Estudio electroquímico de la reactividad de quinonas en acetonitrilo. Influencia de la estructura molecular
The quinone functional group is commonly found in the nature as a component of structures of chemical systems presenting biological activity. Such activity arises from the capacity that quinones present to generate stable radical species during their electrochemical reduction. Since the stability of...
| Autor: | |
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2006 |
| País: | México |
| Institución: | Universidad Autónoma Metropolitana |
| Repositorio: | Repositorio Institucional de la UAM Iztapalapa |
| Idioma: | español |
| OAI Identifier: | oai:bindani.izt.uam.mx:mc87pq840 |
| Acceso en línea: | https://doi.org/10.24275/uami.mc87pq840 |
| Access Level: | acceso abierto |
| Palabra clave: | info:eu-repo/classification/LEM/Quinone info:eu-repo/classification/LEM/Electrochemistry info:eu-repo/classification/LEM/Quinona info:eu-repo/classification/LEM/Electroquímica info:eu-repo/classification/cti/2 |
| Sumario: | The quinone functional group is commonly found in the nature as a component of structures of chemical systems presenting biological activity. Such activity arises from the capacity that quinones present to generate stable radical species during their electrochemical reduction. Since the stability of such radical species (semiquinones) is determined, among other factors, by their chemical structures, it is important to consider the effect that a given modification in the chemical environment of the quinone, has in the processes of generation of these radical species. For these purposes, the employment of electrochemical methods of analysis provides a powerful alternative to analyze the effect that the molecular modification has during the semiquinone formation. The present work focuses in the study of the reactivity of the reduction process quinone-semiquinone in acetonitrile, in order to evaluate the way in which the chemical structure modifies the semiquinone reactivity. This study is divided in four chapters. The first chapter deal with the possibility of establish a quantitative relationship between the effect of the substituent effect of groups present in the quinone, with the halfwave potential values (E1/2) associated to the process of interest. For this purpose two different models were employed: the first ones employed the Hammett-Zuman formalism, while for the second it was used a strategy of quantum chemical calculations to estimate the Electron Affinity (EA) for the quinone-semiquinone process in gas phase. The limitations presented for both models were related to the innacuracy that they have for describing the influence of the reactivity of the semiquinone in the E1/2 values for a series of 28 quinones (of the type 1,4-benzoquinone, 1,4-naphthoquinone y 9,10-anthraquinone). For this reason, it was required to evaluate the magnitude of the substituente effect in the electron transfer kinetics, with the aim of improving the models for describing the quinone reactivity, which was the subject of the second chapter. In the second chapter, the kinetic parameters (ks: apparent rate constant for electron transfer and transfer coefficient), asociated with the electronic transfer quinone-semiquinone, were evaluated employing a novel procedure involving the use of the Laplace transform method. With this methodology, the transfer coefficient () was independently evaluated for the studied quinones. Both kinetic parameters presented notable differences between each quinone family (1,4-benzoquinone vs 1,4- naphthoquinone vs 9,10-anthraquinone). From the data of the apparent rate constant -ksan estimation of the activation free energy (G ‡ ) was performed, finding that the corresponding values are influenced by the nature of the substituents present in each quinone family. The estimation of the inner (i) and outer (o) components of the reorganization energy () was performed to evaluate their contributions in the G ‡ values, within the framework of the microscopic electron transfer theory of Marcus and Hush. From experimental spin density data of the electrogenerated semiquinone, obtained from spectroelectrochemical – Electron Spin Resonance (ESR) methods; it was possible e to observe that the differences in reactivity occuring for the studied quinones, were determined by the spin density localization that occurs in a particular species. This effect of spin density localization is highly related to the symmetry of the electrogenerated semiquinonas and has consequences on the transfer coefficient for the corresponding reduction process. In the third chapter, the presence of Intramolecular Hydrogen Bond (IHB) interactions as structural modification features for quinones was studied. The study of a select group of and -hydroxyquinones, showed that, both the process of formation and stability of IHB bonds, and coupled chemical reactions to the electron transfer, modify the energy, as E1/2, and the kinetic parameters associated to the electrochemical reduction of the studied hydroxyquinones. The experimental stability of the electrogenerated semiquinones from -hydroxyquinones, evaluated by spectroelectrochemical-ESR experiments, suggested that reactivity of such species is dependent on the symmetry of distribution of the spin density. Likewise, the electrochemical analysis of a synthetic - hydroxquinone 2HNQ (2-hydroxy-1,4- naphtoquinone) by cyclic voltammetry and double step potential chronoamperometry, revealed that selfprotonation processes are coupled to the first electron transfer. One of the products of this reduction step is the deprotonated quinone, which is reduced at the second reduction signal observed for this compound, generating a radical dianion species. The electrogenerated radical dianion was detected employing spectroelectrochemical-ESR experiments and its spectrum suggests that the first carbonyl quinone group that is reduced in the one adjacent to the deprotonated hydroxy functionality. This strategy of analysis allowed establishing a methodology to characterize the reactivity of the radical dianion species electrogenerated during the electrochemical reduction of natural occuring -hydroxyquinones (Perezone, Horminone y 7-O- methylConacytone). Therefore, in the fourth chapter the relationship between the structural ESR data with the charge transfer properties associated to the formation of radical dianions is studied. For Perezone y la Horminone, the structure and stability of the electrogenerated radical dianions is modulated by the presence of and IHB type interactions. However, in the case of 7-O-methyl-Conacytone, the formation of the stable radical dianion was found to be associated to the generation of two radical species. This behavior is compatible with the one observed upon the electrochemical study of a methyl derivative at the -hydroxy functionality of this quinone (7, 12, 20- O-trimethyl-Conacytone). The study of this last compound revealed that during the first electron transfer process, the first semiquinona electrogenerated is reactive enough to generate another radical species leading to a two-electron voltammetric peak. The obtained results allowed a broader vision of the quinone-semiquinone electron transfer process in terms of the molecular structure and also allow proposing some features related to the design of new quinonoid structures with different electrochemical properties. |
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