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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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Area of Science:

  • Organic Chemistry
  • Quantum Chemistry
  • Materials Science

Background:

  • Recent studies revealed conjugate polycyclic hydrocarbons with unusual properties, behaving as pure diradical singlets despite accepting electron pairing.
  • These "entangled" molecules possess spatially entangled singly occupied molecular orbitals (SOMOs), distinguishing them from diradicaloids and disjoint diradicals.

Purpose of the Study:

  • To extend the study of entangled molecules to a wider range of architectures with degenerate Hückel SOMOs.
  • To formulate a topological criterion for predicting entangled pure diradical behavior in conjugate hydrocarbons.
  • To investigate a second set of molecules with degenerate Hückel SOMOs and analyze factors influencing diradical stability.

Main Methods:

  • Theoretical investigation using the topological Hückel Hamiltonian to analyze molecular orbital degeneracy.
  • Comparison of energies between pure diradical and closed-shell wave functions.
  • Formulation of a topological criterion based on molecular structure.

Main Results:

  • Identified a series of architectures exhibiting pure diradical wave functions with symmetry-keeping geometries and lower energy than closed-shell counterparts.
  • Developed a topological criterion to predict entangled pure diradical behavior in conjugate hydrocarbons.
  • Proposed molecules with degenerate Hückel SOMOs where aromaticity conservation is key to diradical stability.

Conclusions:

  • Entangled molecules represent a distinct class of diradicals with unique electronic and structural properties.
  • The developed topological criterion provides a predictive tool for identifying such molecules.
  • Aromaticity of six-membered rings plays a crucial role in stabilizing diradical solutions in these systems.