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NMR Spectroscopy: Spin–Spin Coupling01:08

NMR Spectroscopy: Spin–Spin Coupling

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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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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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Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)

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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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Atomic Nuclei: Magnetic Resonance01:05

Atomic Nuclei: Magnetic Resonance

1.0K
The number of nuclear spins aligned in the lower energy state is slightly greater than those in the higher energy state. In the presence of an external magnetic field, as the spins precess at the Larmor frequency, the excess population results in a net magnetization oriented along the z axis. When a pulse or a short burst of radio waves at the Larmor frequency is applied along the x axis, the coupling of frequencies causes resonance and flips the nuclear spins of the excess population from the...
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Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)

1.3K
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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¹H NMR Signal Multiplicity: Splitting Patterns01:13

¹H NMR Signal Multiplicity: Splitting Patterns

6.4K
When protons A and X are coupled, their nuclear spin energy levels are slightly modified. This is because the energy required to excite proton A to a spin state parallel to proton X is slightly different from the energy required for it to become anti-parallel to spin X. Consequently, there are two possible excitation frequencies for A (A1 and A2), depending on the spin state of X, and vice versa. The mutual nature of coupling implies that the difference between frequencies A1 and A2, indicated...
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Measurement of Coherence Decay in GaMnAs Using Femtosecond Four-wave Mixing
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Nonreciprocal Magnetic Coupling Using Nonlinear Meta-Atoms.

Xiaoguang Zhao1,2, Ke Wu1, Chunxu Chen1

  • 1Department of Mechanical Engineering Boston University Boston MA 02215 USA.

Advanced Science (Weinheim, Baden-Wurttemberg, Germany)
|October 12, 2020
PubMed
Summary
This summary is machine-generated.

Researchers developed a novel method to break Lorentz reciprocity using coupled linear and nonlinear meta-atoms. This passive approach, validated by an RF isolator, offers a new pathway for designing advanced radiofrequency and optical devices.

Keywords:
coupled mode theorymagnetic couplingmeta‐atomsnonlinearnonreciprocity

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Area of Science:

  • Physics
  • Metamaterials
  • Nonlinear Optics

Background:

  • Breaking Lorentz reciprocity is crucial for devices like isolators and circulators.
  • Traditional methods involve external excitation (magnetic fields, modulation).
  • Nonlinear effects offer a passive alternative to break reciprocity.

Purpose of the Study:

  • To present a coupled system of linear and nonlinear meta-atoms for achieving nonreciprocity.
  • To develop an analytical model based on coupled mode theory (CMT) for design and optimization.
  • To demonstrate the system's functionality through an experimental RF isolator.

Main Methods:

  • Designing a coupled system of linear and nonlinear meta-atoms.
  • Developing an analytical model using coupled mode theory (CMT).
  • Experimental fabrication and testing of a radiofrequency (RF) isolator.

Main Results:

  • Achieved nonreciprocity in a coupled meta-atom system.
  • Experimental RF isolator demonstrated a forward-to-backward propagation contrast of approximately 20 dB.
  • The CMT model provides a generalizable framework for predicting limitations of nonlinearity-based nonreciprocity.

Conclusions:

  • The presented coupled meta-atom system effectively breaks Lorentz reciprocity passively.
  • The CMT model facilitates the design and optimization of such systems.
  • This approach has broad applicability in integrated photonics, optical metamaterials, and metasurfaces.