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Related Concept Videos

Double Resonance Techniques: Overview01:12

Double Resonance Techniques: Overview

Double resonance techniques in Nuclear Magnetic Resonance (NMR) spectroscopy involve the simultaneous application of two different frequencies or radiofrequency pulses to manipulate and observe two distinct nuclear spins. One important application of double resonance is spin decoupling, which selectively suppresses coupling with one type of nucleus while observing the NMR signal from another nucleus, simplifying the spectrum and enhancing resolution.
Spin decoupling is usually achieved by...
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...
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...
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,...
NMR Spectroscopy: Spin–Spin Coupling01:08

NMR Spectroscopy: Spin–Spin Coupling

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 in...

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Microwave Photonics Systems Based on Whispering-gallery-mode Resonators
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Multimode vibrational couplings in resonant positron annihilation.

Sergio d'A Sanchez1, Marco A P Lima, Márcio T do N Varella

  • 1Departamento de Física, Universidade Federal do Paraná, CP 19044, 81531-090 Curitiba, PR, Brazil.

Physical Review Letters
|October 11, 2011
PubMed
Summary

Resonant positron annihilation mechanisms are clarified by including positron-induced potential energy surface distortions. These distortions, previously overlooked, contribute to vibrational couplings and offer a new pathway for understanding annihilation processes.

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Area of Science:

  • Atomic and Molecular Physics
  • Quantum Chemistry
  • Positron Interactions

Background:

  • Mechanisms of multimode vibrational couplings in resonant positron annihilation remain poorly understood.
  • Existing theories may not fully capture the intricacies of these resonant processes.

Purpose of the Study:

  • To elucidate the role of positron-induced potential energy surface distortions in resonant positron annihilation.
  • To investigate the contribution of these distortions to vibrational couplings.
  • To compare existing annihilation theories with a more comprehensive model.

Main Methods:

  • Theoretical modeling of positron-induced distortions of potential energy surfaces.
  • Analysis of energy transfer into single- and multiquantum vibrations.
  • Comparative analysis of established and Feshbach annihilation theories.

Main Results:

  • Positron-induced distortions of the potential energy surface are identified as a source of resonant positron annihilation.
  • These distortions facilitate energy transfer into vibrational modes, a pathway previously disregarded.
  • The currently accepted annihilation model is shown to be a specific instance of the Feshbach annihilation theory.

Conclusions:

  • Positron-induced potential energy surface distortions are a crucial, previously overlooked, mechanism in resonant positron annihilation.
  • A unified framework, incorporating these distortions, provides a more complete understanding of annihilation pathways.
  • The Feshbach annihilation theory offers a more general and encompassing description of resonant positron annihilation.