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Hydrogen transfer vs proton transfer in 7-hydroxy-quinoline.(NH3)3: a CASSCF/CASPT2 study
Antonio Fernández-Ramos1, Emilio Martínez-Núñez, Saulo A Vázquez
1Departamento de Química Física, Facultade de Química, Universidade de Santiago de Compostela, Santiago de Compostela, Spain.
The Journal of Physical Chemistry. A
|June 15, 2007
Summary
Investigating excited-state tautomerization in 7-hydroxyquinoline.(NH3)3, this study reveals two competitive mechanisms. Hydrogen-atom transfer is favored under Cs-symmetry constraints, while proton transfer dominates without constraints.
Area of Science:
- Quantum Chemistry
- Photochemistry
- Molecular Dynamics
Background:
- Tautomerization is a fundamental process in photochemistry.
- The excited-state tautomerization of 7-hydroxyquinoline is of significant interest.
- Previous studies suggested a hydrogen-atom transfer mechanism.
Purpose of the Study:
- To investigate the enol to keto tautomerization in the lowest singlet excited state of the 7-hydroxyquinoline.(NH3)3 cluster.
- To explore and compare two distinct reaction mechanisms.
- To elucidate the role of nonadiabatic transitions and symmetry constraints.
Main Methods:
- Multiconfigurational Complete Active Space Self-Consistent Field (CASSCF) calculations.
- Multi-state Complete Active Space Second-Order Perturbation Theory (CASPT2) calculations.
- Exploration of reaction pathways under Cs-symmetry constraints and without constraints.
Main Results:
- Two competing tautomerization mechanisms were identified: hydrogen-atom transfer and proton transfer.
- Under Cs-symmetry constraints, hydrogen-atom transfer via nonadiabatic transitions between 1pipi* and pisigma* states is favored.
- Without constraints, proton transfer along the 1pipi* surface is predicted, involving ammonia molecule displacement.
Conclusions:
- Both hydrogen-atom and proton transfer mechanisms are competitive in the excited-state tautomerization of 7-hydroxyquinoline.(NH3)3.
- Proton transfer appears slightly more favorable when symmetry constraints are relaxed.
- The study highlights the sensitivity of reaction pathways to computational constraints and electronic states.
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