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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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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...
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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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Atomic Nuclei: Nuclear Spin01:08

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All atomic particles possess an intrinsic angular momentum, or 'spin'. Electrons, protons, and neutrons each have a spin value of ½, although protons and neutrons in nuclei may have higher half-integer spins owing to energetic factors.
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Atomic Nuclei: Nuclear Spin State Population Distribution01:14

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Near absolute zero temperatures, in the presence of a magnetic field, the majority of nuclei prefer the lower energy spin-up state to the higher energy spin-down state. As temperatures increase, the energy from thermal collisions distributes the spins more equally between the two states. The Boltzmann distribution equation gives the ratio of the number of spins predicted in the spin −½ (N−) and spin +½ (N+) states.
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Observation of a four-spin solid effect.

Kong Ooi Tan1, Robert G Griffin1

  • 1Francis Bitter Magnet Laboratory and Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.

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|May 7, 2022
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Summary
This summary is machine-generated.

Researchers observed the four-spin solid effect (4SSE), a new dynamic nuclear polarization mechanism, by exciting forbidden transitions in electron-nuclear spin systems using specific microwave frequencies. This finding advances nuclear magnetic resonance signal enhancement techniques.

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Area of Science:

  • Magnetic Resonance
  • Quantum Mechanics
  • Chemical Physics

Background:

  • Continuous wave dynamic nuclear polarization enhances Nuclear Magnetic Resonance (NMR) signals.
  • The two-spin solid effect (2SSE) and three-spin solid effect (3SSE) are known mechanisms involving electron-nuclear spin systems.
  • These effects rely on microwave irradiation to induce state mixing and forbidden transitions.

Purpose of the Study:

  • To report the first direct observation of the four-spin solid effect (4SSE).
  • To investigate the conditions and mechanisms underlying 4SSE.
  • To validate theoretical models and simulations for 4SSE.

Main Methods:

  • Experimental observation of forbidden double- and quadruple-quantum transitions.
  • Utilizing trityl radicals in a glycerol-water mixture at specific magnetic field (0.35 T), NMR frequency (15 MHz), and microwave frequency (9.8 GHz) at 80 K.
  • Derivation of the 4SSE effective Hamiltonian, matching conditions, and transition probabilities.

Main Results:

  • Direct observation of the four-spin solid effect (4SSE) at microwave frequencies matching ωμw = ω0S ± 3ω0I.
  • Experimental data aligns with theoretical predictions and numerical simulations.
  • Demonstrated feasibility of 4SSE in a relevant experimental setup.

Conclusions:

  • The four-spin solid effect (4SSE) is experimentally confirmed as a valid dynamic nuclear polarization mechanism.
  • The study provides a theoretical framework and experimental validation for 4SSE.
  • This work expands the understanding of multi-spin effects in dynamic nuclear polarization and NMR signal enhancement.