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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.
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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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.
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...
1.3K
Spin–Spin Coupling: One-Bond Coupling01:17

Spin–Spin Coupling: One-Bond Coupling

1.2K
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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Spin–Spin Coupling Constant: Overview01:08

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1.2K
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...
1.2K
¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

2.3K
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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Nuclear spin coupling crossover in dense molecular hydrogen.

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High-pressure studies reveal molecular hydrogen loses its distinct ortho- and para- spin isomers above 70 GPa. This nuclear spin crossover phenomenon in quantum solids demonstrates a fundamental change in hydrogen

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Area of Science:

  • Quantum Solid-State Physics
  • Nuclear Magnetic Resonance Spectroscopy
  • High-Pressure Science

Background:

  • Molecular hydrogen (H2) exhibits unique spin isomers, ortho- and para-hydrogen, due to coupled rotational and nuclear spin properties.
  • High pressures significantly increase intermolecular interactions, potentially hindering molecular rotation and altering spin properties.
  • Previous high-pressure experiments could not directly measure nuclear spin states above 100 GPa.

Purpose of the Study:

  • To investigate the effect of extreme pressure on the nuclear spin statistics of molecular hydrogen.
  • To observe and characterize the behavior of ortho- and para-hydrogen at pressures up to 123 GPa.
  • To explore the potential for a nuclear spin crossover phenomenon in quantum solids.

Main Methods:

  • In-situ high-pressure nuclear magnetic resonance (NMR) spectroscopy.
  • Experiments conducted on molecular hydrogen in its hexagonal phase I.
  • Measurements performed at room temperature up to 123 GPa.

Main Results:

  • Confirmed the presence of ortho- and para-hydrogen spin isomers at lower pressures.
  • Observed a crossover in nuclear spin statistics above 70 GPa.
  • Evidence of a transition from a spin-1 quadrupolar to a spin-1/2 dipolar system, indicating loss of spin isomer distinction.

Conclusions:

  • Extreme pressure causes a loss of spin isomer distinction in molecular hydrogen.
  • A novel nuclear spin crossover phenomenon has been identified in quantum solids.
  • These findings provide direct experimental evidence of pressure-induced changes in nuclear spin statistics.