should carefully consider helium abundance when deriving axion constraints from stellar data.