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Radical Reactivity: Overview01:11

Radical Reactivity: Overview

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Radicals, the highly reactive species, gain stability by undergoing three different reactions. The first reaction involves a radical-radical coupling, in which a radical combines with another radical, forming a spin‐paired molecule. The second reaction is between a radical and a spin‐paired molecule, generating a new radical and a new spin‐paired molecule. The third reaction is radical decomposition in a unimolecular reaction, forming a new radical and a spin‐paired...
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Radical Formation: Overview01:03

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A bond can be broken either by heterolytic bond cleavage to form ions or homolytic bond cleavage to yield radicals. A fishhook arrow is used to represent the motion of a single electron in homolytic bond cleavage. There are two main sources from which radicals can be formed:
Radicals from spin-paired molecules:
Radicals can be obtained from spin-paired molecules either by homolysis or electron transfer. While two radicals are formed in the former, an electron is added in the...
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Radical Formation: Addition00:47

Radical Formation: Addition

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Radicals can be formed by adding a radical to a spin-paired molecule. This is typically observed with unsaturated species, where the addition of a radical across the π bond leads to the production of a new radical by dissolving the π bond. For example, the addition of a Br radical to an alkene yields a carbon-centered radical.
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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...
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This lesson delves into the geometry of a radical, which is influenced by the electronic structure of the molecule. The principle is similar to that of a lone pair, where the unpaired electron influences the geometry at the radical center.
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Radical reactions can occur either intermolecularly or intramolecularly. In an intermolecular radical reaction, a nucleophilic radical adds to an electrophilic alkene or vice versa. In such reactions, the radical and generally the alkene, which is also called the radical trap, are two different molecules. Additionally, for such intermolecular reactions to occur, the radical trap must be active, present in an excess concentration, and the radical starting material must have a weak...
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On Entangled Singlet Pure Diradicals.

Georges Trinquier1, Grégoire David2, Elohan Veillon1

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Researchers identified novel "entangled pure diradicals," which are conjugated hydrocarbons with unique electronic structures. These molecules exhibit pure diradical character, distinct from traditional diradicals, offering new avenues in molecular design.

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Area of Science:

  • Theoretical Chemistry
  • Organic Chemistry
  • Quantum Chemistry

Background:

  • Conjugated hydrocarbons can exhibit diradical character.
  • Jahn-Teller distortions are common in systems with degenerate molecular orbitals.
  • Distinguishing pure diradicals from diradicaloids is crucial for understanding electronic properties.

Purpose of the Study:

  • To investigate a class of conjugated hydrocarbons predicted to be singlet diradicals.
  • To explore the electronic structure and properties of these molecules, particularly their diradical character.
  • To propose a new classification and design principles for these unique molecular systems.

Main Methods:

  • Topological Hückel Hamiltonian analysis.
  • Density Functional Theory (DFT) calculations.
  • Complete Active Space Self-Consistent Field (CASSCF) calculations with second-order perturbation theory.

Main Results:

  • Identified conjugated hydrocarbons with two degenerate singly occupied molecular orbitals (SOMOs).
  • Confirmed pure diradical character, free from ionic valence-bond components, due to SOMO symmetry.
  • Observed lower energy for open-shell spin-unrestricted solutions compared to closed-shell ones.
  • Demonstrated that aromaticity prevents Jahn-Teller distortions.
  • Proposed the term "entangled pure diradicals" for systems where SOMOs have large amplitudes on neighboring atoms.

Conclusions:

  • The studied molecules are confirmed as entangled pure diradicals, distinct from disjoint diradicals and diradicaloids.
  • Aromaticity plays a key role in stabilizing these diradical systems.
  • Prescription rules for designing such entangled pure diradicals were provided.