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Substituent effects on "hyperconjugative" aromaticity and antiaromaticity in planar cyclopolyenes
Israel Fernández1, Judy I Wu, Paul von Ragué Schleyer
1Departamento de Química Orgánica I, Facultad de Ciencias Químicas, Universidad Complutense de Madrid , 28040, Madrid, Spain, and Department of Chemistry, Center for Computational Chemistry, University of Georgia , Athens, Georgia 30602, United States.
Organic Letters
|June 4, 2013
Summary
Hyperconjugation significantly impacts ring aromaticity. The study shows how CH2 and C(SiH3)2 groups contribute pseudo π electrons, influencing aromaticity based on Hückel
Area of Science:
- Computational chemistry
- Organic chemistry
- Quantum chemistry
Background:
- Aromaticity is a key concept in chemistry, influencing molecular stability and reactivity.
- Hyperconjugation, the interaction of sigma bonds with adjacent pi systems, can modify electronic properties.
- Understanding these effects is crucial for predicting molecular behavior.
Purpose of the Study:
- To quantify the influence of hyperconjugation on the aromaticity of various cyclic systems.
- To investigate how different substituents (R = H, SiH3, F) affect hyperconjugative aromaticity.
- To extend the Hückel rule to systems with hyperconjugative electron contributions.
Main Methods:
- Computed aromatic stabilization energies (ASEs).
- Dissected nucleus-independent chemical shifts (NICSπzz) were used to assess aromaticity.
- Calculations focused on planar conformations of (CnHn)CR2 rings (n=3, 5, 7, 9).
Main Results:
- CH2 and C(SiH3)2 groups provide "pseudo" π electrons, enhancing hyperconjugation.
- CF2 groups act as partially vacant p orbitals, influencing the electronic structure.
- Compounds were classified as aromatic or antiaromatic based on the Hückel rule and the number of pseudo π electrons.
Conclusions:
- Hyperconjugation plays a significant role in determining the aromaticity of cyclic systems.
- The Hückel rule can be extended to predict "hyperconjugative" aromaticity and antiaromaticity.
- Substituent choice critically affects the electronic properties and aromatic character of these rings.
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