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Aromaticity in spin-polarized systems: can rings be simultaneously alpha aromatic and beta antiaromatic?
Marcos Mandado1, Ana M Graña, Ignacio Pérez-Juste
1Department of Physical Chemistry, University of Vigo, As Lagoas (Marcosende) s/n, Vigo, Galicia 36310, Spain.
The Journal of Chemical Physics
|December 3, 2008
Summary
Understanding aromaticity in open-shell systems requires examining alpha and beta electron contributions. This study reveals a general aromaticity rule, extending conflicting aromaticities to radical systems.
Area of Science:
- Quantum Chemistry
- Theoretical Chemistry
- Organic Chemistry
Background:
- Aromaticity is a key concept in chemistry, traditionally applied to closed-shell systems.
- Open-shell systems, like radicals, present unique challenges in defining and quantifying aromaticity.
- Global aromaticity indices may obscure nuanced behaviors in systems with unpaired electrons.
Purpose of the Study:
- To partition aromaticity indices into alpha and beta electron contributions for open-shell systems.
- To develop a generalized aromaticity rule applicable to radical systems.
- To investigate the phenomenon of conflicting aromaticities in open-shell molecules.
Main Methods:
- Calculation of multicenter electron delocalization indices.
- Computation of nucleus-independent chemical shift (NICS) indices.
- Partitioning of these indices into alpha and beta contributions.
- Estimation of aromatic stabilization energy (ASE).
Main Results:
- A full understanding of aromaticity in open-shell systems necessitates dissecting global properties into alpha and beta terms.
- The 4n+2 and 4n aromaticity rules for singlet and triplet annulenes, respectively, unify under a general rule when alpha and beta terms are analyzed separately.
- Demonstrated the existence of radical systems exhibiting simultaneous alpha-aromaticity and beta-antiaromaticity, or vice versa.
- ASE values for these radical systems were slightly positive or negative, indicating a small overall aromatic or antiaromatic character.
Conclusions:
- A generalized aromaticity rule, considering alpha and beta contributions, is proposed for open-shell systems.
- The concept of conflicting aromaticities is extended to radical species.
- The study provides a more comprehensive framework for understanding aromaticity in complex electronic systems.
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