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Related Concept Videos

¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

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 π orbitals.
Electron Paramagnetic Resonance (EPR) Spectroscopy: Organic Radicals01:17

Electron Paramagnetic Resonance (EPR) Spectroscopy: Organic Radicals

Ideally, an unpaired electron shows a single peak in the EPR spectrum due to the transition between the two spin energy states. However, coupling interactions can occur between the spins of the unpaired electron and any neighboring spin-active nuclei. This hyperfine coupling results in hyperfine splitting, where the EPR signal is split into multiplets. The signals split into 2nI + 1 peaks, where n is the number of equivalent nuclei and I is the nuclear spin. These splitting patterns provide...
¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
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...
π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter of the molecule. This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside. The protons in benzene are deshielded and exhibit high chemical shifts in the range 6.5–8.5 ppm. The shielding effect at the center of the ring is evident in complex aromatic molecules, such as annulenes. In...
π Electron Effects on Chemical Shift: Overview01:27

π Electron Effects on Chemical Shift: Overview

An applied magnetic field causes loosely bound π-electrons in organic molecules to circulate, producing a local or induced diamagnetic field over a large spatial volume. As the molecules tumble in solution, the field generated by π-electrons in spherical substituents results in a zero net field. However, the net field generated by π-electrons in non-spherical substituents is not zero. The effect of this induced field depends on the orientation of the molecule with respect to B0, resulting in...

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Related Experiment Video

Updated: Jul 15, 2026

Resonance Raman Spectroscopy of Extreme Nanowires and Other 1D Systems
07:44

Resonance Raman Spectroscopy of Extreme Nanowires and Other 1D Systems

Published on: April 28, 2016

Resonant channel coupling in electron scattering by pyrazine.

Carl Winstead1, Vincent McKoy

  • 1A.A. Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91125, USA.

Physical Review Letters
|May 16, 2007
PubMed
Summary

Electron scattering reveals pyrazine has two shape resonances and a third mixed with core-excited states. This coupling may explain radiation damage in DNA and RNA molecules.

Area of Science:

  • Quantum chemistry
  • Molecular physics
  • Electron scattering

Background:

  • Pyrazine is a nitrogen-containing heterocyclic aromatic compound.
  • Low-energy electron scattering can reveal important information about molecular electronic structure.
  • Resonances in electron scattering are often indicative of temporary negative ions.

Purpose of the Study:

  • To investigate the nature of low-energy electron resonances in pyrazine.
  • To determine the extent of mixing between shape and core-excited resonances.
  • To assess the implications of these findings for pi-ring systems, including DNA and RNA.

Main Methods:

  • Detailed analysis of electron scattering data for pyrazine.
  • Theoretical calculations to model resonance states.

More Related Videos

Photoelectron Imaging of Anions Illustrated by 310 Nm Detachment of F−
06:53

Photoelectron Imaging of Anions Illustrated by 310 Nm Detachment of F−

Published on: July 27, 2018

Related Experiment Videos

Last Updated: Jul 15, 2026

Resonance Raman Spectroscopy of Extreme Nanowires and Other 1D Systems
07:44

Resonance Raman Spectroscopy of Extreme Nanowires and Other 1D Systems

Published on: April 28, 2016

Photoelectron Imaging of Anions Illustrated by 310 Nm Detachment of F−
06:53

Photoelectron Imaging of Anions Illustrated by 310 Nm Detachment of F−

Published on: July 27, 2018

  • Investigation of resonant channel coupling mechanisms.

    • Main Results:

    • The first two low-energy resonances in pyrazine are identified as predominantly single-channel shape resonances.
    • The third resonance is found to be significantly mixed with core-excited resonances.
    • This mixing involves triplet states of the pyrazine anion.

    Conclusions:

    • Resonant channel coupling is a significant phenomenon in pyrazine.
    • Such coupling is likely prevalent in other pi-ring molecules, such as DNA and RNA nucleobases.
    • This mechanism may contribute to radiation-induced damage in biological molecules.