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α/β Conflicting Aromaticity Under the Microscope: Study of Pro-Aromatic Radicals
Raúl Lago-Saavedra1, Ignacio Pérez-Juste1, Marcos Mandado1
1Department of Physical Chemistry, University of Vigo, Lagoas-Marcosende s/n, 36310, Vigo, Spain.
Summary
Pro-aromatic radicals exhibit conflicting alpha/beta aromaticity, with one spin component being aromatic and the other anti-aromatic. Substituents can slightly modulate this aromatic character.
Area of Science:
- Computational Chemistry
- Organic Chemistry
- Quantum Chemistry
Background:
- Aromaticity is a fundamental concept in chemistry, crucial for understanding molecular stability and reactivity.
- Pro-aromatic radicals present unique electronic structures due to unpaired electrons, potentially leading to complex aromaticity patterns.
- Understanding the interplay of electron spins and aromaticity is key to predicting chemical behavior.
Purpose of the Study:
- To investigate the aromaticity of pro-aromatic radicals and their substituted derivatives.
- To analyze the distinct contributions of alpha and beta electrons to aromaticity.
- To interpret aromaticity in radicals using established quantum chemical indices.
Main Methods:
- Utilized multicenter delocalization indices (MCI) to quantify aromaticity.
- Employed nuclear-independent chemical shifts (NICS) to confirm aromatic and anti-aromatic character.
- Separately analyzed the contributions of alpha and beta electrons to MCI and NICS values.
- Interpreted results based on the 2n+1/2n aromaticity rule.
Main Results:
- All investigated monocyclic radicals displayed conflicting alpha/beta aromaticity.
- One spin component showed significant MCI values, indicating aromaticity, while the other showed negligible or negative values.
- NICS indices corroborated the MCI findings, confirming aromaticity in one spin component and anti-aromaticity/non-aromaticity in the other.
- Bicyclic radicals showed aromatic fused benzene rings, while donor rings exhibited conflicting aromaticity.
- Electron-withdrawing groups slightly enhanced aromaticity in the aromatic spin component and reduced anti-aromaticity in the other.
Conclusions:
- Pro-aromatic radicals inherently possess conflicting alpha/beta aromaticity.
- The electronic nature of substituents influences the degree of aromaticity and anti-aromaticity in these systems.
- MCI and NICS are effective tools for characterizing the complex aromaticity in radical species.
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