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Published on: July 19, 2019
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Excited-state proton transfer relieves antiaromaticity in molecules
Chia-Hua Wu1, Lucas José Karas1, Henrik Ottosson2
1Department of Chemistry, University of Houston, Houston, TX 77004.
Summary
Baird's rule explains excited-state proton transfer (ESPT) in organic molecules. Proton transfer relieves antiaromaticity in excited states, a phenomenon confirmed by NICS calculations and observed in various compounds.
Area of Science:
- Organic Chemistry
- Photochemistry
- Quantum Chemistry
Background:
- Excited-state proton transfer (ESPT) is a key photochemical process in organic compounds.
- Baird's rule provides a theoretical framework for understanding aromaticity and antiaromaticity in excited states.
- Antiaromaticity in excited states can drive chemical reactions to relieve strain.
Purpose of the Study:
- To elucidate the role of excited-state antiaromaticity in driving ESPT reactions.
- To validate Baird's rule using computational methods and experimental observations.
- To investigate the influence of light on hydrogen bond strengths in ESPT systems.
Main Methods:
- Computational chemistry, specifically nucleus-independent chemical shift (NICS) calculations.
- Analysis of experimental data, including Stokes' shifts and photochemical behavior.
- Theoretical application of Baird's rule to various ESPT reaction mechanisms.
Main Results:
- NICS calculations confirm significant antiaromaticity in the excited states of ESPT-competent molecules.
- ESPT reactions were observed to occur primarily in antiaromatic excited states (e.g., S1), not aromatic ones (e.g., S2).
- Baird's rule accurately predicts the modulation of hydrogen bond strengths upon photoexcitation, correlating with changes in antiaromaticity.
Conclusions:
- Excited-state antiaromaticity is a crucial driving force for ESPT reactions, as explained by Baird's rule.
- The interplay between electronic structure, antiaromaticity, and proton transfer is fundamental to understanding excited-state dynamics.
- Baird's rule offers predictive power for photochemical reactions and light-induced changes in molecular interactions.
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