Phenothiazine radical-cations

Electron transfer equilibria with iodide ions and the determination of one-electron redox potentials by pulse radiolysis

verfasst von: Detlef Bahnemann, Klaus Dieter Asmus, Robin L. Willson
Abstract: The establishment of an equilibrium reaction between iodine radical-anions, I₂^-., the protonated form of the phenothiazine, promethazine, PZH⁺, its related radical-cation, PZH^2+., and iodide ions, I^-, has been observed directly by pulse radiolysis [reaction (i)]. From measurements of the radical-anion and radical-I ₂^-. + PZH⁺ ⇌ PZH^2+. + 2l ^- (i) cation equilibrium concentrations at different iodide and promethazine concentrations a value for the equilibrium constant K₃ = 94 mol dm^-3 has been attained. A closely similar value K₃ = 85 mol dm^-3 has been derived from a kinetic analysis of the radical concentrations as equilibrium was approached. Taking the mean value K₃ = 90 ± 5 mol dm^-3 and a reported value of K ₈ = 8.85 × 10^-6 mol dm^-3 for the equilibrium reaction (ii) the difference in redox potentials given by equation (iii) can be calculated. Values of I₂^-. ⇌ I ^. + I^- (ii) ΔE° = E°(PZH^2+./ PZH⁺) - E°(I^./I^-) = -0.415 V (iii) E°(PZH^2+./PZH⁺) = +0.865 V and E°(l ^./l^-) = +1.31 V have been reported previously. Our experimental result agrees very well with the calculated ΔE° = -0.445 V based on these independently derived redox potentials. Measurements of absolute rate constants for the reaction of PZH^2+. and related radical-cations from chlorpromazine (3) and metiazinic acid (2) with ascorbate (4) (AH^-) and α-tocopherol (5) (α-T) indicate that such electron transfer equilibria are likely to exist also with other phenothiazine cations and strongly reducing compounds. The following bimolecular rate constants have been obtained: k(PZH^2+. + AH^-) = 8.2 × 10⁸, k(PZH^2+. + α-T) = 1.1 × 10 ⁸, k(MZ^±. + AH^-) = 9.0 × 10 ⁷, and k(MZ^±. + α-T) = 1.3 × 10 ⁸ mol^-1 dm³ s^-1.
Externe Organisation(en): Helmholtz-Zentrum Berlin für Materialien und Energie GmbH
Brunel University
Typ: Artikel
Journal: Journal of the Chemical Society, Perkin Transactions 2
Seiten: 1669-1673
Anzahl der Seiten: 5
ISSN: 1472-779X
Publikationsdatum: 1983
Publikationsstatus: Veröffentlicht
Peer-reviewed: Ja
ASJC Scopus Sachgebiete: Allgemeine Chemie
Elektronische Version(en): https://doi.org/10.1039/P29830001669 (Zugang: Unbekannt)

BibTeX

@article{f48330885a1d448386b45f056774a6d5,
title = "Phenothiazine radical-cations: Electron transfer equilibria with iodide ions and the determination of one-electron redox potentials by pulse radiolysis",
abstract = "The establishment of an equilibrium reaction between iodine radical-anions, I2-., the protonated form of the phenothiazine, promethazine, PZH+, its related radical-cation, PZH2+., and iodide ions, I-, has been observed directly by pulse radiolysis [reaction (i)]. From measurements of the radical-anion and radical-I 2-. + PZH+ ⇌ PZH2+. + 2l - (i) cation equilibrium concentrations at different iodide and promethazine concentrations a value for the equilibrium constant K3 = 94 mol dm-3 has been attained. A closely similar value K3 = 85 mol dm-3 has been derived from a kinetic analysis of the radical concentrations as equilibrium was approached. Taking the mean value K3 = 90 ± 5 mol dm-3 and a reported value of K 8 = 8.85 × 10-6 mol dm-3 for the equilibrium reaction (ii) the difference in redox potentials given by equation (iii) can be calculated. Values of I2-. ⇌ I . + I- (ii) ΔE° = E°(PZH2+./ PZH+) - E°(I./I-) = -0.415 V (iii) E°(PZH2+./PZH+) = +0.865 V and E°(l ./l-) = +1.31 V have been reported previously. Our experimental result agrees very well with the calculated ΔE° = -0.445 V based on these independently derived redox potentials. Measurements of absolute rate constants for the reaction of PZH2+. and related radical-cations from chlorpromazine (3) and metiazinic acid (2) with ascorbate (4) (AH-) and α-tocopherol (5) (α-T) indicate that such electron transfer equilibria are likely to exist also with other phenothiazine cations and strongly reducing compounds. The following bimolecular rate constants have been obtained: k(PZH2+. + AH-) = 8.2 × 108, k(PZH2+. + α-T) = 1.1 × 10 8, k(MZ±. + AH-) = 9.0 × 10 7, and k(MZ±. + α-T) = 1.3 × 10 8 mol-1 dm3 s-1.",
author = "Detlef Bahnemann and Asmus, {Klaus Dieter} and Willson, {Robin L.}",
note = "Copyright: Copyright 2015 Elsevier B.V., All rights reserved.",
year = "1983",
doi = "10.1039/P29830001669",
language = "English",
pages = "1669--1673",
journal = "Journal of the Chemical Society, Perkin Transactions 2",
issn = "1472-779X",
publisher = "Chemical Society",
number = "9",
}