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Microwave-assisted Intramolecular Dehydrogenative Diels-Alder Reactions for the Synthesis of Functionalized Naphthalenes/Solvatochromic Dyes
Published on: April 1, 2013
The Dewar benzene radical cation and its ring-opening reaction
Thomas Bally1, Stephan Matzinger, Pawel Bednarek
1Department of Chemistry, University of Fribourg, Switzerland. Thomas.Bally@unifr.ch
Journal of the American Chemical Society
|June 15, 2006
Summary
The radical cation of Dewar benzene (1*+) was observed using optical spectroscopy. Its charge resonance band suggests the odd electron resides in a pi-molecular orbital, indicating a stable structure at 77 K.
Area of Science:
- Physical Chemistry
- Spectroscopy
- Computational Chemistry
Background:
- Radical cations are reactive intermediates crucial in chemical reactions.
- Dewar benzene is a strained isomer of benzene with unique electronic properties.
- Understanding the stability and reactivity of radical cations provides insights into reaction mechanisms.
Purpose of the Study:
- To generate and characterize the radical cation of Dewar benzene (1*+) using optical spectroscopy.
- To investigate the electronic structure and stability of 1*+ in cryogenic matrices.
- To elucidate the reaction mechanism for the decay of 1*+ to the radical cation of benzene (2*+).
Main Methods:
- Generation and observation of the radical cation of Dewar benzene (1*+) in cryogenic matrices.
- Optical spectroscopy to identify characteristic charge resonance bands.
- Theoretical calculations including TD-DFT and CASPT2 to predict transition energies and geometries.
- Analysis of potential energy surfaces and reaction pathways involving avoided crossings.
Main Results:
- The radical cation of Dewar benzene (1*+) was successfully generated and observed.
- A charge resonance band at 600 nm was identified, similar to norbornadiene radical cation, indicating pi-MO localization of the odd electron.
- Theoretical methods (TD-DFT, CASPT2) underestimated the charge resonance transition energy, suggesting geometric discrepancies.
- A kinetic barrier of at least 7-8 kcal/mol separates 1*+ from its decay product, 2*+, enabling observation at 77 K.
- The ring-opening of 1*+ to 2*+ proceeds via a multistep process involving two avoided crossings and pseudodiabatic transitions.
Conclusions:
- The radical cation of Dewar benzene (1*+) possesses a stable structure in cryogenic matrices due to a significant kinetic barrier.
- The electronic structure is characterized by the odd electron residing in a pi-molecular orbital.
- The decay mechanism to the radical cation of benzene (2*+) involves complex surface hopping between electronic states.
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