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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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Structure of Conjugated Dienes01:16

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Introduction
Conjugated dienes are compounds characterized by the presence of alternating double and single bonds. In a conjugated system like 1,3-butadiene, the unhybridized 2p orbital on each carbon overlaps continuously, allowing the π electrons to be delocalized across the entire molecule. In contrast, this type of overlap does not occur in cumulated and isolated dienes, such as 2,3-pentadiene and 1,4-pentadiene, respectively. Instead, the π electrons remain localized between the double...
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Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

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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.
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Network Covalent Solids02:18

Network Covalent Solids

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Network covalent solids contain a three-dimensional network of covalently bonded atoms as found in the crystal structures of nonmetals like diamond, graphite, silicon, and some covalent compounds, such as silicon dioxide (sand) and silicon carbide (carborundum, the abrasive on sandpaper). Many minerals have networks of covalent bonds.
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The Cope rearrangement is classified as a [3,3] sigmatropic shift in 1,5-dienes, leading to a more stable, isomeric 1,5-diene. The reaction involves a concerted movement of six electrons, four from two π bonds and two from a σ bond, via an energetically favorable chair-like transition state.
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Newman Projections02:06

Newman Projections

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Different notations are used to represent the three-dimensional structure of molecules on two-dimensional surfaces. One of the most commonly used representations is the dash-wedge formula. The dashed wedges, solid wedges, and the plane lines indicate the groups situated behind the plane, coming out of the plane, and in the plane, respectively.
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Microscopic Visualization of Porous Nanographenes Synthesized through a Combination of Solution and On-Surface Chemistry
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Cove-Edged Nanographenes with Localized Double Bonds.

Yanwei Gu1, Rafael Muñoz-Mármol2, Shaofei Wu1

  • 1Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore, Singapore.

Angewandte Chemie (International Ed. in English)
|February 4, 2020
PubMed
Summary

Two novel cove-edged nanographenes (NGs), CN1 and CN2, were synthesized and characterized. Their unique contorted structures feature localized double bonds, resulting in small band gaps and moderate fluorescence, making them promising for electronic applications.

Keywords:
aromaticitycontorted aromaticscove edgenanographenesoptoelectronics

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Area of Science:

  • Materials Science
  • Organic Chemistry
  • Nanotechnology

Background:

  • Nanographenes (NGs) are carbon-based nanomaterials with tunable electronic properties.
  • Cove-edged NGs offer unique structural and electronic characteristics.
  • Efficient synthesis of large-size NGs remains a challenge.

Purpose of the Study:

  • To report the efficient synthesis of two large-size cove-edged nanographenes, CN1 and CN2.
  • To investigate the structural, electronic, and optical properties of these novel NGs.
  • To explore the impact of contorted geometry and localized double bonds on their properties.

Main Methods:

  • X-ray crystallography for structural analysis.
  • Bond length analysis and theoretical calculations (NICS, ACID) for electronic structure.
  • Raman spectroscopy to identify olefinic double bonds.
  • Photoluminescence spectroscopy to determine fluorescence quantum yield.

Main Results:

  • Successful synthesis of CN1 and CN2 with contorted backbones due to steric repulsion.
  • Structural analysis confirmed localized C=C double bonds and aromatic sextet rings (Clar's formula).
  • Small band gaps (1.84 eV for CN1, 1.37 eV for CN2) and moderate fluorescence quantum yields (35% for CN1, 50% for CN2) were observed.
  • Contorted geometry suppressed aggregation in solution.

Conclusions:

  • The study presents efficient synthesis of novel cove-edged nanographenes with unique structural features.
  • The presence of localized double bonds significantly reduces the band gap, enabling potential electronic applications.
  • The contorted geometry enhances fluorescence properties and solution processability.