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Related Concept Videos

IR Absorption Frequency: Delocalization01:04

IR Absorption Frequency: Delocalization

Electron delocalization refers to the distribution of electrons across multiple atoms within a molecule rather than being confined to a single atom or bond. This phenomenon is common in systems with conjugated bonds—structures where alternating single and double bonds allow π-electrons to move freely across the network. The movement of electrons stabilizes the molecule and can affect various chemical properties, including vibrational frequencies observed in IR spectroscopy.
In IR spectroscopy,...
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Resonance and Hybrid Structures

According to the theory of resonance, if two or more Lewis structures with the same arrangement of atoms can be written for a molecule, ion, or radical, the actual distribution of electrons is an average of that shown by the various Lewis structures.
Resonance Structures and Resonance Hybrids
The Lewis structure of a nitrite anion (NO2−) may actually be drawn in two different ways, distinguished by the locations of the N–O and N=O bonds.
Stability of Conjugated Dienes01:28

Stability of Conjugated Dienes

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A comparison of the enthalpies of hydrogenation of dienes reveals that conjugated dienes release less heat on hydrogenation, rendering them more stable than their nonconjugated analogs.
Valence Bond Theory and Hybridized Orbitals02:38

Valence Bond Theory and Hybridized Orbitals

According to valence bond theory, a covalent bond results when: (1) an orbital on one atom overlaps an orbital on a second atom, and (2) the single electrons in each orbital combine to form an electron pair. The strength of a covalent bond depends on the extent of overlap of the orbitals involved. Maximum overlap is possible when the orbitals overlap on a direct line between the two nuclei.
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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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Hybridization of Atomic Orbitals I

The mathematical expression known as the wave function, ψ, contains information about each orbital and the wavelike properties of electrons in an isolated atom. When atoms are bound together in a molecule, the wave functions combine to produce new mathematical descriptions that have different shapes. This process of combining the wave functions for atomic orbitals is called hybridization and is mathematically accomplished by the linear combination of atomic orbitals. The new orbitals that...

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Excitonic Hamiltonians for Calculating Optical Absorption Spectra and Optoelectronic Properties of Molecular Aggregates and Solids
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Understanding conjugation and hyperconjugation from electronic delocalization measures.

Ferran Feixas1, Eduard Matito, Jordi Poater

  • 1Institut de Química Computacional and Departament de Química, Universitat de Girona, Girona, Catalonia, Spain. ferran.feixas@gmail.com

The Journal of Physical Chemistry. A
|September 22, 2011
PubMed
Summary
This summary is machine-generated.

Conjugation and hyperconjugation are key organic chemistry concepts. This study uses electron delocalization to quantify these effects, resolving debates about stabilization in molecules like 1,3-butadiene.

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Area of Science:

  • Organic Chemistry
  • Quantum Chemistry
  • Computational Chemistry

Background:

  • Conjugation and hyperconjugation are crucial for explaining organic chemistry phenomena but are difficult to measure experimentally.
  • Previous studies relied on isodesmic reactions and energy decomposition analysis, with some methods yielding controversial results for specific molecules.
  • Electronic delocalization measures have emerged as a powerful tool for understanding chemical bonding and aromaticity.

Purpose of the Study:

  • To investigate conjugation and hyperconjugation using electronic delocalization measures within the Quantum Theory of Atoms in Molecules (QTAIM).
  • To re-evaluate the controversial stabilization effects in 1,3-butadiene and 1,3-butadiyne.
  • To clarify the interplay between conjugation and hyperconjugation in these systems and their hydrogenated counterparts.

Main Methods:

  • Calculation of electronic delocalization measures using the Quantum Theory of Atoms in Molecules (QTAIM).
  • Analysis of stabilization effects in 1,3-butadiene, 1,3-butadiyne, and their hydrogenated products (1-butene, 1-butyne).
  • Comparison of results with previous experimental and theoretical assessments, including the Kistiakowsky scheme.

Main Results:

  • Electronic delocalization measures confirm the presence of conjugation in both 1,3-butadiene and 1,3-butadiyne.
  • Hyperconjugation stabilization was identified in the hydrogenated products, 1-butene and 1-butyne.
  • The study resolves the discrepancy regarding the extent of conjugation in 1,3-butadiyne, showing that hyperconjugation in 1-butyne previously masked it.

Conclusions:

  • Electronic delocalization measures provide a reliable method for quantifying conjugation and hyperconjugation.
  • The apparent lack of conjugation in 1,3-butadiyne by some methods is due to the masking effect of hyperconjugation in reference molecules.
  • This work clarifies the nature and extent of electronic stabilization in conjugated systems and their saturated analogues.