TY - JOUR

T1 - Phenothiazine radical-cations

T2 - Electron transfer equilibria with iodide ions and the determination of one-electron redox potentials by pulse radiolysis

AU - Bahnemann, Detlef

AU - Asmus, Klaus Dieter

AU - Willson, Robin L.

N1 - Copyright: Copyright 2015 Elsevier B.V., All rights reserved.

PY - 1983

Y1 - 1983

N2 - 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.

AB - 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.

UR - http://www.scopus.com/inward/record.url?scp=0007689742&partnerID=8YFLogxK

U2 - 10.1039/P29830001669

DO - 10.1039/P29830001669

M3 - Article

AN - SCOPUS:0007689742

SP - 1669

EP - 1673

JO - Journal of the Chemical Society, Perkin Transactions 2

JF - Journal of the Chemical Society, Perkin Transactions 2

SN - 1472-779X

IS - 9

ER -