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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.
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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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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.
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Isomerism in Complexes
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Coordination compounds and complexes exhibit different colors, geometries, and magnetic behavior, depending on the metal atom/ion and ligands from which they are composed. In an attempt to explain the bonding and structure of coordination complexes, Linus Pauling proposed the valence bond theory, or VBT, using the concepts of hybridization and the overlapping of the atomic orbitals. According to VBT, the central metal atom or ion (Lewis acid) hybridizes to provide empty orbitals of suitable...
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Overview of Valence Bond Theory
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Bond-Length Alternation as a Structural Coordinate for Electronic Regime Crossover in Indophenines.

Jônatas Faleiro Berbigier1, Dwight S Seferos1,2

  • 1Department of Chemistry, University of Toronto, Lash Miller Chemical Laboratories, Toronto, Ontario M5S 3H6, Canada.

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Bond-length alternation (BLA) quantifies diradical character in conjugated systems. Reduced BLA in indophenines drives a transition from closed-shell to diradicaloid electronic structures, impacting molecular properties.

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Area of Science:

  • * Organic Chemistry
  • * Computational Chemistry
  • * Materials Science

Background:

  • * Diradical character in conjugated π-systems is often qualitatively assessed via quinoidal distortion.
  • * Quantitative structural descriptors linking molecular geometry to electronic ground states are limited.
  • * Understanding these structure-property relationships is crucial for designing novel organic materials.

Purpose of the Study:

  • * To establish bond-length alternation (BLA) as a quantitative descriptor for electronic regimes in donor-acceptor indophenines.
  • * To correlate BLA with the transition from closed-shell to diradicaloid electronic structures.
  • * To investigate the impact of BLA on frontier orbital degeneracy and spin polarization.

Main Methods:

  • * Utilized broken-symmetry density functional calculations.
  • * Analyzed bond-length alternation (BLA) as a key geometric parameter.
  • * Computed singlet-triplet energy separation (ΔEST), fractional frontier natural occupations, and spin polarization (⟨S2⟩).

Main Results:

  • * Progressive reduction in BLA correlates with a shift from closed-shell to diradicaloid character.
  • * A narrow BLA window marks a rapid decrease in ΔEST, increased natural occupations, and enhanced spin polarization.
  • * Frontier natural orbitals show a loss of bonding-antibonding complementarity and emergence of quasi-degenerate pairs.

Conclusions:

  • * Bond-length alternation (BLA) is a practical geometric descriptor for classifying electronic regimes in indophenines.
  • * The study reveals a clear structure-electronic correlation driven by BLA.
  • * Findings suggest similar correlations may exist in other quinoidal π-systems, aiding material design.