title: Hydrogen transfer vs proton transfer in 7-hydroxy-quinoline.(NH3)3: a CASSCF/CASPT2 study.
abstract: Multiconfigurational CASSCF and CASPT2 calculations were performed to investigate the enol --> keto tautomerization in the lowest singlet excited state of the 7-hydroxyquinoline.(NH3)3 cluster. Two different reaction mechanisms were explored. The first one corresponds to that proposed previously by Tanner et al. (Science 2003, 302, 1736) on the basis of experimental observations and CASSCF optimizations under Cs-symmetry constraints. This mechanism comprises four consecutive steps and involves nonadiabatic transitions between the valence 1pipi* state and a pisigma* Rydberg-type state, resulting in hydrogen-atom transfer. Single-point CASPT2 calculations corroborate that for Cs-symmetry pathways hydrogen-atom transfer is clearly preferred over proton transfer. The second mechanism, predicted by CASSCF optimizations without constraints, implies proton transfer along a pathway on the 1pipi* surface in which one or more ammonia molecules depart significantly from the molecular plane defined by the hydroxyquinoline ring. The results suggest that both mechanisms may be competitive with proton transfer being somewhat favorable over hydrogen-atom transfer.
authors.affiliation: Departamento de Química Física, Facultade de Química, Universidade de Santiago de Compostela, Santiago de Compostela, Spain.
authors.forename: Antonio
authors.initials: A
authors.lastname: Fernández-Ramos
authors.forename: Emilio
authors.initials: E
authors.lastname: Martínez-Núñez
authors.forename: Saulo A
authors.initials: SA
authors.lastname: Vázquez
authors.forename: Miguel A
authors.initials: MA
authors.lastname: Ríos
authors.forename: Carlos M
authors.initials: CM
authors.lastname: Estévez
authors.forename: Manuela
authors.initials: M
authors.lastname: Merchán
authors.forename: Luis
authors.initials: L
authors.lastname: Serrano-Andrés
mesh_terms: D002414:Cations, Monovalent / Q000737:chemistry; D006859:Hydrogen / Q000737:chemistry*; D006912:Hydroxyquinolines / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D013237:Stereoisomerism
chemical_list: D002414:Cations, Monovalent; D006912:Hydroxyquinolines; D006859:Hydrogen