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Related Concept Videos

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...
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.
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,...
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...
Vicinal Diols via Reductive Coupling of Aldehydes or Ketones: Pinacol Coupling Overview01:27

Vicinal Diols via Reductive Coupling of Aldehydes or Ketones: Pinacol Coupling Overview

Wilhelm Rudolph Fittig discovered the pinacol coupling reaction in 1859. It is a radical dimerization reaction and involves the reductive coupling of aldehydes or ketones in the presence of hydrocarbon solvent to yield vicinal diols.
Induced Electric Dipoles01:28

Induced Electric Dipoles

A permanent electric dipole orients itself along an external electric field. This rotation can be quantified by defining the potential energy because the external torque does work in rotating it. Then, the potential energy is minimum at the parallel configuration and maximum at the antiparallel configuration. While the former is a stable equilibrium, the latter is an unstable equilibrium.
Since the absolute value of potential energy holds no physical meaning, its zero value can be chosen as per...

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Ultrahigh Density Array of Vertically Aligned Small-molecular Organic Nanowires on Arbitrary Substrates
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Vibronic coupling in quantum wires: applications to polydiacetylene.

H Yamagata1, F C Spano

  • 1Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, USA.

The Journal of Chemical Physics
|August 10, 2011
PubMed
Summary

This study presents a theory for vibronic coupling in one-dimensional semiconductors, revealing properties similar to Frenkel exciton J-aggregates. Exciton properties in conjugated polymers can be determined from photoluminescence spectra, matching experimental data.

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Area of Science:

  • Condensed matter physics
  • Materials science
  • Quantum chemistry

Background:

  • Understanding photophysical properties of conjugated polymers is crucial for optoelectronic applications.
  • Vibronic coupling significantly influences exciton behavior in low-dimensional materials.

Purpose of the Study:

  • To develop a theoretical framework for vibronic coupling in direct band gap, one-dimensional semiconductors.
  • To elucidate the photophysical properties of isolated, defect-free conjugated polymers.
  • To establish a connection between theoretical models and experimental observations.

Main Methods:

  • A Holstein-like Hamiltonian was employed within a multi-particle basis set.
  • Absorption and emission spectra due to Wannier-Mott excitons were evaluated.
  • Analysis of photoluminescence spectra, specifically the ratio of 0-0 and 0-1 line strengths (I(0-0)/I(0-1)).

Main Results:

  • The photophysical properties of one-dimensional quantum wires were found to resemble Frenkel exciton J-aggregates.
  • Exciton coherence length and effective mass were determined from the I(0-0)/I(0-1) ratio.
  • A T(-1/2) dependence for I(0-0)/I(0-1) was observed, consistent with experimental data for polydiacetylenes.

Conclusions:

  • The developed theory accurately describes vibronic coupling and photophysical properties in conjugated polymers.
  • The study provides a method to extract exciton parameters from readily measurable spectral features.
  • The findings offer insights into the behavior of excitons in one-dimensional quantum systems.