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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.
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...
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...
¹H NMR: Pople Notation01:09

¹H NMR: Pople Notation

The Pople nomenclature system classifies spin systems based on the difference between their chemical shifts. Coupled spins are denoted by capital letters with subscripts indicating the number of equivalent nuclei. When the coupled nuclei have well-separated chemical shifts, they are assigned letters that are far apart in the alphabet, such as A and X. When the difference in chemical shifts is small, coupled nuclei are named using adjacent letters of the alphabet (AB, MN, or XY).
A proton...

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Using Cyclic Voltammetry, UV-Vis-NIR, and EPR Spectroelectrochemistry to Analyze Organic Compounds
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Probing electronic superexchange coupling at isolated poly-p-phenylene molecules.

Weihua Wang1, Shiyong Wang, Xiuyuan Li

  • 1Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China.

Journal of the American Chemical Society
|June 1, 2010
PubMed
Summary

Superexchange coupling in molecular wires decays exponentially, confirmed by scanning tunneling microscopy. This technique precisely measures coupling strength and molecular states, offering new insights into charge transfer influenced by conformation and environment.

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Area of Science:

  • Condensed Matter Physics
  • Molecular Electronics
  • Surface Science

Background:

  • Superexchange coupling is crucial for charge transport in molecular wires.
  • Understanding this coupling at the single-molecule level is essential for molecular electronics.
  • Previous studies lacked atomic precision in characterizing coupling and molecular states simultaneously.

Purpose of the Study:

  • To probe superexchange coupling in poly-p-phenylene molecular wires.
  • To experimentally verify the predicted exponential decay of coupling strength.
  • To develop a method for quantifying the influence of molecular conformation and environment on charge transfer.

Main Methods:

  • Utilized scanning tunneling microscopy/spectroscopy (STM/STS) at cryogenic temperatures.
  • Measured energy splitting of molecular states from orbital dimerization to characterize coupling strength.
  • Achieved atomic precision in elucidating molecular internal and external states.

Main Results:

  • Confirmed the theoretically predicted exponential decay of superexchange coupling.
  • Determined a decay constant of 0.10 +/- 0.02 A(-1) at the single-molecule level.
  • Demonstrated the ability to correlate molecular conformation and environmental interactions with coupling strength.

Conclusions:

  • Scanning tunneling microscopy/spectroscopy provides a powerful tool for single-molecule characterization of electronic coupling.
  • The findings validate theoretical models of superexchange in molecular systems.
  • This approach offers a new pathway to engineer molecular wires with controlled charge transfer properties.