title: Inner reorganization during the radical-biradical transition in a nor-beta-lapachone derivative possessing two redox centers.
abstract: In this work, the electrochemical behaviour of an antitumoral nitro o-quinone derivative obtained from 3-bromo-nor-beta-lapachone was studied. Cyclic voltammetric experiments, in acetonitrile solution, revealed that both quinone and nitro functions are reduced independently as quasi-reversible one-electron transfer processes in this order. Depending on the reduction potential, a radical anion or a biradical dianion is obtained. The formation of these paramagnetic species was confirmed by performing in situ Electrochemical-Electron Spin Resonance (E-ESR) experiments. Analysis of the kinetics of electron transfer associated to those electron uptake processes, in terms of the Marcus-Hush-Levich model, revealed differences in the reorganization energy (lambda((k))) for both steps (lambda((I)): 1.07-1.11 eV; lambda((II)): 1.21-1.30 eV). By evaluating the conformations of the radical and biradical systems by calculations at the BLYP//TZVP level of theory, it was found that the inner component, for the second reduction process (lambda((II))) was approximately 72% of lambda((II)), reflecting modifications in the molecular structure during the radical anion-biradical dianion transition. This change is also reflected in the differences presented by line widths of the ESR signals of both electrogenerated radical and diradical species.
authors.affiliation: Departamento de Química, Centro de Investigación y Estudios Avanzados, Av. Instituto Politécnico Nacional No. 2508, San Pedro Zacatenco, C. P. 07360. México.
authors.forename: Dulce María
authors.initials: DM
authors.lastname: Hernández
authors.forename: Maria Aline B F
authors.initials: MA
authors.lastname: de Moura
authors.forename: Drochss Pettry
authors.initials: DP
authors.lastname: Valencia
authors.forename: Felipe J
authors.initials: FJ
authors.lastname: González
authors.forename: Ignacio
authors.initials: I
authors.lastname: González
authors.forename: Fabiane C
authors.initials: FC
authors.lastname: de Abreu
authors.forename: Eufrânio N
authors.initials: EN
authors.lastname: da Silva Júnior
authors.forename: Vitor F
authors.initials: VF
authors.lastname: Ferreira
authors.forename: Antônio Ventura
authors.initials: AV
authors.lastname: Pinto
authors.forename: Marilia O F
authors.initials: MO
authors.lastname: Goulart
authors.forename: Carlos
authors.initials: C
authors.lastname: Frontana
mesh_terms: D004563:Electrochemistry; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000737:chemistry; D015394:Molecular Structure; D009285:Naphthoquinones / Q000737:chemistry*; D010084:Oxidation-Reduction
chemical_list: D005609:Free Radicals; D009285:Naphthoquinones; C014638:beta-lapachone